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#81
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Microwave Spectrum - Amplitude Modulation
"Free space attenuation has no frequency dependency."
You must be joking! Free space attenuation is dependant on frequency and can be written as: Attenuation in dB = 36.6 + 20 log F + 20 log d, where F is frequency in MHz and d is distance in miles. Believe me, I really would like free speace attenuation to be frequency independant, (I work with radio planning of 2GHz WCDMA mobile systems) but unfortunately the dependancy is there, when increasing the frequency 10 times the attenuation increases 20dB for the same distance. Regards Hans Don Pearce wrote: On Sun, 22 Feb 2004 17:35:28 +0100, Peter Völpel wrote: Don Pearce schrieb: 2. I have been told by TV engineers that the higher frequencies don't have near the coverage per watt that the low ones do. With this changeover in the US to digital the FCC is assigning new digital channels by lottery. There was a local TV station that had a VHF channel on analog and got assigned a UHF for the digital. They now are using twice the KWs and are spending twice the money on electricity running their new transmitter to get the same coverage area they had with the VHF. Can you clarify? Mike Metzger Although coverage per watt is not really an issue, the problem comes with no line-of-sight coverage, particularly beyond the horizon. As you go higher in frequency you need more power to penetrate shadow areas. Certainly in cities that means really high frequencies need a lot of power. Satellite TV works because the elevation of the satellite is so high that just about anybody can get line-of-sight from somewhere on their house. the issue are not shadow aereas, that is a question of the satellites transmitantenna beamwith. I was talking about shadow areas from terrestrial transmissions, not satellite. The problem is caused by the higher siganlpath attenuation which raises with the frequency. Free space attenuation has no frequency dependency. Beside, attenuation on dustparticles over big cities, reindrops, fog etc. is higher on higher frequencies This is true, but easily allowed for. regards Peter d _____________________________ http://www.pearce.uk.com |
#82
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Microwave Spectrum - Amplitude Modulation
On Wed, 25 Feb 2004 14:33:53 +0900, Tube wrote:
"Free space attenuation has no frequency dependency." You must be joking! Free space attenuation is dependant on frequency and can be written as: Attenuation in dB = 36.6 + 20 log F + 20 log d, where F is frequency in MHz and d is distance in miles. Believe me, I really would like free speace attenuation to be frequency independant, (I work with radio planning of 2GHz WCDMA mobile systems) but unfortunately the dependancy is there, when increasing the frequency 10 times the attenuation increases 20dB for the same distance. Regards Hans Free space attenuation has no frequency dependency. How many times do I need to say it? The 20logF term in your equation describes the size of the receiving antenna, which is assumed to be frequency-dependent - it is unrelated to attenuation in free space. The power flux density at any distance is equal to the transmitted power divided by the area of a sphere of radius equal to the distance. There is no frequency term. Try this on-paper experiment . Assume a perfect dish receiving antenna of a size of your choice. Now work out the signal power you get from it at various frequencies, using the same transmitted power. Interesting result? There is no change in received power with frequency. You want less power at the higher frequency? You have to make the antenna smaller. d _____________________________ http://www.pearce.uk.com |
#83
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Microwave Spectrum - Amplitude Modulation
On Wed, 25 Feb 2004 14:33:53 +0900, Tube wrote:
"Free space attenuation has no frequency dependency." You must be joking! Free space attenuation is dependant on frequency and can be written as: Attenuation in dB = 36.6 + 20 log F + 20 log d, where F is frequency in MHz and d is distance in miles. Believe me, I really would like free speace attenuation to be frequency independant, (I work with radio planning of 2GHz WCDMA mobile systems) but unfortunately the dependancy is there, when increasing the frequency 10 times the attenuation increases 20dB for the same distance. Regards Hans Free space attenuation has no frequency dependency. How many times do I need to say it? The 20logF term in your equation describes the size of the receiving antenna, which is assumed to be frequency-dependent - it is unrelated to attenuation in free space. The power flux density at any distance is equal to the transmitted power divided by the area of a sphere of radius equal to the distance. There is no frequency term. Try this on-paper experiment . Assume a perfect dish receiving antenna of a size of your choice. Now work out the signal power you get from it at various frequencies, using the same transmitted power. Interesting result? There is no change in received power with frequency. You want less power at the higher frequency? You have to make the antenna smaller. d _____________________________ http://www.pearce.uk.com |
#84
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Microwave Spectrum - Amplitude Modulation
On Wed, 25 Feb 2004 14:33:53 +0900, Tube wrote:
"Free space attenuation has no frequency dependency." You must be joking! Free space attenuation is dependant on frequency and can be written as: Attenuation in dB = 36.6 + 20 log F + 20 log d, where F is frequency in MHz and d is distance in miles. Believe me, I really would like free speace attenuation to be frequency independant, (I work with radio planning of 2GHz WCDMA mobile systems) but unfortunately the dependancy is there, when increasing the frequency 10 times the attenuation increases 20dB for the same distance. Regards Hans Free space attenuation has no frequency dependency. How many times do I need to say it? The 20logF term in your equation describes the size of the receiving antenna, which is assumed to be frequency-dependent - it is unrelated to attenuation in free space. The power flux density at any distance is equal to the transmitted power divided by the area of a sphere of radius equal to the distance. There is no frequency term. Try this on-paper experiment . Assume a perfect dish receiving antenna of a size of your choice. Now work out the signal power you get from it at various frequencies, using the same transmitted power. Interesting result? There is no change in received power with frequency. You want less power at the higher frequency? You have to make the antenna smaller. d _____________________________ http://www.pearce.uk.com |
#85
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Microwave Spectrum - Amplitude Modulation
On Wed, 25 Feb 2004 14:33:53 +0900, Tube wrote:
"Free space attenuation has no frequency dependency." You must be joking! Free space attenuation is dependant on frequency and can be written as: Attenuation in dB = 36.6 + 20 log F + 20 log d, where F is frequency in MHz and d is distance in miles. Believe me, I really would like free speace attenuation to be frequency independant, (I work with radio planning of 2GHz WCDMA mobile systems) but unfortunately the dependancy is there, when increasing the frequency 10 times the attenuation increases 20dB for the same distance. Regards Hans Free space attenuation has no frequency dependency. How many times do I need to say it? The 20logF term in your equation describes the size of the receiving antenna, which is assumed to be frequency-dependent - it is unrelated to attenuation in free space. The power flux density at any distance is equal to the transmitted power divided by the area of a sphere of radius equal to the distance. There is no frequency term. Try this on-paper experiment . Assume a perfect dish receiving antenna of a size of your choice. Now work out the signal power you get from it at various frequencies, using the same transmitted power. Interesting result? There is no change in received power with frequency. You want less power at the higher frequency? You have to make the antenna smaller. d _____________________________ http://www.pearce.uk.com |
#86
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Microwave Spectrum - Amplitude Modulation
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#87
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Microwave Spectrum - Amplitude Modulation
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#88
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Microwave Spectrum - Amplitude Modulation
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#89
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Microwave Spectrum - Amplitude Modulation
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#90
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Microwave Spectrum - Amplitude Modulation
Tube wrote: "Free space attenuation has no frequency dependency." You must be joking! I don't think he was. Are you? Free space attenuation is dependant on frequency and can be written as: Attenuation in dB = 36.6 + 20 log F + 20 log d, where F is frequency in MHz and d is distance in miles. Believe me, I really would like free speace attenuation to be frequency independant, (I work with radio planning of 2GHz WCDMA mobile systems) but unfortunately the dependancy is there, when increasing the frequency 10 times the attenuation increases 20dB for the same distance. Oh dear. How many times has smaller receiver antenna aperture size been confused with some actual loss in free space or a vacuum? (It is as hard to defeat the energy conservation law in a vacuum -- or basically lossless air -- as it is anywhere else.) I wish "they" wouldn't call this effect "free space loss," because that is not what it is. |
#91
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Microwave Spectrum - Amplitude Modulation
Tube wrote: "Free space attenuation has no frequency dependency." You must be joking! I don't think he was. Are you? Free space attenuation is dependant on frequency and can be written as: Attenuation in dB = 36.6 + 20 log F + 20 log d, where F is frequency in MHz and d is distance in miles. Believe me, I really would like free speace attenuation to be frequency independant, (I work with radio planning of 2GHz WCDMA mobile systems) but unfortunately the dependancy is there, when increasing the frequency 10 times the attenuation increases 20dB for the same distance. Oh dear. How many times has smaller receiver antenna aperture size been confused with some actual loss in free space or a vacuum? (It is as hard to defeat the energy conservation law in a vacuum -- or basically lossless air -- as it is anywhere else.) I wish "they" wouldn't call this effect "free space loss," because that is not what it is. |
#92
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Microwave Spectrum - Amplitude Modulation
Tube wrote: "Free space attenuation has no frequency dependency." You must be joking! I don't think he was. Are you? Free space attenuation is dependant on frequency and can be written as: Attenuation in dB = 36.6 + 20 log F + 20 log d, where F is frequency in MHz and d is distance in miles. Believe me, I really would like free speace attenuation to be frequency independant, (I work with radio planning of 2GHz WCDMA mobile systems) but unfortunately the dependancy is there, when increasing the frequency 10 times the attenuation increases 20dB for the same distance. Oh dear. How many times has smaller receiver antenna aperture size been confused with some actual loss in free space or a vacuum? (It is as hard to defeat the energy conservation law in a vacuum -- or basically lossless air -- as it is anywhere else.) I wish "they" wouldn't call this effect "free space loss," because that is not what it is. |
#93
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Microwave Spectrum - Amplitude Modulation
Tube wrote: "Free space attenuation has no frequency dependency." You must be joking! I don't think he was. Are you? Free space attenuation is dependant on frequency and can be written as: Attenuation in dB = 36.6 + 20 log F + 20 log d, where F is frequency in MHz and d is distance in miles. Believe me, I really would like free speace attenuation to be frequency independant, (I work with radio planning of 2GHz WCDMA mobile systems) but unfortunately the dependancy is there, when increasing the frequency 10 times the attenuation increases 20dB for the same distance. Oh dear. How many times has smaller receiver antenna aperture size been confused with some actual loss in free space or a vacuum? (It is as hard to defeat the energy conservation law in a vacuum -- or basically lossless air -- as it is anywhere else.) I wish "they" wouldn't call this effect "free space loss," because that is not what it is. |
#94
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Microwave Spectrum - Amplitude Modulation
Curious wrote: Why aren't there amplitude modulated stations in the high megahertz/gigahertz range? I don't know what specific "stations" you are refering to. In the US, the "standard" radio services have been the familiar AM band (@ around 1 MHz) and FM band (@ around 100 MHz). This restriction is simply due to the sequence of history and governmental decree. FM was not invented until the 1930's, so as an accident of history it simply ended up at a higher frequency than the AM band; not because of some of the technological issues some have claimed in this thread. Google Edwin Armstrong for some interesting old radio history. He was a giant. There are multitudinous "stations" that use AM well into the GHz range. For example, our cheezy little 802.11a WLAN 5 GHz stuff uses OFDM in which each subcarrier is both AM an PM modulated. Taking advantage of both degrees of freedom (amplitude *and* phase) seems to allow higher bit rates. That means you could get your hi-fi across the cheezy OFDM link with no troubles. AM transmitters are a tough challenge though. FM transmitters are bone head simple and power efficient, but you won't get the same data rates as a system that takes advantage of both phase and amplitude. What artifacts would arise? None really. Of course, amplitude sensitive detection is sensitive to amplitude noise, as is phase/freq detection to phase noise. |
#95
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Microwave Spectrum - Amplitude Modulation
Curious wrote: Why aren't there amplitude modulated stations in the high megahertz/gigahertz range? I don't know what specific "stations" you are refering to. In the US, the "standard" radio services have been the familiar AM band (@ around 1 MHz) and FM band (@ around 100 MHz). This restriction is simply due to the sequence of history and governmental decree. FM was not invented until the 1930's, so as an accident of history it simply ended up at a higher frequency than the AM band; not because of some of the technological issues some have claimed in this thread. Google Edwin Armstrong for some interesting old radio history. He was a giant. There are multitudinous "stations" that use AM well into the GHz range. For example, our cheezy little 802.11a WLAN 5 GHz stuff uses OFDM in which each subcarrier is both AM an PM modulated. Taking advantage of both degrees of freedom (amplitude *and* phase) seems to allow higher bit rates. That means you could get your hi-fi across the cheezy OFDM link with no troubles. AM transmitters are a tough challenge though. FM transmitters are bone head simple and power efficient, but you won't get the same data rates as a system that takes advantage of both phase and amplitude. What artifacts would arise? None really. Of course, amplitude sensitive detection is sensitive to amplitude noise, as is phase/freq detection to phase noise. |
#96
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Microwave Spectrum - Amplitude Modulation
Curious wrote: Why aren't there amplitude modulated stations in the high megahertz/gigahertz range? I don't know what specific "stations" you are refering to. In the US, the "standard" radio services have been the familiar AM band (@ around 1 MHz) and FM band (@ around 100 MHz). This restriction is simply due to the sequence of history and governmental decree. FM was not invented until the 1930's, so as an accident of history it simply ended up at a higher frequency than the AM band; not because of some of the technological issues some have claimed in this thread. Google Edwin Armstrong for some interesting old radio history. He was a giant. There are multitudinous "stations" that use AM well into the GHz range. For example, our cheezy little 802.11a WLAN 5 GHz stuff uses OFDM in which each subcarrier is both AM an PM modulated. Taking advantage of both degrees of freedom (amplitude *and* phase) seems to allow higher bit rates. That means you could get your hi-fi across the cheezy OFDM link with no troubles. AM transmitters are a tough challenge though. FM transmitters are bone head simple and power efficient, but you won't get the same data rates as a system that takes advantage of both phase and amplitude. What artifacts would arise? None really. Of course, amplitude sensitive detection is sensitive to amplitude noise, as is phase/freq detection to phase noise. |
#97
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Microwave Spectrum - Amplitude Modulation
Curious wrote: Why aren't there amplitude modulated stations in the high megahertz/gigahertz range? I don't know what specific "stations" you are refering to. In the US, the "standard" radio services have been the familiar AM band (@ around 1 MHz) and FM band (@ around 100 MHz). This restriction is simply due to the sequence of history and governmental decree. FM was not invented until the 1930's, so as an accident of history it simply ended up at a higher frequency than the AM band; not because of some of the technological issues some have claimed in this thread. Google Edwin Armstrong for some interesting old radio history. He was a giant. There are multitudinous "stations" that use AM well into the GHz range. For example, our cheezy little 802.11a WLAN 5 GHz stuff uses OFDM in which each subcarrier is both AM an PM modulated. Taking advantage of both degrees of freedom (amplitude *and* phase) seems to allow higher bit rates. That means you could get your hi-fi across the cheezy OFDM link with no troubles. AM transmitters are a tough challenge though. FM transmitters are bone head simple and power efficient, but you won't get the same data rates as a system that takes advantage of both phase and amplitude. What artifacts would arise? None really. Of course, amplitude sensitive detection is sensitive to amplitude noise, as is phase/freq detection to phase noise. |
#98
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Microwave Spectrum - Amplitude Modulation
I work in the AM; FM; Analog and Digital TV industry in Australia.
The answer the the burning question, why not AM in higher Freq. Well it comes down to power and the size of the TX mast. For example we have a site transmitting on 747 kHZ with a 600 foot mast running at 10 kW. And another Tx at 1548 kHZ with a 150 foot mast and 50 kW. So the smaller the mast the more power you require to punch out the same radiation pattern. More power = more money. And also in the age of concept of AM I'm sure the engineers didn't know about Giga hertz or it would have been Giga cycles back then. Blame it on Marconi for not pushing the boundaries. |
#99
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Microwave Spectrum - Amplitude Modulation
I work in the AM; FM; Analog and Digital TV industry in Australia.
The answer the the burning question, why not AM in higher Freq. Well it comes down to power and the size of the TX mast. For example we have a site transmitting on 747 kHZ with a 600 foot mast running at 10 kW. And another Tx at 1548 kHZ with a 150 foot mast and 50 kW. So the smaller the mast the more power you require to punch out the same radiation pattern. More power = more money. And also in the age of concept of AM I'm sure the engineers didn't know about Giga hertz or it would have been Giga cycles back then. Blame it on Marconi for not pushing the boundaries. |
#100
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Microwave Spectrum - Amplitude Modulation
I work in the AM; FM; Analog and Digital TV industry in Australia.
The answer the the burning question, why not AM in higher Freq. Well it comes down to power and the size of the TX mast. For example we have a site transmitting on 747 kHZ with a 600 foot mast running at 10 kW. And another Tx at 1548 kHZ with a 150 foot mast and 50 kW. So the smaller the mast the more power you require to punch out the same radiation pattern. More power = more money. And also in the age of concept of AM I'm sure the engineers didn't know about Giga hertz or it would have been Giga cycles back then. Blame it on Marconi for not pushing the boundaries. |
#101
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Microwave Spectrum - Amplitude Modulation
I work in the AM; FM; Analog and Digital TV industry in Australia.
The answer the the burning question, why not AM in higher Freq. Well it comes down to power and the size of the TX mast. For example we have a site transmitting on 747 kHZ with a 600 foot mast running at 10 kW. And another Tx at 1548 kHZ with a 150 foot mast and 50 kW. So the smaller the mast the more power you require to punch out the same radiation pattern. More power = more money. And also in the age of concept of AM I'm sure the engineers didn't know about Giga hertz or it would have been Giga cycles back then. Blame it on Marconi for not pushing the boundaries. |
#102
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Microwave Spectrum - Amplitude Modulation
Emphatic wrote: I work in the AM; FM; Analog and Digital TV industry in Australia. The answer the the burning question, why not AM in higher Freq. Well it comes down to power and the size of the TX mast. For example we have a site transmitting on 747 kHZ with a 600 foot mast running at 10 kW. And another Tx at 1548 kHZ with a 150 foot mast and 50 kW. So the smaller the mast the more power you require to punch out the same radiation pattern. Utter nonsense. More power = more money. And also in the age of concept of AM I'm sure the engineers didn't know about Giga hertz or it would have been Giga cycles back then. Utter nonsense. Blame it on Marconi for not pushing the boundaries. Utter nonsense. He did push the boundaries _for the time_; that is why he is remembered. |
#103
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Microwave Spectrum - Amplitude Modulation
Emphatic wrote: I work in the AM; FM; Analog and Digital TV industry in Australia. The answer the the burning question, why not AM in higher Freq. Well it comes down to power and the size of the TX mast. For example we have a site transmitting on 747 kHZ with a 600 foot mast running at 10 kW. And another Tx at 1548 kHZ with a 150 foot mast and 50 kW. So the smaller the mast the more power you require to punch out the same radiation pattern. Utter nonsense. More power = more money. And also in the age of concept of AM I'm sure the engineers didn't know about Giga hertz or it would have been Giga cycles back then. Utter nonsense. Blame it on Marconi for not pushing the boundaries. Utter nonsense. He did push the boundaries _for the time_; that is why he is remembered. |
#104
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Microwave Spectrum - Amplitude Modulation
Emphatic wrote: I work in the AM; FM; Analog and Digital TV industry in Australia. The answer the the burning question, why not AM in higher Freq. Well it comes down to power and the size of the TX mast. For example we have a site transmitting on 747 kHZ with a 600 foot mast running at 10 kW. And another Tx at 1548 kHZ with a 150 foot mast and 50 kW. So the smaller the mast the more power you require to punch out the same radiation pattern. Utter nonsense. More power = more money. And also in the age of concept of AM I'm sure the engineers didn't know about Giga hertz or it would have been Giga cycles back then. Utter nonsense. Blame it on Marconi for not pushing the boundaries. Utter nonsense. He did push the boundaries _for the time_; that is why he is remembered. |
#105
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Microwave Spectrum - Amplitude Modulation
Emphatic wrote: I work in the AM; FM; Analog and Digital TV industry in Australia. The answer the the burning question, why not AM in higher Freq. Well it comes down to power and the size of the TX mast. For example we have a site transmitting on 747 kHZ with a 600 foot mast running at 10 kW. And another Tx at 1548 kHZ with a 150 foot mast and 50 kW. So the smaller the mast the more power you require to punch out the same radiation pattern. Utter nonsense. More power = more money. And also in the age of concept of AM I'm sure the engineers didn't know about Giga hertz or it would have been Giga cycles back then. Utter nonsense. Blame it on Marconi for not pushing the boundaries. Utter nonsense. He did push the boundaries _for the time_; that is why he is remembered. |
#106
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Microwave Spectrum - Amplitude Modulation
I stand corrected, at least partially. HighDef appears to be a subset of the
new DTV (Digital TV) standard, and the FCC is allowing broadcasters to choose how to use their alloted channel space. Either a single HDTV broadcast or multiple low-res programs. Anyway, here's a link: http://www.fcc.gov/cgb/consumerfacts/digitaltv.html Mike Don't know if this matters on the propagation subject, but I wasn't referring to high def. I know that is a completely different animal. I was talking about the same old NTSC resolution but transmitted in digital format. The FCC is apparently trying to free up some spectrum for other stuff (can't remember what) and figures they can cram more TV channels into less space by going digital. As I said, they're assigning the new digital channels by lottery and this poor station I'm aware of got a UHF channel where their analog is on VHF. I'd be interested to know what other digital format the FCC is behind. If have a web pointer or even just a keyword I could search on, I'd be interested. --Randy |
#107
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Microwave Spectrum - Amplitude Modulation
I stand corrected, at least partially. HighDef appears to be a subset of the
new DTV (Digital TV) standard, and the FCC is allowing broadcasters to choose how to use their alloted channel space. Either a single HDTV broadcast or multiple low-res programs. Anyway, here's a link: http://www.fcc.gov/cgb/consumerfacts/digitaltv.html Mike Don't know if this matters on the propagation subject, but I wasn't referring to high def. I know that is a completely different animal. I was talking about the same old NTSC resolution but transmitted in digital format. The FCC is apparently trying to free up some spectrum for other stuff (can't remember what) and figures they can cram more TV channels into less space by going digital. As I said, they're assigning the new digital channels by lottery and this poor station I'm aware of got a UHF channel where their analog is on VHF. I'd be interested to know what other digital format the FCC is behind. If have a web pointer or even just a keyword I could search on, I'd be interested. --Randy |
#108
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Microwave Spectrum - Amplitude Modulation
I stand corrected, at least partially. HighDef appears to be a subset of the
new DTV (Digital TV) standard, and the FCC is allowing broadcasters to choose how to use their alloted channel space. Either a single HDTV broadcast or multiple low-res programs. Anyway, here's a link: http://www.fcc.gov/cgb/consumerfacts/digitaltv.html Mike Don't know if this matters on the propagation subject, but I wasn't referring to high def. I know that is a completely different animal. I was talking about the same old NTSC resolution but transmitted in digital format. The FCC is apparently trying to free up some spectrum for other stuff (can't remember what) and figures they can cram more TV channels into less space by going digital. As I said, they're assigning the new digital channels by lottery and this poor station I'm aware of got a UHF channel where their analog is on VHF. I'd be interested to know what other digital format the FCC is behind. If have a web pointer or even just a keyword I could search on, I'd be interested. --Randy |
#109
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Microwave Spectrum - Amplitude Modulation
I stand corrected, at least partially. HighDef appears to be a subset of the
new DTV (Digital TV) standard, and the FCC is allowing broadcasters to choose how to use their alloted channel space. Either a single HDTV broadcast or multiple low-res programs. Anyway, here's a link: http://www.fcc.gov/cgb/consumerfacts/digitaltv.html Mike Don't know if this matters on the propagation subject, but I wasn't referring to high def. I know that is a completely different animal. I was talking about the same old NTSC resolution but transmitted in digital format. The FCC is apparently trying to free up some spectrum for other stuff (can't remember what) and figures they can cram more TV channels into less space by going digital. As I said, they're assigning the new digital channels by lottery and this poor station I'm aware of got a UHF channel where their analog is on VHF. I'd be interested to know what other digital format the FCC is behind. If have a web pointer or even just a keyword I could search on, I'd be interested. --Randy |