A friend asked me if FM HD is data that is carried on a regular FM signal like the old SCA? I know SCA is / was analog but it was part of the FM signal. I quickly told him it was but then I started to think a couple of hours latter. Some HD FM stations have HD transmitters along with their analog transmitters. Harris is pushing it's transmitters that have HD "built in". I was correct that HD is a digital signal that is part of the main analog FM signal like the 19K stereo tone and RDS?
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Am I correct?
- Thread starter secondchoice
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Here's a link to a Google Book page that talks about hybrid implimentation and referrs to the three methods that can be used (high, mid, and low). Hopefully it helps!
secondchoice said:
HD and analog are separate signals and use separate transmitters (whether both in the same box or not). In some cases, the HD signal is generated from a physically separate transmitter and even a different antenna from the analog signal.
Hybrid Digital sits so far outside the allowed occupied bandwidth it kills anything 200 kHz away. Want to prove it? Take one of those nifty cigarette lighter RF amplifiers you plug your iPod into (if you don't have a direct input to your system, like I don't) and tune it 200 kHz above or below your favorite Hybrid Digital radio station, and see if your iPod is even heard just a little bit. And this is FCC approved.
RadeoEngineer said:
An even more interesting thing to do is tune your FM transmitter to the same frequency as a station's analog signal and put the transmitter close enough to an HD Radio that it jams the analog signal so all you hear is the FM transmitter. The station will lock in HD and the radio will go from playing your FM transmitter to the HD-1 stream of that station. Then tune the FM transmitter + or - 0.02mhz and watch the HD signal dropout.
This should not happen. If the receiver can get at least one set of HD sidebands - upper or lower - it should lock, in fact, someone makes a device for stations with first adjacency issues which allow one set of sidebands to be as low as 1% of the analogue power with the other set as high as 10% of the analogue power so that each side can put out as much power as is legal.spunker88 said:
secondchoice said:
SCA is arguably "part of the FM signal" because it is a subcarrier -- an FM modulated signal at ultrasonic frequencies that themselves are modulated by the main FM signal. However, for IBOC (tradename HD Radio) the distinction you're trying to draw is a bit arbitrary. The frequencies used are even more ultrasonic than the SCA frequencies (generally above 100 kHz, while SCA is generally below 100 kHz), but they do not go through the FM modulator like SCA does. The IBOC signal, which is created by wide-bandwidth amplitude modulation, is added to the FM-modulated signal either before or after the power amplifier that is the transmitter. In other words, it is possible (though not required) that the classic FM and the IBOC portions of the signal go through the same transmitter and/or the same transmission antenna. Whether or not you call that "part of the FM signal" is up to you.
So the HD data "rides" on an AM signal at the FM's frequency? Is kind of like the old TV, voice on FM and the picture on AM?jhardis said:
secondchoice said:
No, it is nothing like analog television.
IBOC is "at FM's frequency" in the sense that it's centered at the FM's frequency. However, the power is generally more than 100 kHz away. Also, calling it "AM" is perhaps misleading. It's OFDM (orthogonal frequency division multiplexing) -- hundreds of subcarriers at different frequencies that indicate data by changing phase. However, rather than having hundreds of separate oscillators at these different frequencies, the OFDM signal is generated by creating a complex, wide band, time-domain ("AM") signal whose Fourier transform in the frequency domain is that which is required.
HD FM, HD MW and HD TV all use QAM (Quadrature Amplitude Modulation) to generate their HD signals. It would be impossible to use frequency modulation to do this. The spectrum needed to contain the significant sidebands required to FM a digital scheme would be totally impractable. You could use the same PLL to demodulate both the "FM" and "AM" signals and then send them to DSP circuitry optimized for the properties of each "band".
QAM is so clean that you have channel 31 and 32 (both full power signals in Boston) broadcasting from the same tower! Ditto channels 12 and 13 in Providence. The "HD carriers" everyone complains about exist solely to cancel out any adverse effect HD may cause to it's own HOST analog channel. True IBOC emits no carrier. Take an analog TV and tune it any HDTV channel and tell me how much "noise" you see generated. Same deal for FM and MW HD when their respective analogs are finally sunset. Kinda blows a little hole in those "adjacent interference" theories true IBOC supposedly engenders, n'est ce pas?
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QAM is so clean that you have channel 31 and 32 (both full power signals in Boston) broadcasting from the same tower! Ditto channels 12 and 13 in Providence. The "HD carriers" everyone complains about exist solely to cancel out any adverse effect HD may cause to it's own HOST analog channel. True IBOC emits no carrier. Take an analog TV and tune it any HDTV channel and tell me how much "noise" you see generated. Same deal for FM and MW HD when their respective analogs are finally sunset. Kinda blows a little hole in those "adjacent interference" theories true IBOC supposedly engenders, n'est ce pas?
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iyiyi said:
Dream on. Even Cheap Channel wouldn't spend the money or take the risk of turning off the analog.
Chuck said:
Correct. So I am to assume that poking two adjacent 6 Mhz, full power, HD 19Mbs TV signals on the same stick is a simple task. Yet cleanly running a 30 Khz pure IBOC signal in the MW band is impossible? Perhaps those chuckleheads at CC, CBS, NPR, GM and etc. could invest their coin more wisely by purchasing a large bridge spanning the East River between Manhattan and Brooklyn and charge tolls...
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iyiyi said:
May well be true... however... Why would CC continue to spend millions on developing HD radio and more millions devising formats for the HDs if HD is such a dog?
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Excellent question, but I doubt it has anything to do with them shutting off all their analogs and being the pioneering all digital facilities for which there are no receivers and no interest from the public to acquire should they magically appear. Garbage programming is still garbage programming no matter the bandwidth or dynamic range.
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