FM stereo sub-carrier?

[colly0410]

Been thinking: (Stop laughing I can think  ) The stereo sub-carrier at 38 Khz on FM radio is double sideband AM with a suppressed carrier. My question is could an FM sub-carrier have been used instead? I'm thinking along the lines of SECAM V NTSC/PAL if you know what I mean.. 

[Synchrodyne]

The short answer is yes.  The NSRC (National Stereo Radio Committee) and the FCC did consider both FM-FM and FM-AM proposals (including FM-AM with an SSB subcarrier).  In fact some of the FM-FM proposals, notably that from Crosby, predated the FM-AM proposals.  But there were cogent reasons for choosing FM-AM, i.e. the Zenith-GE system.  Primary was the fact that within the constraint of ±75 kHz maximum deviation, the fact that an AM difference signal subcarrier was interleaved ensured that there was minimal effect on modulation depth, and hence signal-to-noise ratio and service area, as seen by mono receivers.  With an FM subcarrier, whatever maximum modulation depth was assigned to the subcarrier was necessarily subtracted from that available for the mono baseband signal, thus effectively reducing transmitter range.  Additionally, the use of an AM subcarrier still allowed room for the SCA service, which provided essential income for many US FM stations.  That was not always the case with FM subcarriers, particularly if the subcarrier deviation was made large enough to provide a good signal-to-noise ratio for the difference channel.  The Crosby proposal (which was runner-up in the FCC “contest”) used a 50 kHz FM subcarrier with ±25 kHz deviation, and occupying 25 to 75 kHz.  That meant that there was no room for an SCA channel within the 75 kHz maximum modulation frequency then allowed by the FCC.  (Later- early 1980s I think - that upper limit was relaxed to 99 kHz.

Note that FM subcarriers are used for the American SCA service provided by FM radio.  Also, the Japanese (EIAJ) TV stereo sound system used an FM subcarrier alternatively for the stereo difference channel and the second audio programme (SAP) channel.  In part that was because for SAP, where the main channel-to/from-subcarrier cross-talk requirements were more stringent than for stereo, FM provided greater isolation.

The American TV stereo sound system used an AM subcarrier for the stereo difference signal and an FM subcarrier for SAP.  The BTSC (Broadcast Television System Committee) and the FCC considered proposals with both AM and FM stereo subcarriers, in the latter case a system derived from Japanese practice.  In this case, interleaving was much less of an issue because additional deviation was to be made available, and the subcarrier channels would be companded.  AM was chosen in part because it had noticeably lower distortion than what was a relatively narrow-band FM channel.

For some additional background on the FM stereo case, perhaps look here:  http://www.vintage-radio.net/forum/showt...hp?t=56858.


Cheers,

Steve

[colly0410]

Thanks for the link, I must have missed that post as can't remember seeing it before, very interesting..

[colly0410]

I'd forgotten about the SCA hotel & shop muzak system, wonder why they never used it here?

[colly0410]

I'm trying to figure out what advantage doubling the 19Khz pilot tone to reinsert the carrier to the 38Khz DSSC has , why not just have a bit of a carrier at 38Khz? I'm presuming it would take too much power from the main FM carrier, or am I on the wrong track?

[Synchrodyne]

As I understand it, the pilot tone, which was allowed 10% of the available deviation, was placed at 19 kHz to aid in its noise-free recovery.  Given the triangular noise spectrum of FM, then for a given deviation (±7.5 kHz in this case), the noise at 19 kHz was 6 dB lower than the noise at 38 kHz.  Also, at 19 kHz, the pilot tone is surrounded by a reasonable amount of “white space”, so that the circuits used to select, amplify and process it were unlikely to be affected by nearby signals.  On the other hand, at 38 kHz, it would have been surrounded by nearby difference signal sidebands that might well have been significantly greater in magnitude, and might therefore upset recovery circuits.  Conceivably a pilot tone at 38 kHz would have required greater amplitude, and something like 20% of the available deviation to be adequately robust, thus leaving 80% rather than 90% of the deviation available for the programme signals.

Back in the late 1950s, I think that the assumption would have been made that receivers would most likely have used sharply tuned LC filters to extract the pilot tone.  So keeping the pilot tone clear of noise and nearby sidebands was a key desideratum.  PLL techniques, which might be better able to deal with adverse situations, were known and doable with valves (and to some extent were used in colour TV receivers for colour subcarrier recovery), but were seen as too complex and too costly for general use in domestic receivers.  In fact PLL-based decoding did not arrive for general use in FM stereo receivers until the IC age, starting with the RCA CA3090 in 1971 and soon followed by the Fairchild µA758 and Motorola MC1310.  And these were second-generation IC decoders, the first generation having been represented by the Motorola MC1304/5.

Note though that the American MTS system also included a pilot tone, even though by then it could assumed that PLL-type decoders would be used.  In this case the pilot tone was at line frequency, namely 15 734 Hz.  So clearly the view was that despite circuit improvements over 20 years or so, a half-frequency pilot tone was still preferable.


Cheers,

Steve

[Synchrodyne]

I'd forgotten about the SCA hotel & shop muzak system, wonder why they never used it here?

The BBC did some work on the use of SCA, more I think as a possible extra programme channel than a subscription service.  The results are recorded in BBC Research Department Report RD 1976/14, available here:  http://downloads.bbc.co.uk/rd/pubs/reports/1976-14.pdf.


Whether the IBA did any corresponding work I don't know, but it seems not unlikely.

Cheers,

Steve       

[Terry]

We do, however, have RDS which uses a 57kHz subcarrier (derived from the third harmonic of the stereo pilot).

Presumably this requires far less bandwidth than SCA so can be placed 10kHz closer to the stereo information without causing problems.

[colly0410]

Thanks for info. I was sort of on the right track with ' excess power being stolen from the main FM carrier.' If 10% is stolen from the main carrier no one notices - but 20%? Time to send for the cops. Never knew about B the BBC SCA gubins, will have a read through it in a bit, time for a mug of tea anyway so I'll put the kettle on...

[Synchrodyne]

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