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cabling explained
Mr Lampen of Belden Electronics Division answered this posts on
"rec.audio.high-end" some time back. I hope this answers your questions in regards to cable quality and length of runs whether interconnect or speaker. John In article , Lou Anschuetz writes: OTOH, I can understand how he comes to the conclusions he comes to. As someone with 25 years in IT, I have thrown away dozens of supposedly competently designed SCSI/serial cables that don't work. SCSI cables in particular are darn hard to to get right for the newer higher speeds - and it all seems to come down to how the cable(s) is insulated. I also agree that this doesn't necessarily have anything to do with audio transmission. However, IMHO, this business about insulation continues to bedevil me. Since it does play havoc with digital signal transmission, it seems to me hard to leap to an absolute conclusion that it has no effect in the analog audio world. No hard science on this yet, but it is a very active field of research... Well, I'm on a plane flying from Beijing to Hong Kong (and on to Melbourne, Australia) listening to Chinese Pop music in my headphones, and nothing to do, so I thought I would just jump in here! Why do some cables work on signal X but not with signal Y? The answer is simple: wavelength. (The answer is simple, the explanation is a little more complicated.) Every signal, data, audio, video, etc, etc. occupies one or more frequencies. Each frequency has a wavelength, that is, there is an electromagnetic wave moving down the cable that has a specific length. This can be calculated by dividing 300,000,000 by that frequency. (The answer comes out in meters, so us backwards USA types have to multiply by 3.28 to get to feet.) If you look at 20 kHz (or you can pick any frequency you want), that gives you a wavelength of 15,000 meters (about 9 miles). What that means is that the impedance of the cable (how it reacts to frequencies) is of no consequence. Most engineers agree that the ACTUAL critical distance is 1/4 wavelength which is, in this case 2.25 miles, still too far to have any effect. This distance is also affect by the plastic around the conductors. This affects the speed "velocity" of the signal. (Signals only move at the speed of light in a vaccum, i.e. outer space. If you ever have a chance to check out your speakers there, be sure and give me a complete test report!) Let's say your interconnect cables were the worst ever made (a "velocity of propagation" of 50%). You would multiply the wavelength by 50% so we're down to 1.12 miles. Anybody with one mile speaker cables? data cable? mic cables? video cables? audio cable? Well, yes, there are people with AUDIO cables of that length or more. They call themselves the TELEPHONE COMPANY. And the maximum distance between your home and your central office is 13,000 ft. (over 2 miles). But the twisted pairs they use (even fancy new ones, Category 3) are not the correct impedance (too expensive). Luckily, the audio on your phone ends at 3500 Hz. You can calculate the bandwidth and see you can go VERY far before you need to have impedance-specific twisted pairs. I'll be honest and tell you that I don't know the actual occupied bandwidth of a 56k data signal. (I think it's NRZI Manchester Coded, that would make it 28kHz bandwidth.) I'm sure you note that this puts it on the edge for performance if you are far from the central office, and you can't always get dialup that fast. And you'll understand why in the USA, the FCC now mandates that all telephone wiring must be Category 3 or better. Category 3 is impedance-specific (100 ohms) data cable. But there are other reasons the FCC made this a standard besides distance. The signals either go back to the source or simply stop in the cable (called "standing waves") and then radiate all that reflected energegy into cables around them. That's why the phone company is trying to use data cable, to avoid "alien crosstalk" (between cables). That's why you can use crappy interconnect cables that come free with your receiver and it all sounds pretty good. They aren't long enough to make a difference. As long as you have continuity (electrical signal flow), you'll be just fine. But what if you use that cable for DIGITAL (i.e. S/PDIF). Suddenly we're sampling that audio at 44.1kHz (like a CD), and the bandwidth is defined as 128 times the sampling (5.6448 MHz, if memory serves). Let's just use 6 MHz to make it easy. (If you don't want easy, do it yourself.) 300,000,000 divided by 6,000,000 = 50 meters. 1/4-wavelength = 12.5 meters or about 44 feet. How long is your cable? 3 ft? 6 ft? No problem. It's still not LONG enough to make a difference. Now, if you want to send S/PDIF to the other end of the house, that might be 40 ft. or even more. What would you do? Be sure and use cable which is the correct impedance. S/PDIF requires 75 ohm cable (75 ohm cable has the lowest signal loss "attenuation" so it is used for many applications where low loss is required). What if you get a really long piece of crappy cable? What impedance is it? Who knows!! And what will happen is that the signal, which will require a 75 ohm "transmission line" and not see it. The farther it is away from 75 ohm the more the signal will be REFLECTED back to the source and not get to the other end. This is called "return loss" in the cable world. You send me the actual impedance of your cable and I can calculate the mismatch and the return loss. But, if you get the right 75 ohm cable, how far can you go? It depends on the size of the wire (gage) in the middle. Let's assume it is a 20 AWG center (such as Belden 1505A, the world's most popular coax cable and a nice generic size).. Then the cable could go 716 ft. (This is based on a source voltage of 0.5v and a minimum received voltage of 0.2v, at a bandwidth of 5.6448 MHz.) Yup, 700 feet!!! (Snippy aside: try going 700 ft. on a piece of toslink plastic fiber!) And in the digital format, you can be real close to the maximum and the signal (i.e. data/audio) sounds just perfect. But add in one patch cord, or one less than perfect connector, or just a few more feet of cable, and you could be down the slippery slope of the "digital cliff". So, how many of you have to run 700 ft.??? Not many, I assume. This is not to say you shouldn't have cable with low capacitance. Digital signals need low capacitance to keep their sharp edges (i.e. the clock). But (amazing fact that no cable manufacturer ever told you) once you choose the construction (i.e. plastic/dielectric/velocity) and the impedance (i.e. 75 ohms) the capacitance is AUTOMATIC. High velocity foam coax 75 ohm = 15 pF/ft. Old solid polyethylene 75 ohm coax = 20 pF/ft. And this is why you should avoid CATV coax, because it's not all copper in that center conductor (it's copper-clad steel for high-frequency-only applications where only the skin of the conductor is working.) This means the resistance is 5 to 7 times higher than an all-copper center and the distance you can go is 5 to 7 times shorter. (OK, we're down to 100 ft. and you're going 6 ft., but I wouldn't use it.) And all those SCSI cables (at the beginning of this thread) suffer from the same problem. Most of them use generic multiconductors (just pull one apart if you don't believe me). The impedance of a pair of wires is determined by their size (gage), the distance between them and the quality (dielectric contact) of the material inbetween. So, while the plastic is important, these cables fail at high frequencies because of the DISTANCE between conductors, which varies all over the place. Open up a SCSI 2 or 3 cable, what do you see? Ahhhh, twisted pairs!!! And the better the pairs (tighter impedance specs) the less the mismatch, the less the return loss, the greater the received signal strength, the less the bit errors. Flat cable can also help maintain impedance specs, at least better than just a bunch of loose wires. As frequencies go higher and higher, every dimension becomes more critical. Just take your car on a racetrack if you don't believe me. You need a race car? Buy a race car! You need a high-frequency cable? Buy one made, tested, verified, and guaranteed for whatever frequency band you wish. OK, class, how did I do? Steve Lampen Belden Electronics Division |
#2
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cabling explained
Very nice..
Adair |
#3
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cabling explained
Midlant wrote:
Mr Lampen of Belden Electronics Division answered this posts on "rec.audio.high-end" some time back. I hope this answers your questions in regards to cable quality and length of runs whether interconnect or speaker. John In article , Lou Anschuetz writes: OTOH, I can understand how he comes to the conclusions he comes to. As someone with 25 years in IT, I have thrown away dozens of supposedly competently designed SCSI/serial cables that don't work. SCSI cables in particular are darn hard to to get right for the newer higher speeds - and it all seems to come down to how the cable(s) is insulated. I also agree that this doesn't necessarily have anything to do with audio transmission. However, IMHO, this business about insulation continues to bedevil me. Since it does play havoc with digital signal transmission, it seems to me hard to leap to an absolute conclusion that it has no effect in the analog audio world. No hard science on this yet, but it is a very active field of research... Well, I'm on a plane flying from Beijing to Hong Kong (and on to Melbourne, Australia) listening to Chinese Pop music in my headphones, and nothing to do, so I thought I would just jump in here! Why do some cables work on signal X but not with signal Y? The answer is simple: wavelength. (The answer is simple, the explanation is a little more complicated.) Every signal, data, audio, video, etc, etc. occupies one or more frequencies. Each frequency has a wavelength, that is, there is an electromagnetic wave moving down the cable that has a specific length. This can be calculated by dividing 300,000,000 by that frequency. (The answer comes out in meters, so us backwards USA types have to multiply by 3.28 to get to feet.) If you look at 20 kHz (or you can pick any frequency you want), that gives you a wavelength of 15,000 meters (about 9 miles). What that means is that the impedance of the cable (how it reacts to frequencies) is of no consequence. Most engineers agree that the ACTUAL critical distance is 1/4 wavelength which is, in this case 2.25 miles, still too far to have any effect. This distance is also affect by the plastic around the conductors. This affects the speed "velocity" of the signal. (Signals only move at the speed of light in a vaccum, i.e. outer space. If you ever have a chance to check out your speakers there, be sure and give me a complete test report!) Let's say your interconnect cables were the worst ever made (a "velocity of propagation" of 50%). You would multiply the wavelength by 50% so we're down to 1.12 miles. Anybody with one mile speaker cables? data cable? mic cables? video cables? audio cable? Well, yes, there are people with AUDIO cables of that length or more. They call themselves the TELEPHONE COMPANY. And the maximum distance between your home and your central office is 13,000 ft. (over 2 miles). But the twisted pairs they use (even fancy new ones, Category 3) are not the correct impedance (too expensive). Luckily, the audio on your phone ends at 3500 Hz. You can calculate the bandwidth and see you can go VERY far before you need to have impedance-specific twisted pairs. I'll be honest and tell you that I don't know the actual occupied bandwidth of a 56k data signal. (I think it's NRZI Manchester Coded, that would make it 28kHz bandwidth.) I'm sure you note that this puts it on the edge for performance if you are far from the central office, and you can't always get dialup that fast. And you'll understand why in the USA, the FCC now mandates that all telephone wiring must be Category 3 or better. Category 3 is impedance-specific (100 ohms) data cable. But there are other reasons the FCC made this a standard besides distance. The signals either go back to the source or simply stop in the cable (called "standing waves") and then radiate all that reflected energegy into cables around them. That's why the phone company is trying to use data cable, to avoid "alien crosstalk" (between cables). That's why you can use crappy interconnect cables that come free with your receiver and it all sounds pretty good. They aren't long enough to make a difference. As long as you have continuity (electrical signal flow), you'll be just fine. But what if you use that cable for DIGITAL (i.e. S/PDIF). Suddenly we're sampling that audio at 44.1kHz (like a CD), and the bandwidth is defined as 128 times the sampling (5.6448 MHz, if memory serves). Let's just use 6 MHz to make it easy. (If you don't want easy, do it yourself.) 300,000,000 divided by 6,000,000 = 50 meters. 1/4-wavelength = 12.5 meters or about 44 feet. How long is your cable? 3 ft? 6 ft? No problem. It's still not LONG enough to make a difference. Now, if you want to send S/PDIF to the other end of the house, that might be 40 ft. or even more. What would you do? Be sure and use cable which is the correct impedance. S/PDIF requires 75 ohm cable (75 ohm cable has the lowest signal loss "attenuation" so it is used for many applications where low loss is required). What if you get a really long piece of crappy cable? What impedance is it? Who knows!! And what will happen is that the signal, which will require a 75 ohm "transmission line" and not see it. The farther it is away from 75 ohm the more the signal will be REFLECTED back to the source and not get to the other end. This is called "return loss" in the cable world. You send me the actual impedance of your cable and I can calculate the mismatch and the return loss. But, if you get the right 75 ohm cable, how far can you go? It depends on the size of the wire (gage) in the middle. Let's assume it is a 20 AWG center (such as Belden 1505A, the world's most popular coax cable and a nice generic size).. Then the cable could go 716 ft. (This is based on a source voltage of 0.5v and a minimum received voltage of 0.2v, at a bandwidth of 5.6448 MHz.) Yup, 700 feet!!! (Snippy aside: try going 700 ft. on a piece of toslink plastic fiber!) And in the digital format, you can be real close to the maximum and the signal (i.e. data/audio) sounds just perfect. But add in one patch cord, or one less than perfect connector, or just a few more feet of cable, and you could be down the slippery slope of the "digital cliff". So, how many of you have to run 700 ft.??? Not many, I assume. This is not to say you shouldn't have cable with low capacitance. Digital signals need low capacitance to keep their sharp edges (i.e. the clock). But (amazing fact that no cable manufacturer ever told you) once you choose the construction (i.e. plastic/dielectric/velocity) and the impedance (i.e. 75 ohms) the capacitance is AUTOMATIC. High velocity foam coax 75 ohm = 15 pF/ft. Old solid polyethylene 75 ohm coax = 20 pF/ft. And this is why you should avoid CATV coax, because it's not all copper in that center conductor (it's copper-clad steel for high-frequency-only applications where only the skin of the conductor is working.) This means the resistance is 5 to 7 times higher than an all-copper center and the distance you can go is 5 to 7 times shorter. (OK, we're down to 100 ft. and you're going 6 ft., but I wouldn't use it.) And all those SCSI cables (at the beginning of this thread) suffer from the same problem. Most of them use generic multiconductors (just pull one apart if you don't believe me). The impedance of a pair of wires is determined by their size (gage), the distance between them and the quality (dielectric contact) of the material inbetween. So, while the plastic is important, these cables fail at high frequencies because of the DISTANCE between conductors, which varies all over the place. Open up a SCSI 2 or 3 cable, what do you see? Ahhhh, twisted pairs!!! And the better the pairs (tighter impedance specs) the less the mismatch, the less the return loss, the greater the received signal strength, the less the bit errors. Flat cable can also help maintain impedance specs, at least better than just a bunch of loose wires. As frequencies go higher and higher, every dimension becomes more critical. Just take your car on a racetrack if you don't believe me. You need a race car? Buy a race car! You need a high-frequency cable? Buy one made, tested, verified, and guaranteed for whatever frequency band you wish. OK, class, how did I do? Steve Lampen Belden Electronics Division THANK YOU FOR THIS GREAT POST! You are deserving of a great thank-you man. I was trying to figure out cabling and wiring and was going to post about it before I know it you post it all here together. Great information and very easy to understand. You should be a teacher if it pays better. EFFENDI |
#4
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cabling explained
Very good explanation. Thank you.
-Bill "Midlant" wrote in message news:v_fsb.20629$oB3.15068@lakeread03... Mr Lampen of Belden Electronics Division answered this posts on "rec.audio.high-end" some time back. I hope this answers your questions in regards to cable quality and length of runs whether interconnect or speaker. John In article , Lou Anschuetz writes: OTOH, I can understand how he comes to the conclusions he comes to. As someone with 25 years in IT, I have thrown away dozens of supposedly competently designed SCSI/serial cables that don't work. SCSI cables in particular are darn hard to to get right for the newer higher speeds - and it all seems to come down to how the cable(s) is insulated. I also agree that this doesn't necessarily have anything to do with audio transmission. However, IMHO, this business about insulation continues to bedevil me. Since it does play havoc with digital signal transmission, it seems to me hard to leap to an absolute conclusion that it has no effect in the analog audio world. No hard science on this yet, but it is a very active field of research... Well, I'm on a plane flying from Beijing to Hong Kong (and on to Melbourne, Australia) listening to Chinese Pop music in my headphones, and nothing to do, so I thought I would just jump in here! Why do some cables work on signal X but not with signal Y? The answer is simple: wavelength. (The answer is simple, the explanation is a little more complicated.) Every signal, data, audio, video, etc, etc. occupies one or more frequencies. Each frequency has a wavelength, that is, there is an electromagnetic wave moving down the cable that has a specific length. This can be calculated by dividing 300,000,000 by that frequency. (The answer comes out in meters, so us backwards USA types have to multiply by 3.28 to get to feet.) If you look at 20 kHz (or you can pick any frequency you want), that gives you a wavelength of 15,000 meters (about 9 miles). What that means is that the impedance of the cable (how it reacts to frequencies) is of no consequence. Most engineers agree that the ACTUAL critical distance is 1/4 wavelength which is, in this case 2.25 miles, still too far to have any effect. This distance is also affect by the plastic around the conductors. This affects the speed "velocity" of the signal. (Signals only move at the speed of light in a vaccum, i.e. outer space. If you ever have a chance to check out your speakers there, be sure and give me a complete test report!) Let's say your interconnect cables were the worst ever made (a "velocity of propagation" of 50%). You would multiply the wavelength by 50% so we're down to 1.12 miles. Anybody with one mile speaker cables? data cable? mic cables? video cables? audio cable? Well, yes, there are people with AUDIO cables of that length or more. They call themselves the TELEPHONE COMPANY. And the maximum distance between your home and your central office is 13,000 ft. (over 2 miles). But the twisted pairs they use (even fancy new ones, Category 3) are not the correct impedance (too expensive). Luckily, the audio on your phone ends at 3500 Hz. You can calculate the bandwidth and see you can go VERY far before you need to have impedance-specific twisted pairs. I'll be honest and tell you that I don't know the actual occupied bandwidth of a 56k data signal. (I think it's NRZI Manchester Coded, that would make it 28kHz bandwidth.) I'm sure you note that this puts it on the edge for performance if you are far from the central office, and you can't always get dialup that fast. And you'll understand why in the USA, the FCC now mandates that all telephone wiring must be Category 3 or better. Category 3 is impedance-specific (100 ohms) data cable. But there are other reasons the FCC made this a standard besides distance. The signals either go back to the source or simply stop in the cable (called "standing waves") and then radiate all that reflected energegy into cables around them. That's why the phone company is trying to use data cable, to avoid "alien crosstalk" (between cables). That's why you can use crappy interconnect cables that come free with your receiver and it all sounds pretty good. They aren't long enough to make a difference. As long as you have continuity (electrical signal flow), you'll be just fine. But what if you use that cable for DIGITAL (i.e. S/PDIF). Suddenly we're sampling that audio at 44.1kHz (like a CD), and the bandwidth is defined as 128 times the sampling (5.6448 MHz, if memory serves). Let's just use 6 MHz to make it easy. (If you don't want easy, do it yourself.) 300,000,000 divided by 6,000,000 = 50 meters. 1/4-wavelength = 12.5 meters or about 44 feet. How long is your cable? 3 ft? 6 ft? No problem. It's still not LONG enough to make a difference. Now, if you want to send S/PDIF to the other end of the house, that might be 40 ft. or even more. What would you do? Be sure and use cable which is the correct impedance. S/PDIF requires 75 ohm cable (75 ohm cable has the lowest signal loss "attenuation" so it is used for many applications where low loss is required). What if you get a really long piece of crappy cable? What impedance is it? Who knows!! And what will happen is that the signal, which will require a 75 ohm "transmission line" and not see it. The farther it is away from 75 ohm the more the signal will be REFLECTED back to the source and not get to the other end. This is called "return loss" in the cable world. You send me the actual impedance of your cable and I can calculate the mismatch and the return loss. But, if you get the right 75 ohm cable, how far can you go? It depends on the size of the wire (gage) in the middle. Let's assume it is a 20 AWG center (such as Belden 1505A, the world's most popular coax cable and a nice generic size).. Then the cable could go 716 ft. (This is based on a source voltage of 0.5v and a minimum received voltage of 0.2v, at a bandwidth of 5.6448 MHz.) Yup, 700 feet!!! (Snippy aside: try going 700 ft. on a piece of toslink plastic fiber!) And in the digital format, you can be real close to the maximum and the signal (i.e. data/audio) sounds just perfect. But add in one patch cord, or one less than perfect connector, or just a few more feet of cable, and you could be down the slippery slope of the "digital cliff". So, how many of you have to run 700 ft.??? Not many, I assume. This is not to say you shouldn't have cable with low capacitance. Digital signals need low capacitance to keep their sharp edges (i.e. the clock). But (amazing fact that no cable manufacturer ever told you) once you choose the construction (i.e. plastic/dielectric/velocity) and the impedance (i.e. 75 ohms) the capacitance is AUTOMATIC. High velocity foam coax 75 ohm = 15 pF/ft. Old solid polyethylene 75 ohm coax = 20 pF/ft. And this is why you should avoid CATV coax, because it's not all copper in that center conductor (it's copper-clad steel for high-frequency-only applications where only the skin of the conductor is working.) This means the resistance is 5 to 7 times higher than an all-copper center and the distance you can go is 5 to 7 times shorter. (OK, we're down to 100 ft. and you're going 6 ft., but I wouldn't use it.) And all those SCSI cables (at the beginning of this thread) suffer from the same problem. Most of them use generic multiconductors (just pull one apart if you don't believe me). The impedance of a pair of wires is determined by their size (gage), the distance between them and the quality (dielectric contact) of the material inbetween. So, while the plastic is important, these cables fail at high frequencies because of the DISTANCE between conductors, which varies all over the place. Open up a SCSI 2 or 3 cable, what do you see? Ahhhh, twisted pairs!!! And the better the pairs (tighter impedance specs) the less the mismatch, the less the return loss, the greater the received signal strength, the less the bit errors. Flat cable can also help maintain impedance specs, at least better than just a bunch of loose wires. As frequencies go higher and higher, every dimension becomes more critical. Just take your car on a racetrack if you don't believe me. You need a race car? Buy a race car! You need a high-frequency cable? Buy one made, tested, verified, and guaranteed for whatever frequency band you wish. OK, class, how did I do? Steve Lampen Belden Electronics Division |
#5
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cabling explained
Unfortunately, I can't share the enthusiasm of the other posters
responding in this thread. The post says absolutely nothing to us in the audio world, and is in fact completely irrelevant. He's looking at the issue from a microwave transmission line perspective which, as he pointed out in his first paragraph, doesn't apply to audio. One of the most comprehensive articles I've read on the subject of speaker cables was written by Fred Davis in the '80s and published by the Journal of the Audio Engineering Society. I've placed a reprint at the following website: http://www.geocities.com/audiotechpa...ractions. pdf Davis essentially examines the relevant electrical properties of several different cables (everything from $100/meter cables to jumper cables he found in his garage to lamp cord) and performs an analysis of their interactions with loudspeakers and amplifiers. Also noteworthy is that Davis looks at capacitance and inductance properties in the cables and compares them to various aspects of their construction which, unlike the assertion made in the original post, depends on many more factors than just the dielectric properties. Rather, the geometrical properties and other electrical properties come into play. Midlant wrote: Mr Lampen of Belden Electronics Division answered this posts on "rec.audio.high-end" some time back. I hope this answers your questions in regards to cable quality and length of runs whether interconnect or speaker. John In article , Lou Anschuetz writes: OTOH, I can understand how he comes to the conclusions he comes to. As someone with 25 years in IT, I have thrown away dozens of supposedly competently designed SCSI/serial cables that don't work. SCSI cables in particular are darn hard to to get right for the newer higher speeds - and it all seems to come down to how the cable(s) is insulated. I also agree that this doesn't necessarily have anything to do with audio transmission. However, IMHO, this business about insulation continues to bedevil me. Since it does play havoc with digital signal transmission, it seems to me hard to leap to an absolute conclusion that it has no effect in the analog audio world. No hard science on this yet, but it is a very active field of research... Well, I'm on a plane flying from Beijing to Hong Kong (and on to Melbourne, Australia) listening to Chinese Pop music in my headphones, and nothing to do, so I thought I would just jump in here! Why do some cables work on signal X but not with signal Y? The answer is simple: wavelength. (The answer is simple, the explanation is a little more complicated.) Every signal, data, audio, video, etc, etc. occupies one or more frequencies. Each frequency has a wavelength, that is, there is an electromagnetic wave moving down the cable that has a specific length. This can be calculated by dividing 300,000,000 by that frequency. (The answer comes out in meters, so us backwards USA types have to multiply by 3.28 to get to feet.) If you look at 20 kHz (or you can pick any frequency you want), that gives you a wavelength of 15,000 meters (about 9 miles). What that means is that the impedance of the cable (how it reacts to frequencies) is of no consequence. Most engineers agree that the ACTUAL critical distance is 1/4 wavelength which is, in this case 2.25 miles, still too far to have any effect. This distance is also affect by the plastic around the conductors. This affects the speed "velocity" of the signal. (Signals only move at the speed of light in a vaccum, i.e. outer space. If you ever have a chance to check out your speakers there, be sure and give me a complete test report!) Let's say your interconnect cables were the worst ever made (a "velocity of propagation" of 50%). You would multiply the wavelength by 50% so we're down to 1.12 miles. Anybody with one mile speaker cables? data cable? mic cables? video cables? audio cable? Well, yes, there are people with AUDIO cables of that length or more. They call themselves the TELEPHONE COMPANY. And the maximum distance between your home and your central office is 13,000 ft. (over 2 miles). But the twisted pairs they use (even fancy new ones, Category 3) are not the correct impedance (too expensive). Luckily, the audio on your phone ends at 3500 Hz. You can calculate the bandwidth and see you can go VERY far before you need to have impedance-specific twisted pairs. I'll be honest and tell you that I don't know the actual occupied bandwidth of a 56k data signal. (I think it's NRZI Manchester Coded, that would make it 28kHz bandwidth.) I'm sure you note that this puts it on the edge for performance if you are far from the central office, and you can't always get dialup that fast. And you'll understand why in the USA, the FCC now mandates that all telephone wiring must be Category 3 or better. Category 3 is impedance-specific (100 ohms) data cable. But there are other reasons the FCC made this a standard besides distance. The signals either go back to the source or simply stop in the cable (called "standing waves") and then radiate all that reflected energegy into cables around them. That's why the phone company is trying to use data cable, to avoid "alien crosstalk" (between cables). That's why you can use crappy interconnect cables that come free with your receiver and it all sounds pretty good. They aren't long enough to make a difference. As long as you have continuity (electrical signal flow), you'll be just fine. But what if you use that cable for DIGITAL (i.e. S/PDIF). Suddenly we're sampling that audio at 44.1kHz (like a CD), and the bandwidth is defined as 128 times the sampling (5.6448 MHz, if memory serves). Let's just use 6 MHz to make it easy. (If you don't want easy, do it yourself.) 300,000,000 divided by 6,000,000 = 50 meters. 1/4-wavelength = 12.5 meters or about 44 feet. How long is your cable? 3 ft? 6 ft? No problem. It's still not LONG enough to make a difference. Now, if you want to send S/PDIF to the other end of the house, that might be 40 ft. or even more. What would you do? Be sure and use cable which is the correct impedance. S/PDIF requires 75 ohm cable (75 ohm cable has the lowest signal loss "attenuation" so it is used for many applications where low loss is required). What if you get a really long piece of crappy cable? What impedance is it? Who knows!! And what will happen is that the signal, which will require a 75 ohm "transmission line" and not see it. The farther it is away from 75 ohm the more the signal will be REFLECTED back to the source and not get to the other end. This is called "return loss" in the cable world. You send me the actual impedance of your cable and I can calculate the mismatch and the return loss. But, if you get the right 75 ohm cable, how far can you go? It depends on the size of the wire (gage) in the middle. Let's assume it is a 20 AWG center (such as Belden 1505A, the world's most popular coax cable and a nice generic size).. Then the cable could go 716 ft. (This is based on a source voltage of 0.5v and a minimum received voltage of 0.2v, at a bandwidth of 5.6448 MHz.) Yup, 700 feet!!! (Snippy aside: try going 700 ft. on a piece of toslink plastic fiber!) And in the digital format, you can be real close to the maximum and the signal (i.e. data/audio) sounds just perfect. But add in one patch cord, or one less than perfect connector, or just a few more feet of cable, and you could be down the slippery slope of the "digital cliff". So, how many of you have to run 700 ft.??? Not many, I assume. This is not to say you shouldn't have cable with low capacitance. Digital signals need low capacitance to keep their sharp edges (i.e. the clock). But (amazing fact that no cable manufacturer ever told you) once you choose the construction (i.e. plastic/dielectric/velocity) and the impedance (i.e. 75 ohms) the capacitance is AUTOMATIC. High velocity foam coax 75 ohm = 15 pF/ft. Old solid polyethylene 75 ohm coax = 20 pF/ft. And this is why you should avoid CATV coax, because it's not all copper in that center conductor (it's copper-clad steel for high-frequency-only applications where only the skin of the conductor is working.) This means the resistance is 5 to 7 times higher than an all-copper center and the distance you can go is 5 to 7 times shorter. (OK, we're down to 100 ft. and you're going 6 ft., but I wouldn't use it.) And all those SCSI cables (at the beginning of this thread) suffer from the same problem. Most of them use generic multiconductors (just pull one apart if you don't believe me). The impedance of a pair of wires is determined by their size (gage), the distance between them and the quality (dielectric contact) of the material inbetween. So, while the plastic is important, these cables fail at high frequencies because of the DISTANCE between conductors, which varies all over the place. Open up a SCSI 2 or 3 cable, what do you see? Ahhhh, twisted pairs!!! And the better the pairs (tighter impedance specs) the less the mismatch, the less the return loss, the greater the received signal strength, the less the bit errors. Flat cable can also help maintain impedance specs, at least better than just a bunch of loose wires. As frequencies go higher and higher, every dimension becomes more critical. Just take your car on a racetrack if you don't believe me. You need a race car? Buy a race car! You need a high-frequency cable? Buy one made, tested, verified, and guaranteed for whatever frequency band you wish. OK, class, how did I do? Steve Lampen Belden Electronics Division |
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cabling explained
No, he set out the needs for RF then compared it to audio to make his
point. Audio is not as high frequency as RF so don't use CATV coax and since the wavelengths are so long, don't worry too much about cable properties having a majore impact on the system. We run cabling in such short lenghts that it really doesn't matter. I read the link you posted and he summed it up the same way. "12 AWG seems more than adequate, even for demanding systems, high power Speaker levels, and reasonable lengths. The effects of 3.1-m cables are subtle, so many situations may not warrant the use of special cables. Low-inductance cables will provide the best performance when driving reactive loads, especially with amplifiers having low damping factor, and when flat response is critical, when long cable lengths are required, or when perfection is sought. Though not as linear as flat cables, 12 AWG wire works well and exceeds the high frequency performance of other two-conductor cables tested." Remember, he's writing in the context of home audio which some customers are very demanding of and even go to such lenghts as building listening rooms desinged around Cardas' principle among others. A very quiet listening environment, unlike car audio. And, home audio speakers aren't pressed into car doors, rear shelves, etc. They reveal a whole lot more that even the best car audio set up can deliver due to acoustics. Room interaction is a major player when it comes to great sound. John "Mark Zarella" wrote in message ... Unfortunately, I can't share the enthusiasm of the other posters responding in this thread. The post says absolutely nothing to us in the audio world, and is in fact completely irrelevant. He's looking at the issue from a microwave transmission line perspective which, as he pointed out in his first paragraph, doesn't apply to audio. One of the most comprehensive articles I've read on the subject of speaker cables was written by Fred Davis in the '80s and published by the Journal of the Audio Engineering Society. I've placed a reprint at the following website: http://www.geocities.com/audiotechpa...ractions. pdf Davis essentially examines the relevant electrical properties of several different cables (everything from $100/meter cables to jumper cables he found in his garage to lamp cord) and performs an analysis of their interactions with loudspeakers and amplifiers. Also noteworthy is that Davis looks at capacitance and inductance properties in the cables and compares them to various aspects of their construction which, unlike the assertion made in the original post, depends on many more factors than just the dielectric properties. Rather, the geometrical properties and other electrical properties come into play. Midlant wrote: Mr Lampen of Belden Electronics Division answered this posts on "rec.audio.high-end" some time back. I hope this answers your questions in regards to cable quality and length of runs whether interconnect or speaker. John In article , Lou Anschuetz writes: OTOH, I can understand how he comes to the conclusions he comes to. As someone with 25 years in IT, I have thrown away dozens of supposedly competently designed SCSI/serial cables that don't work. SCSI cables in particular are darn hard to to get right for the newer higher speeds - and it all seems to come down to how the cable(s) is insulated. I also agree that this doesn't necessarily have anything to do with audio transmission. However, IMHO, this business about insulation continues to bedevil me. Since it does play havoc with digital signal transmission, it seems to me hard to leap to an absolute conclusion that it has no effect in the analog audio world. No hard science on this yet, but it is a very active field of research... Well, I'm on a plane flying from Beijing to Hong Kong (and on to Melbourne, Australia) listening to Chinese Pop music in my headphones, and nothing to do, so I thought I would just jump in here! Why do some cables work on signal X but not with signal Y? The answer is simple: wavelength. (The answer is simple, the explanation is a little more complicated.) Every signal, data, audio, video, etc, etc. occupies one or more frequencies. Each frequency has a wavelength, that is, there is an electromagnetic wave moving down the cable that has a specific length. This can be calculated by dividing 300,000,000 by that frequency. (The answer comes out in meters, so us backwards USA types have to multiply by 3.28 to get to feet.) If you look at 20 kHz (or you can pick any frequency you want), that gives you a wavelength of 15,000 meters (about 9 miles). What that means is that the impedance of the cable (how it reacts to frequencies) is of no consequence. Most engineers agree that the ACTUAL critical distance is 1/4 wavelength which is, in this case 2.25 miles, still too far to have any effect. This distance is also affect by the plastic around the conductors. This affects the speed "velocity" of the signal. (Signals only move at the speed of light in a vaccum, i.e. outer space. If you ever have a chance to check out your speakers there, be sure and give me a complete test report!) Let's say your interconnect cables were the worst ever made (a "velocity of propagation" of 50%). You would multiply the wavelength by 50% so we're down to 1.12 miles. Anybody with one mile speaker cables? data cable? mic cables? video cables? audio cable? Well, yes, there are people with AUDIO cables of that length or more. They call themselves the TELEPHONE COMPANY. And the maximum distance between your home and your central office is 13,000 ft. (over 2 miles). But the twisted pairs they use (even fancy new ones, Category 3) are not the correct impedance (too expensive). Luckily, the audio on your phone ends at 3500 Hz. You can calculate the bandwidth and see you can go VERY far before you need to have impedance-specific twisted pairs. I'll be honest and tell you that I don't know the actual occupied bandwidth of a 56k data signal. (I think it's NRZI Manchester Coded, that would make it 28kHz bandwidth.) I'm sure you note that this puts it on the edge for performance if you are far from the central office, and you can't always get dialup that fast. And you'll understand why in the USA, the FCC now mandates that all telephone wiring must be Category 3 or better. Category 3 is impedance-specific (100 ohms) data cable. But there are other reasons the FCC made this a standard besides distance. The signals either go back to the source or simply stop in the cable (called "standing waves") and then radiate all that reflected energegy into cables around them. That's why the phone company is trying to use data cable, to avoid "alien crosstalk" (between cables). That's why you can use crappy interconnect cables that come free with your receiver and it all sounds pretty good. They aren't long enough to make a difference. As long as you have continuity (electrical signal flow), you'll be just fine. But what if you use that cable for DIGITAL (i.e. S/PDIF). Suddenly we're sampling that audio at 44.1kHz (like a CD), and the bandwidth is defined as 128 times the sampling (5.6448 MHz, if memory serves). Let's just use 6 MHz to make it easy. (If you don't want easy, do it yourself.) 300,000,000 divided by 6,000,000 = 50 meters. 1/4-wavelength = 12.5 meters or about 44 feet. How long is your cable? 3 ft? 6 ft? No problem. It's still not LONG enough to make a difference. Now, if you want to send S/PDIF to the other end of the house, that might be 40 ft. or even more. What would you do? Be sure and use cable which is the correct impedance. S/PDIF requires 75 ohm cable (75 ohm cable has the lowest signal loss "attenuation" so it is used for many applications where low loss is required). What if you get a really long piece of crappy cable? What impedance is it? Who knows!! And what will happen is that the signal, which will require a 75 ohm "transmission line" and not see it. The farther it is away from 75 ohm the more the signal will be REFLECTED back to the source and not get to the other end. This is called "return loss" in the cable world. You send me the actual impedance of your cable and I can calculate the mismatch and the return loss. But, if you get the right 75 ohm cable, how far can you go? It depends on the size of the wire (gage) in the middle. Let's assume it is a 20 AWG center (such as Belden 1505A, the world's most popular coax cable and a nice generic size).. Then the cable could go 716 ft. (This is based on a source voltage of 0.5v and a minimum received voltage of 0.2v, at a bandwidth of 5.6448 MHz.) Yup, 700 feet!!! (Snippy aside: try going 700 ft. on a piece of toslink plastic fiber!) And in the digital format, you can be real close to the maximum and the signal (i.e. data/audio) sounds just perfect. But add in one patch cord, or one less than perfect connector, or just a few more feet of cable, and you could be down the slippery slope of the "digital cliff". So, how many of you have to run 700 ft.??? Not many, I assume. This is not to say you shouldn't have cable with low capacitance. Digital signals need low capacitance to keep their sharp edges (i.e. the clock). But (amazing fact that no cable manufacturer ever told you) once you choose the construction (i.e. plastic/dielectric/velocity) and the impedance (i.e. 75 ohms) the capacitance is AUTOMATIC. High velocity foam coax 75 ohm = 15 pF/ft. Old solid polyethylene 75 ohm coax = 20 pF/ft. And this is why you should avoid CATV coax, because it's not all copper in that center conductor (it's copper-clad steel for high-frequency-only applications where only the skin of the conductor is working.) This means the resistance is 5 to 7 times higher than an all-copper center and the distance you can go is 5 to 7 times shorter. (OK, we're down to 100 ft. and you're going 6 ft., but I wouldn't use it.) And all those SCSI cables (at the beginning of this thread) suffer from the same problem. Most of them use generic multiconductors (just pull one apart if you don't believe me). The impedance of a pair of wires is determined by their size (gage), the distance between them and the quality (dielectric contact) of the material inbetween. So, while the plastic is important, these cables fail at high frequencies because of the DISTANCE between conductors, which varies all over the place. Open up a SCSI 2 or 3 cable, what do you see? Ahhhh, twisted pairs!!! And the better the pairs (tighter impedance specs) the less the mismatch, the less the return loss, the greater the received signal strength, the less the bit errors. Flat cable can also help maintain impedance specs, at least better than just a bunch of loose wires. As frequencies go higher and higher, every dimension becomes more critical. Just take your car on a racetrack if you don't believe me. You need a race car? Buy a race car! You need a high-frequency cable? Buy one made, tested, verified, and guaranteed for whatever frequency band you wish. OK, class, how did I do? Steve Lampen Belden Electronics Division |
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No, he set out the needs for RF then compared it to audio to make his
point. But what point is that exactly? He's comparing apples and oranges. Or rather, contrasting apples and oranges. Audio is not as high frequency as RF so don't use CATV coax There's nothing wrong with coax. Many people use coax for their preamp signal cables. The input impedance of amplifiers and processors are high enough to make any effects of increased capacitance negligible. I've yet to see a practical application where it can be used for speaker wiring - where capacitance can actually be relevant. and since the wavelengths are so long, don't worry too much about cable properties having a majore impact on the system. We run cabling in such short lenghts that it really doesn't matter. I read the link you posted and he summed it up the same way. Yes, transmission line theory can be neglected in car audio. I don't know of anyone using any elements of microwave theory to support using different kinds of wire in car audio though, so I don't know why the author insisted on addressing this. "12 AWG seems more than adequate, even for demanding systems, high power Speaker levels, and reasonable lengths. The effects of 3.1-m cables are subtle, so many situations may not warrant the use of special cables. Low-inductance cables will provide the best performance when driving reactive loads, especially with amplifiers having low damping factor, and when flat response is critical, when long cable lengths are required, or when perfection is sought. Though not as linear as flat cables, 12 AWG wire works well and exceeds the high frequency performance of other two-conductor cables tested." How did the author come to these conclusions? None of what he said prior to this statement was relevant to it. He said little about the reactance of the cable and its effect on the output. Remember, he's writing in the context of home audio which some customers are very demanding of and even go to such lenghts as building listening rooms desinged around Cardas' principle among others. A very quiet listening environment, unlike car audio. And, home audio speakers aren't pressed into car doors, rear shelves, etc. They reveal a whole lot more that even the best car audio set up can deliver due to acoustics. Room interaction is a major player when it comes to great sound. I realize this, which makes me wonder even further why he addressed the issue like he did. Yes, home users are demanding. So what does this have to do with telling people about the differences between two cables? If I were to explain the differences, it would be as Fred Davis did: an impedance explanation of attenuation. That is, how a voltage drop occurs, and then compare the relative magnitudes based on the electrical properties of the cable. The author you posted did not do this, but instead talked about transmission line theory. Since virtually no one was citing transmission line theory as a reason for using one cable over another, I find that it was completely unnecessary to do so. Therefore, it hardly serves as a comprehensive explanation of why cables are different, and more importantly, what the relative magnitude of these differences are. I don't mean to criticize your posting this on here, because I think it's an interesting analysis probably worth reading by anyone who wants to know one of the differences between the requirements for transmission of a very high frequency and very low frequency signal (though a bit misleading in a couple other areas). However, I just wanted to point out that it's not an explanation of what the differences between cables are and how such differences may or may not be significant. |
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"Mark Zarella" wrote in message .. . No, he set out the needs for RF then compared it to audio to make his point. But what point is that exactly? He's comparing apples and oranges. Or rather, contrasting apples and oranges. Audio is not as high frequency as RF so don't use CATV coax There's nothing wrong with coax. Many people use coax for their preamp signal cables. The input impedance of amplifiers and processors are high enough to make any effects of increased capacitance negligible. I've yet to see a practical application where it can be used for speaker wiring - where capacitance can actually be relevant. CATV Coax is only copper clad steal (or maybe only some of it is). That's why Belden doesn't recommend it. Other coax where the center conductor is solid copper is fine for audio. The difference for them is that RF is of such high frequency that it only travels on the outer most portion of the wire where audio uses much more of the conductor since it's in a lower frequency range. I do have an email from Belden stating wire part numbers for analog IC's, digital IC's, and speaker cabling, the latter surprised me as it isn't all that thick which is more in line with Mr Davis's article. I can email it to you or post it here for the ng if anyone is interested in building their own. (personaly I'm cheap and use www.knukonceptz.com cables.) About a year and a half ago, I came across and EE who happened to enjoy music and had a pretty decent system. He used nothing but coax for his IC"s. He stated the same as everyone else, we use such short lengths that it doesn't matter what we use as long as its well built. He accidentally pinched his turntable's IC so built another one out of boredom one day. He was shocked at the way the music sounded with the new one. All his highs were back. I guess the pinched dielectric caused havoc with the cables capacitance or other property. Yes, transmission line theory can be neglected in car audio. I don't know of anyone using any elements of microwave theory to support using different kinds of wire in car audio though, so I don't know why the author insisted on addressing this. Beats me. Mr Davis wrote it in 1991. I don't think theory has changed since then. From what I gathered reading the article, I think Mr Davis went from known fact into opinion, then back again, switching several times. His brain was probably working faster than his fingers could type and he got ahead of himself. Proofreading it, his mind filled in the blanks for him. I've seen that happen many times at work. (Lots of PhD types.) "12 AWG seems more than adequate, even for demanding systems, high power Speaker levels, and reasonable lengths. The effects of 3.1-m cables are subtle, so many situations may not warrant the use of special cables. Low-inductance cables will provide the best performance when driving reactive loads, especially with amplifiers having low damping factor, and when flat response is critical, when long cable lengths are required, or when perfection is sought. Though not as linear as flat cables, 12 AWG wire works well and exceeds the high frequency performance of other two-conductor cables tested." How did the author come to these conclusions? None of what he said prior to this statement was relevant to it. He said little about the reactance of the cable and its effect on the output. Like I said above, I don't know. It's from Mr Davis' article that you posted. I snipped the rest. Honestly, both authors were wrote about transmission line IRT amp through cable to speaker. He has quite a formula set up. I appreciate you stating you weren't intending to flame me. Thanks. I posted the article since I thought there was a need for it. One thing about audio is it is full of people waiting to take our money. Mark up on cables is ridiculous. A salesman of an audio shop (which shall remain anon) said that the stores customer were following cable mfg's advice on allocating a certain % on cables when buying a system that the store wasn't selling its higher end gear but more mid-fi which had less profit margin. So the owner contacted one of the cable companies who agreed to build him cables with his name on it for a per unit cost of $14.65. These were the same cables they were selling for $695.00! He could sell them for anything he wanted to (cheap) and get people to spend money on better equipment, where it mattered the most. The store was able to sell higher quality gear making its customers appreciate what it was they were spending thier hard earned money on, turn a profit, stay in business and continue selling good quality audio instead of the lower end mass market Circuit City stuff. As much as I hate it, there is a difference. Spend as much as you can on speakers then go from there. Some say start with the source, but it's still only as good as the speakers. I have been blown away by pretty medicore gear when hooked up to really great speakers. On AudioQuest's site, in their FAQ section, they state speaker cable length differences of 25% will make a difference. I don't know how since the signal is traveling so fast and with the various other problems we face with acoustics, I think any difference there may be measurable but not discernable. John |
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cabling explained
CATV Coax is only copper clad steal (or maybe only some of it is).
That's why Belden doesn't recommend it. Other coax where the center conductor is solid copper is fine for audio. So what? It doesn't matter. The input impedance of a typical amplifier is between 10k to 50k ohms. You could add a 100 ohm resistor in there and it wouldn't make any difference. It's a simple voltage divider. Attenuation is roughly proportional to the ratio between wire impedance and input impedance. As you can see, a 100 ohm wire impedance would present attenuation of the signal of less than 1%. You can simply compensate for this with the gain adjustment, or just ignore it as it's indetectable to humans anyway, and in fact would probably be within the noise level. And this is with a 100 ohm resistor! If you were to truly go from solid copper wire to solid steel wire, you'd be increasing the resistance of a 20 foot wire from somewhere on the order of 0.1 ohms to 0.2 ohms. In other words, resistivity of the wire does not matter. The difference for them is that RF is of such high frequency that it only travels on the outer most portion of the wire where audio uses much more of the conductor since it's in a lower frequency range. But who cares about RF? I do have an email from Belden stating wire part numbers for analog IC's, digital IC's, and speaker cabling, the latter surprised me as it isn't all that thick which is more in line with Mr Davis's article. I can email it to you or post it here for the ng if anyone is interested in building their own. (personaly I'm cheap and use www.knukonceptz.com cables.) About a year and a half ago, I came across and EE who happened to enjoy music and had a pretty decent system. He used nothing but coax for his IC"s. I'd never use typical CATV coax in a car just because it's a pain to work with - solid wire breaks rather easily and isn't very bendable. He stated the same as everyone else, we use such short lengths that it doesn't matter what we use as long as its well built. He accidentally pinched his turntable's IC so built another one out of boredom one day. He was shocked at the way the music sounded with the new one. All his highs were back. I guess the pinched dielectric caused havoc with the cables capacitance or other property. It's the power of suggestion. Even if the cable was pinched, it would have little bearing on the capacitance of the wire - just the resistance. If the resistance skyrocketed, it would not differentially attenuate the highs. Yes, transmission line theory can be neglected in car audio. I don't know of anyone using any elements of microwave theory to support using different kinds of wire in car audio though, so I don't know why the author insisted on addressing this. Beats me. Mr Davis wrote it in 1991. I don't think theory has changed since then. Ah, I thought the quote you provided was from the post written by the other guy. That's why it made no sense in his context. From what I gathered reading the article, I think Mr Davis went from known fact into opinion, then back again, switching several times. His brain was probably working faster than his fingers could type and he got ahead of himself. Proofreading it, his mind filled in the blanks for him. I've seen that happen many times at work. (Lots of PhD types.) Well, some of what he said was purely conjecture. I actually disagree with Davis' interpretation of his results as it applies to audio since it doesn't address detectability by humans. The part of the article worth reading is the part where he measured the electrical properties of the cables and noted their interactions with amplifiers. That's the purely scientific aspect of the article. Like most scientific articles, the interpretation at the end of the paper is subject to debate, but the results aren't unless you take issue with his methodology. On AudioQuest's site, in their FAQ section, they state speaker cable length differences of 25% will make a difference. I don't know how since the signal is traveling so fast and with the various other problems we face with acoustics, I think any difference there may be measurable but not discernable. I've spent a considerable amount of time criticizing Audioquest's FAQ in here. I think they're also the company that tries to convince people that the skin effect is relevant in audio and comes up with some bogus claims. I won't buy from them. But I probably wouldn't buy that overpriced crap anyway. |
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