17-02-2017, 08:44 PM
It may be noted that the Russian polar modulation FM stereo system did use a reduced subcarrier rather than a separate pilot tone. In this case the subcarrier was at 31.25 kHz. The (L-R) subcarrier itself was suppressed 14 dB by a notch filter, and was allowed 20% modulation. Thus its sidebands were allowed up to 80% modulation, as was the (L+R) baseband. From a stereo noise perspective at lower received signal strengths, it was probably not as good as the Zenith-GE system, although having the subcarrier at 31.25 kHz rather than 38 kHz would have gained a dB or two.
The apparent reason for doing it this way was to allow for very simple and low cost decoders. Following the FM discriminator there was a 31.25 kHz peaking filter to restore the subcarrier to its full level, followed by a pair of opposite polarity diode demodulators. Subcarrier modulation phasing was arranged so that these opposite polarity demodulators respectively produce the left and right signals. But more sophisticated decoding could also be used. I’d guess, although I don’t actually know, that the transmitter subcarrier notch filter would have been specified with a definite and not-too-sharp characteristic to allow the construction of economic, chronically stable receiver reciprocal peaking filters, which would likely have had broadish 3 dB points incursing somewhat into the sidebands.
The 31.25 kHz subcarrier frequency was twice the TV (625/50) line frequency, chosen so that the polar modulation system could also be used for TV sound, although I don’t know that in practice it ever was thus used.
The Siemens FM stereo proposal used a 30 kHz subcarrier of which only the lower sideband was transmitted, along with a very reduced carrier that was allowed only 6% modulation. One of its reported problems (at the receiver end) was that of regenerating a stable subcarrier given its low level and the proximity of the sidebands.
Connecting the dots, it looks as if the 14 dB subcarrier suppression with 20% modulation allowance of the Russian system was about as low as it was practicable to go for a reduced subcarrier without running into subcarrier retrieval difficulties.
Cheers,
Steve
The apparent reason for doing it this way was to allow for very simple and low cost decoders. Following the FM discriminator there was a 31.25 kHz peaking filter to restore the subcarrier to its full level, followed by a pair of opposite polarity diode demodulators. Subcarrier modulation phasing was arranged so that these opposite polarity demodulators respectively produce the left and right signals. But more sophisticated decoding could also be used. I’d guess, although I don’t actually know, that the transmitter subcarrier notch filter would have been specified with a definite and not-too-sharp characteristic to allow the construction of economic, chronically stable receiver reciprocal peaking filters, which would likely have had broadish 3 dB points incursing somewhat into the sidebands.
The 31.25 kHz subcarrier frequency was twice the TV (625/50) line frequency, chosen so that the polar modulation system could also be used for TV sound, although I don’t know that in practice it ever was thus used.
The Siemens FM stereo proposal used a 30 kHz subcarrier of which only the lower sideband was transmitted, along with a very reduced carrier that was allowed only 6% modulation. One of its reported problems (at the receiver end) was that of regenerating a stable subcarrier given its low level and the proximity of the sidebands.
Connecting the dots, it looks as if the 14 dB subcarrier suppression with 20% modulation allowance of the Russian system was about as low as it was practicable to go for a reduced subcarrier without running into subcarrier retrieval difficulties.
Cheers,
Steve
