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#1
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common mode rejection vs. crosstalk
what's the difference?
i'm looking at specs for the new Emu 1820m. below are the specs. the common mode rejection always seems to be many db less than the crosstalk db. i don't think i understand "common mode rejection". to my slightly-trained eyes, the overall specs look nice. would an experienced person agree? Sample Rates: 44.1, 48, 96, and 192 kHz, from internal crystal or externally supplied clock (no sample rate conversion). Bit Depth: 16 or 24 bits PCI Bus-Mastering DMA subsystem reduces CPU usage Zero-latency direct hardware monitoring w/effects Analogue line inputs-6 Type: servo-balanced, DC-coupled, low-noise input circuitry Level (software selectable): * Professional: +4dBu nominal, 20dBu maximum (balanced) * Consumer: -10dBV nominal, 6dBV maximum (unbalanced) Frequency Response: +/- .05dB, 20Hz – 20 kHz THD+N (1 kHz at -1dBFS): -110dB (.0003%) SNR (A-weighted): 120dB Dynamic Range (1 kHz, A-weighted): 120dB Stereo Crosstalk (1kHz at -1dBFS): -115dB Common-Mode Rejection (60Hz): 40dB Analogue line outputs-8 Type: balanced, low-noise, 3-pole low-pass differential filter Level (software selectable): * Professional: +4dBu nominal, 20dBu maximum (balanced) * Consumer: -10dBV nominal, 6dBV maximum (unbalanced) Frequency Response: 0.0/-.35dB, 20Hz – 20 kHz THD+N (1 kHz at -1dBFS): -105dB (.0006%) SNR (A-weighted): 120dB Dynamic Range (1 kHz, A-weighted): 120dB Stereo Crosstalk (1 kHz at -1dBFS) -115dB Mic pre and line inputs-2 Type: TFPro™ combination microphone preamp and line input Frequency Response: +0.8/-0.1dB, 20Hz – 20 kHz Stereo Crosstalk (1 kHz min gain, -1dBFS): -120dB Line Input: * Gain Range: -12 to +28dB * Max Level: +17dBV (19.2dBu) * THD+N (1 kHz at -1dBFS, min gain): -94dB (.002%) * Dynamic Range (A-weighted, 1kHz min gain): 100dB * SNR (A-weighted, min gain): 100dB Microphone Preamplifier * Gain Range: +10 to +50dB * Max Level: -12dBV (-9.8dBu) * THD+N (1 kHz at -1dBFS, min gain): -95dB (.0018%) * SNR (A-weighted, min gain): 100dB * Input Impedance: 330 ohms * Common-mode Rejection Ratio (60Hz): 80dB Phone input stereo Type: RIAA equalized phono input Frequency Response: +/-0.5dB, 50Hz – 20 kHz THD+N (1 kHz, 10mV RMS unbalanced input): -76dB (.015%) SNR (10mV RMS unbalanced input, A-weighted): 90dB Stereo Crosstalk (1kHz at -1dBFS) : -80dB Maximum level: * Professional: 80mV RMS * Consumer: 20mV RMS Input capacitance: 220 pF Input impedance: 47K ohm Digital I/O S/PDIF: * 2 in/2 out coaxial (transformer coupled) * 2 in/3 out optical (software switched with ADAT) * AES/EBU or S/PDIF format, switchable under software control ADAT: * 8 channels, 24-bit @ 44.1/48kHz * 4 channels, 24-bit @ 96kHz (S-MUX compatible * 2 channels, 24-bit @ 192kHz Firewi * 400 Mbps 1394a port (6-pin) * Compatible with DV cameras, storage peripherals, etc Midi: 2 in, 2 out |
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#2
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xy wrote:
what's the difference? i'm looking at specs for the new Emu 1820m. below are the specs. the common mode rejection always seems to be many db less than the crosstalk db. i don't think i understand "common mode rejection". CMRR is a measure of how well balanced a balanced connection is. It is a measure of how good the rejection of common mode noise is. to my slightly-trained eyes, the overall specs look nice. would an experienced person agree? Anybody can get these sorts of numbers today. How does it sound? --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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#3
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On 2 Jul 2004 13:02:02 -0400, in rec.audio.pro you wrote:
xy wrote: what's the difference? i'm looking at specs for the new Emu 1820m. below are the specs. the common mode rejection always seems to be many db less than the crosstalk db. i don't think i understand "common mode rejection". CMRR is a measure of how well balanced a balanced connection is. It is a measure of how good the rejection of common mode noise is. to my slightly-trained eyes, the overall specs look nice. would an experienced person agree? Anybody can get these sorts of numbers today. How does it sound? --scott yep, and I notice that they only seemed to spec CMRR at line frequency, Really they should give some idea of what its like at 20K. Normaly its easy to get high CMRR at line freq, but it tends to rise at about 6dB/octave Common-Mode Rejection (60Hz): 40dB Generally means that they have cheated the cmrr specification. ie anyone can get 40dB, but in production it could actually be say 60dB. But they give such a loose spec, that you never can complain. One device could be 41dB, and another 75dB, and still be in spec! martin Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank. |
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#4
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thanks for all the replies everyone. i remembered later that "common
mode" often has to do with hum on an ac line, but these descriptions were very clarifying. i haven't heard the box yet. i'm doing lots of organizing in july. and then starting in august, i'm going into listen/buy mode. there are two things that "concern" me about this new emu box. one: the clock..how good could it be at this price point? that can be solved since it has word clock input. but the second concern: the power on the breakout box comes from the ethernet cable (there is no ac mains cord on the breakout box). how much juice can an ethernet cable deliver, and isn't it a bad idea to have power and digital signal going down the same line? i guess a third concern would be the quality of the analog circuits surrounding the converters. but i think it's cool that emu is using the top of the line converters that the expensive digi boxes are using. and the layout of the box is very appealing. |
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#5
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thanks for all the replies everyone. i remembered later that "common
mode" often has to do with hum on an ac line, but these descriptions were very clarifying. i haven't heard the box yet. i'm doing lots of organizing in july. and then starting in august, i'm going into listen/buy mode. there are two things that "concern" me about this new emu box. one: the clock..how good could it be at this price point? that can be solved since it has word clock input. but the second concern: the power on the breakout box comes from the ethernet cable (there is no ac mains cord on the breakout box). how much juice can an ethernet cable deliver, and isn't it a bad idea to have power and digital signal going down the same line? i guess a third concern would be the quality of the analog circuits surrounding the converters. but i think it's cool that emu is using the top of the line converters that the expensive digi boxes are using. and the layout of the box is very appealing. |
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#6
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#7
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"Mike Rivers". Common mode rejection is the ability to reject noise that's common to the two wires of a balanced input. An example is RF interference that's picked up by both wires equally. ** That not a correct example. RF interference is defeated firstly by the use of shielded cable and secondly by the use of filters to reduce such signals at the inputs of the balanced pre-amp. Common mode rejection operates across the audio band and maybe a little beyond but is usually most effective at the lower frequencies since the main aim is to eliminate ground hum from audio systems. A ground hum voltage will appear equally on the two wires and so be rejected. Since a balanced input works on the voltage difference between the two wires, if the same stray signal is picked up equally by both its difference will be zero and it will be rejected. ** A voltage injected into a balanced audio line by external magnetic fields ( like nearby high AC current cables and transformers) creates a hum signal in differential mode that the pre-amp *will* amplify - its CMRR has no effect. This sort of interference is reduced by the fact the two wires are *twisted* inside the cable which reverses the phase of any hum signal picked up every inch or so along the line and hence cancels it out. Where multiple twisted pairs are used in the same cable the twisting reduces crosstalk in the same way as above. "Star Quad" cable uses four twisted wires instead of two to enhance the effect of the twisting and virtually eliminates induced hum problems even when used near to high current AC cabling. ............ Phil |
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#8
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On Sat, 3 Jul 2004 15:07:57 +1000, "Phil Allison"
wrote: This sort of interference is reduced by the fact the two wires are *twisted* inside the cable which reverses the phase of any hum signal picked up every inch or so along the line and hence cancels it out. Where multiple twisted pairs are used in the same cable the twisting reduces crosstalk in the same way as above. Don't try this at home kids. Chris Hornbeck |
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#9
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"Chris Hornbeck"... "Phil Allison" Don't try this at home kids. Chris Hornbeck ** Imbecile. ............ Phil |
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#10
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On Sat, 3 Jul 2004 16:15:50 +1000, "Phil Allison"
wrote: ** Imbecile. Oh, OK, cool. So you're doing what in America is called guerilla theater. I've seen a few great ones, even done one myself. Here's the setup: My beautiful young friend Rebekah in belly dancing drag enters from house rear, dances through the house and onto stage. I'm crouched at stage lip with disposable flash camera. Dance, flash, dance, flash. Argument ensues; gets personal, very personal. Nobody, esp. including sound man has been notified. Rebekah has to wave repeatedly to get dance music to stop. I step up onto the stage; Rebekah grabs the camera; I grab it back and throw it against the wall. Audiences ****s. We alternate (yeah, I know, but it worked) reading: "Life is short, Art is long Opportunity fleeting, Experiment treacherous Judgement difficult. Hippocrates" We called it "I Dream of Rebe" and later did a cell phone piece (didn't work at all) called "My Dinner with Rebe". So how do you envision yours? Chris Hornbeck |
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#11
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On Sat, 3 Jul 2004 15:07:57 +1000, "Phil Allison"
wrote: ** A voltage injected into a balanced audio line by external magnetic fields ( like nearby high AC current cables and transformers) creates a hum signal in differential mode that the pre-amp *will* amplify - its CMRR has no effect. And, by the way, before you go "correcting" someone like Mike Rivers, who's a real engineer as evidenced by the depth and clarity of his thinking, you might get the most basic stuff straight. For a beginning to a clue, think about the wavelength of a hum field. A man apologizes when he's wrong. Deal. Chris Hornbeck |
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#12
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"Chris Hornbeck" "Phil Allison" And, by the way, before you go "correcting" someone like Mike Rivers, who's a real engineer as evidenced by the depth and clarity of his thinking, you might get the most basic stuff straight. ** Mr Rivers is no "real engineer". For a beginning to a clue, think about the wavelength of a hum field. ** Nor are you. A man apologizes when he's wrong. Deal. Chris Hornbeck ** Start now if you like. ........... Phil |
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#13
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Phil Allison wrote:
"Mike Rivers". Common mode rejection is the ability to reject noise that's common to the two wires of a balanced input. An example is RF interference that's picked up by both wires equally. ** That not a correct example. RF interference is defeated firstly by the use of shielded cable and secondly by the use of filters to reduce such signals at the inputs of the balanced pre-amp. Common mode rejection operates across the audio band and maybe a little beyond but is usually most effective at the lower frequencies since the main aim is to eliminate ground hum from audio systems. A ground hum voltage will appear equally on the two wires and so be rejected. So why is it you think that RF can't exist at audible frequencies? Even if it's caused by A/C current and at 50 or 60 Hz, it's still radio noise picked up by the wires because they're acting as antennas, right? - Logan |
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#14
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"Logan Shaw" "Mike Rivers". Common mode rejection is the ability to reject noise that's common to the two wires of a balanced input. An example is RF interference that's picked up by both wires equally. ** That not a correct example. RF interference is defeated firstly by the use of shielded cable and secondly by the use of filters to reduce such signals at the inputs of the balanced pre-amp. Common mode rejection operates across the audio band and maybe a little beyond but is usually most effective at the lower frequencies since the main aim is to eliminate ground hum from audio systems. A ground hum voltage will appear equally on the two wires and so be rejected. So why is it you think that RF can't exist at audible frequencies? ** RF = "radio frequencies". Radio frequencies are those used for radio communication - ie much higher than audio frequencies. The term Mike used was " RF interference" - which refers to unwanted injection of RF energy into a circuit. Even if it's caused by A/C current and at 50 or 60 Hz, it's still radio noise picked up by the wires because they're acting as antennas, right? ** No. The wires inside the cable are acting as an induction loop in magnetic field while frequencies concerned are in the audio range. .............. Phil |
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#15
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Logan Shaw wrote:
Phil Allison wrote: "Mike Rivers". Common mode rejection is the ability to reject noise that's common to the two wires of a balanced input. An example is RF interference that's picked up by both wires equally. ** That not a correct example. RF interference is defeated firstly by the use of shielded cable and secondly by the use of filters to reduce such signals at the inputs of the balanced pre-amp. Common mode rejection operates across the audio band and maybe a little beyond but is usually most effective at the lower frequencies since the main aim is to eliminate ground hum from audio systems. A ground hum voltage will appear equally on the two wires and so be rejected. So why is it you think that RF can't exist at audible frequencies? Even if it's caused by A/C current and at 50 or 60 Hz, it's still radio noise picked up by the wires because they're acting as antennas, right? RF doesn't exist at audible frequencies... and 60 Hz isn't RF. RF is stuff at hundreds of KHz or higher. You can't hear RF directly, you can only hear RF when it gets rectified and turned into audio frequencies by electronics. The two strategies for dealing with RF are to prevent it from getting picked up in the first place, and prevent it from being rectified. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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#16
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#17
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On Sat, 03 Jul 2004 06:33:26 GMT, Logan Shaw
wrote: So why is it you think that RF can't exist at audible frequencies? What's the definition of "RF" on your planet? :-) CubaseFAQ www.laurencepayne.co.uk/CubaseFAQ.htm "Possibly the world's least impressive web site": George Perfect |
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#18
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Laurence Payne wrote:
On Sat, 03 Jul 2004 06:33:26 GMT, Logan Shaw wrote: So why is it you think that RF can't exist at audible frequencies? What's the definition of "RF" on your planet? :-) Here's one possible definition: http://en.wikipedia.org/wiki/Radio_frequency From the text: "The ELF, SLF, ULF, and VLF bands overlap the AF (audio frequency) spectrum, which is approximately 20–20,000 Hz". Also, there's a nice chart of how (basically) the entire spectrum is allocated at http://www.ntia.doc.gov/osmhome/allochrt.html . We used to have a poster of this on the wall at one place I worked. It's a pretty cool chart. Anyway, the chart shows allocated frequencies down to 9 kHz, just as the FCC's Table of Frequency Allocations does. You can find the latter at http://www.fcc.gov/oet/spectrum/ . Of course, even if these frequencies weren't allocated, this would not mean that the phenomenon known as "radio" can't happen at those frequencies. And you can also argue (and I'll agree) that they aren't the most useful frequencies of the spectrum. But the point is that RF can exist at audible frequencies. - Logan |
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#19
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#20
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"Mike Rivers"
Phil Allison: ** That not a correct example. A bad penny always returns. Welcome back, Phil. ** Typical ****head reply from the NG's parrot. RF interference is defeated firstly by the use of shielded cable and secondly by the use of filters to reduce such signals at the inputs of the balanced pre-amp. However, what doesn't get caught by those crude mechanical methods will be reduced by common mode rejection. ** What pig ignorant drivel !! Shielding and filtering are not "crude" and CMR has no effect on FRO interference. That was a bad rebuttal. ** What Rivers posted was as worthless crap. Common mode rejection operates across the audio band and maybe a little beyond but is usually most effective at the lower frequencies since the main aim is to eliminate ground hum from audio systems. A ground hum voltage will appear equally on the two wires and so be rejected. In theory it can operate in any frequency range, even DC. ** Irrelevant reply - as usual for a parrot. Did anyone say anything about audio here? ** Not Mike Rivers anyhow. ** A voltage injected into a balanced audio line by external magnetic fields ( like nearby high AC current cables and transformers) creates a hum signal in differential mode that the pre-amp *will* amplify - its CMRR has no effect. Whoa! This is EXACTLY where common mode rejection is useful in audio circuits. ** More pig ignorant drivel. The two wires in a balanced line form a loop - loops pick up induced hum just perfectly. It's what lets us connect microphones with zip cord. The reason why it doesn't work as well as we'd like it to is that it's rare that the noise voltage at both inputs is rarely exactly the same, so there will always be some difference, which will be amplified. ** The voltage induced in a loop is a differential signal - same as the wanted signal on an audio line. This sort of interference is reduced by the fact the two wires are *twisted* inside the cable which reverses the phase of any hum signal picked up every inch or so along the line and hence cancels it out. Where multiple twisted pairs are used in the same cable the twisting reduces crosstalk in the same way as above. A very twisted explanation. ** More pig ignorant drivel. I suggest that anyone really interested in the theory behind this statement read the book about cable written by Steve Lampen of Belden. It's pretty easy to understand. ** Polly want another cracker ??? "Star Quad" cable uses four twisted wires instead of two to enhance the effect of the twisting and virtually eliminates induced hum problems even when used near to high current AC cabling. This is correct. ** And does kinda prove that twisting of the cable is responsible for the rejection of induced hum from external fields. Knowing who I'm talking to, ** Mike - you have NO idea who you are talking to. I feel compelled to make this statement. ** What a posturing ass you are Mike. I've offered the correct answer, ** Not one thing about that post was correct. explained to other readers why your response isn't quite correct, and tried really hard not to make you look like a jerk this time around. That's all I have to say on the subject until someone changes it. ** LOL - how pathetic. ............ Phil |
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#21
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Newsgroups are kind of like a bar, where folks of all stripe sit around
talking about everything they care to. With one exception: no one gets their lights punched out, no matter how obnoxious. |
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#22
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In article ,
"Phil Allison" wrote: ** A voltage injected into a balanced audio line by external magnetic fields ( like nearby high AC current cables and transformers) creates a hum signal in differential mode that the pre-amp *will* amplify - its CMRR has no effect. Actually, it's current that gets induced by a changing magnetic field, not voltage. This current acts upon whatever impedances are present to then create a voltage. Yeah, it seems like a subtle point, but it completely determines if and what sort of interference will be present. If the balanced line has balanced impedances on both sides, then all the interfering voltage created by the interference current will be common mode and could be completely cancelled out by an ideal receiver. If there is an impedance imbalance, then some of the interference will result in a difference mode signal, one that can never be removed by any balanced receiver. This sort of interference is reduced by the fact the two wires are *twisted* inside the cable which reverses the phase of any hum signal picked up every inch or so along the line and hence cancels it out. Where multiple twisted pairs are used in the same cable the twisting reduces crosstalk in the same way as above. Yes, twisting makes the loop area effectively zero, so there's no mutual coupling and thus no induced current and thus no induced voltage. Regards, Monte McGuire |
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#23
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"Monte McGuire"... "Phil Allison" ** A voltage injected into a balanced audio line by external magnetic fields ( like nearby high AC current cables and transformers) creates a hum signal in differential mode that the pre-amp *will* amplify - its CMRR has no effect. Actually, it's current that gets induced by a changing magnetic field, not voltage. This current acts upon whatever impedances are present to then create a voltage. ** Wrong - a voltage is induced. The current that flows depends on the impedances, conductor cross section etc. The lower they are the more the current - but this is not relevant to the problem since hum voltages are what get amplified and heard. Yeah, it seems like a subtle point, but it completely determines if and what sort of interference will be present. ** That just compounds the first error. If the balanced line has balanced impedances on both sides, then all the interfering voltage created by the interference current will be common mode and could be completely cancelled out by an ideal receiver. If there is an impedance imbalance, then some of the interference will result in a difference mode signal, one that can never be removed by any balanced receiver. ** Wrong - the loop formed by the two signal carrying lines is *the circuit* in which the hum voltage is induced. This is in differential ode - same as the wanted signal. This sort of interference is reduced by the fact the two wires are *twisted* inside the cable which reverses the phase of any hum signal picked up every inch or so along the line and hence cancels it out. Where multiple twisted pairs are used in the same cable the twisting reduces crosstalk in the same way as above. Yes, twisting makes the loop area effectively zero, so there's no mutual coupling and thus no induced current and thus no induced voltage. ** A balanced audio line that is **NOT** twisted is just an induction loop. ............... Phil |
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#24
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On Mon, 5 Jul 2004 14:02:57 +1000, "Phil Allison"
wrote: ** A balanced audio line that is **NOT** twisted is just an induction loop. This, I think, is the source of your misconception, IMO. Just for fun, pretend that you don't believe this, then rework the same steps. Good fortune, Chris Hornbeck |
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#25
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On Mon, 5 Jul 2004 14:02:57 +1000, "Phil Allison"
wrote: ** Wrong - a voltage is induced. The current that flows depends on the impedances, conductor cross section etc. The lower they are the more the current - but this is not relevant to the problem since hum voltages are what get amplified and heard. I retract my earlier post; this is the more fundamental misconception. Maybe thinking in terms of a conventional power electrical generator would help clear things up. All the same rules apply. Good fortune, Chris Hornbeck |
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#26
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#27
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On Mon, 05 Jul 2004 03:46:09 GMT, Monte McGuire
wrote: In article , "Phil Allison" wrote: This sort of interference is reduced by the fact the two wires are *twisted* inside the cable which reverses the phase of any hum signal picked up every inch or so along the line and hence cancels it out. Where multiple twisted pairs are used in the same cable the twisting reduces crosstalk in the same way as above. Yes, twisting makes the loop area effectively zero, so there's no mutual coupling and thus no induced current and thus no induced voltage. Imagine the simpler case of a balanced signal on an untwisted pair line. What is now different in the induced current? First consider the case of the two conductors having *no* spacing. Then the case of spacing significant compared to a wavelength. Next consider the case of the same lines with a single twist centered on a symmetrical hum source. Then with infinite twisting. All this talk about "reversing the phase" etc. is fundamentally flawed, IMO. Chris Hornbeck |
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#28
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"Chris Hornbeck" Imagine the simpler case of a balanced signal on an untwisted pair line. What is now different in the induced current? First consider the case of the two conductors having *no* spacing. Then the case of spacing significant compared to a wavelength. Next consider the case of the same lines with a single twist centered on a symmetrical hum source. Then with infinite twisting. All this talk about "reversing the phase" etc. is fundamentally flawed, IMO. ** For Christ's sake Chris - do a *real test * instead of posting ASININE thought experiments with wrong outcomes. Get a length of insulated wire, connect the ends to pins 2 and 3 of an XLR, plug it into a mic pre and try the effect of having an open loop, closed loop and then twisted tightly all along its length when held close proximity to an AC power transformer. ............ Phil |
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#29
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** A voltage injected into a balanced audio line by external magnetic
fields ( like nearby high AC current cables and transformers) creates a hum signal in differential mode that the pre-amp *will* amplify - its CMRR has no effect. Hi Phil, could you explain this is more detail? I always thought that a magnetic field induced a current (or voltage) into the two wires of a mic cable (for example) in equal magnitude and angle. If I understand correctly, this isn't the case, rather, the hum induces a current/voltage in one of the wires as a postive going signal, and the other as a negative going signal. That is, they are equal and opposite in polarity. Magnetics begins to get a bit beyond me, but I understand its importance in what I deal with everyday. Thanks, Phil. Chris Deckard Saint Louis, Mo. |
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#30
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"mr c deckard" ** A voltage injected into a balanced audio line by external magnetic fields ( like nearby high AC current cables and transformers) creates a hum signal in differential mode that the pre-amp *will* amplify - its CMRR has no effect. Hi Phil, could you explain this is more detail? I always thought that a magnetic field induced a current (or voltage) into the two wires of a mic cable (for example) in equal magnitude and angle. If I understand correctly, this isn't the case, rather, the hum induces a current/voltage in one of the wires as a postive going signal, and the other as a negative going signal. That is, they are equal and opposite in polarity. ** Do the test I suggested with a length of insulated wire, XLR and pre-amp. Then think how a voltage is created in the coil of a dynamic, mic sent down the cable to the pre-amp and is amplified. Then realise that the connecting cable is just an extension of that same coil. Recall that a dynamic mic hums when placed near an AC power transformer ( except for those with effective, internal hum bucking coils). ............. Phil |
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#31
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On Tue, 6 Jul 2004 12:44:00 +1000, "Phil Allison"
wrote: ** Do the test I suggested with a length of insulated wire, XLR and pre-amp. Then think how a voltage is created in the coil of a dynamic, mic sent down the cable to the pre-amp and is amplified. Then realise that the connecting cable is just an extension of that same coil. Recall that a dynamic mic hums when placed near an AC power transformer ( except for those with effective, internal hum bucking coils). Then recall how the hum bucking coil works. Perfect example. (Hint: it's in the same field as the innocent but offending coil). (What the heck, another hint: to be within the "same" field means to be within a small-compared-to-wavelength average distance) (OK, one more final hint: average. How to be reliably average...) Chris Hornbeck |
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#32
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#33
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** Do the test I suggested with a length of insulated wire, XLR and pre-amp. Then think how a voltage is created in the coil of a dynamic, mic sent down the cable to the pre-amp and is amplified. Then realise that the connecting cable is just an extension of that same coil. Recall that a dynamic mic hums when placed near an AC power transformer ( except for those with effective, internal hum bucking coils). ok, got that. but what about when the cable runs next to an AC transformer? that's the part that i don't quite get -- i don't see how it could induce current in one conductor in opposite polarity with the other. btw, i don't understand the voltage vs. current induction thing -- my physics book says a moving B field will induce a /current/ in a coil. what am i missing? thanks, chris deckard saint louis moe |
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#34
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** Do the test I suggested with a length of insulated wire, XLR and pre-amp. Then think how a voltage is created in the coil of a dynamic, mic sent down the cable to the pre-amp and is amplified. Then realise that the connecting cable is just an extension of that same coil. Recall that a dynamic mic hums when placed near an AC power transformer ( except for those with effective, internal hum bucking coils). ok, got that. but what about when the cable runs next to an AC transformer? that's the part that i don't quite get -- i don't see how it could induce current in one conductor in opposite polarity with the other. btw, i don't understand the voltage vs. current induction thing -- my physics book says a moving B field will induce a /current/ in a coil. what am i missing? thanks, chris deckard saint louis moe |
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#35
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This sort of interference is reduced by the fact the two wires are
*twisted* inside the cable which reverses the phase of any hum signal picked up every inch or so along the line and hence cancels it out. Where multiple twisted pairs are used in the same cable the twisting reduces crosstalk in the same way as above. Twisting puts NOTHING out of phase. Twisting assures that both wires are introduced the same noise energy amplitude, so that is can be properly canceled out by the differential input. Twisting improves CMR because the noise signal is more uniform. ........... Phil |
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#36
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"Crumb" This sort of interference is reduced by the fact the two wires are *twisted* inside the cable which reverses the phase of any hum signal picked up every inch or so along the line and hence cancels it out. Where multiple twisted pairs are used in the same cable the twisting reduces crosstalk in the same way as above. Twisting puts NOTHING out of phase. ** Go try it - you fool. See how WRONG you are. .............. Phil |
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#37
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Phil Allison wrote:
"Crumb" This sort of interference is reduced by the fact the two wires are *twisted* inside the cable which reverses the phase of any hum signal picked up every inch or so along the line and hence cancels it out. Where multiple twisted pairs are used in the same cable the twisting reduces crosstalk in the same way as above. Twisting puts NOTHING out of phase. ** Go try it - you fool. See how WRONG you are. There is a nice discussion of this in the ITT Radio Engineer's Handbook. I won't summarize it here because it'll just make Phil go off his nut again, but it's worth looking up. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
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"Chris Hornbeck" Twisting puts NOTHING out of phase. Twisting assures that both wires are introduced the same noise energy amplitude, so that is can be properly cancelled out by the differential input. Twisting improves CMR because the noise signal is more uniform. perzactly. A moment's thought about the wavelength of a prospective hum signal would make phase arguments evaporate, but even a moment's thought is precious. ** So Chris, the resident thought experiment imbecile, has still not tried a real test. The wavelength of the magnetic field is UTTERLY irrelevant. Twisting *only* averages the errors from both conductors' inability to be in exactly the same place. ** Complete bull****. DO - A - REAL - TEST - CHRIS !!!!!!!!!! What are YOU soooooo damn frightened of ????????? Discovering that you are WRONG ?? ................ Phil |
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