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Old 28th Apr 2021, 2:12 am   #21
Synchrodyne
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Default Re: DX FM reception.

Quote:
Originally Posted by SteveCG View Post
BTW I wonder whether Dolby B applied only to the stereo sub-carrier component might have assisted fringe area reception?

That I think would be debatable.

The partial compression of the subcarrier, along with the probable use of a different pre-emphasis constant (25 µs), would have reduced the extent to which the baseband and subcarrier interleaved. This would have required lower levels of maximum modulation for each in order to avoid exceeding the ±67.5 kHz maximum. Perhaps there could have been a trade-off, with a lesser reduction of modulation depth than nominally required accompanied by more severe peak limiting. But the effect of a modulation depth reduction would have been to reduce the mono service area for a given transmitter power output. Maybe, with partial compression of the subcarrier, the stereo service area would have been a higher fraction of the mono service area than for the basic Zenith-GE system, but with minimal, if any net increase in actual service area. Applying a greater maximum modulation depth reduction to the subcarrier than to the baseband might have helped, but at the expense of reasonable compatibility with non-Dolby equipped receivers that were not configured to handle the difference.

One could outpoint that the American Zenith-DBX TV stereo sound system did use a highly compressed subcarrier. In that case though, the subcarrier was allocated an aliquot of ±50 Hz deviation, additional to the ±25 kHz allocated to the baseband. In the TV sound case, there was plenty of bandwidth available to do this, whereas in the FM case, the ±75 kHz maximum deviation was a fairly hard limit.

Incidentally, with Dolby on the sub-carrier only, FDM decoding would be required, rather than TDM, which was the modal choice for the receivers and most decoding ICs. The Philips TDA1005A could do both TDM and FDM, but off-hand I don’t know whether it allowed access to the raw demodulated sub-carrier for separate processing. But the National LM1884, designed for American TV sound, did.

Given the ±75 kHz maximum deviation constraint, in turn derived from channel allocations in the FN band, and the resultant established receiver design, I don’t think that there was a “magic bullet” that would have solved the stereo “problems”. The reasons were well-explained by RW in post #11. Additionally, a graphic illustration was provided by Keller (*):

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I have taken Keller’s graph (d) and roughly added in the Zenith-GE subcarrier, with pre- and de-emphasis also shown:

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One may see that whatever kind of subcarrier was used, it was going to be in a much higher noise zone than the baseband. Having the subcarrier at a low a frequency as reasonably possible was clearly desirable. One may also see the attraction of using only the lower sideband of an AM or AMSC subcarrier. However, transmitting this way would mean losing interleaving, which was probably worth around 6 dB. Also, it would have been impracticable with the realizable consumer technology of the day. Using only the LSB (of a DSB subcarrier) at the receiving end would be a way of taking advantage of its lower noise without giving up interleaving at the transmitting end, and as mentioned previously was actually done.

Maybe therein is a challenge for the those interested in FM stereo DX - design and build a decoder that uses only the LSB of the subcarrier, and so ascertain what improvement may be obtained by this means. I imagine that using some of the AM stereo decoding techniques, including quadrature demodulation and AF phase-shifting for ISB sideband separation would have been applicable. As an example, The Hershberger synchronous detector (Popular Electronics 1982 April p.61ff), adapted for 38 kHz (rather than 455 kHz) operation and for an external carrier input (19 kHz) might be a starting point for circuitry that extracts the LSB from the subcarrier.


(*) V.H.F. Radio Manual; P.R. Keller; Newnes, 1957; see p.29.


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Old 28th Apr 2021, 3:28 am   #22
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Default Re: DX FM reception.

I should note that in my marked-up version of Keller’s diagram, for convenience I showed the subcarrier only on one side of the main carrier; in reality there is a mirror-image of it on the other side.
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Old 28th Apr 2021, 8:15 am   #23
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Default Re: DX FM reception.

Thank you Synchrodyne for the explanation. Given the way broadcasting is moving we are lucky still to have any form of analogue Stereo FM left to think about!
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Old 30th Apr 2021, 5:32 pm   #24
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Default Re: DX FM reception.

Remember reading somewhere that an FM subcarrier was considered but many FM stations in America used higher subcarriers for store music & it wasn't compatible with them. Would an FM subcarrier have had a better S/N ratio though?
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Old 30th Apr 2021, 6:06 pm   #25
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Default Re: DX FM reception.

Yes, some US FM stations used subcarriers to carry 'elevator music' for use in motels, supermarkets and similar.

I spent 6 months working with a bunch of Honeywell techies in Dallas/Fort-Worth back in the early-80s and one of them had built a subcarrier-receiver using one of the LM56x PLL decoder chips to extract this.

Practically, I've always thought that FM is really just a variant of SSB with a 90-degree-shift; in times-past there were FM-detectors using strange valves [EQ90 Nonode?] that used this approach.
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Old 30th Apr 2021, 7:26 pm   #26
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Default Re: DX FM reception.

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Originally Posted by colly0410 View Post
Would an FM subcarrier have had a better S/N ratio though?
FM only gives you an S/N advantage if the bandwidth it occupies is significantly wider than the bandwidth an AM scheme would take up. So no real room for a win. You could get it if you widened the whole channel, but the primary FM modulation would do that for you directly anyway. I'm not sure that FM on FM would yield any advantage. It could get messy with sidebands going down into the sum signal baseband.

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Old 30th Apr 2021, 11:18 pm   #27
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Default Re: DX FM reception.

The Crosby FM stereo system, which was runner-up to the Zenith-GE system in the US FCC evaluation, used an FM subcarrier at 50 kHz, with a maximum deviation of ±25 kHz, meaning a deviation ratio, referred to 15 kHz, of just 1.67. Nominally the subcarrier occupied the band 25 to 75 kHz. It was allowed 50% of the main carrier deviation. Thus because there was no interleaving with an FM subcarrier, the main channel had to be restricted to 50% deviation, meaning a non-negligible reduction in the mono service area for a given S/N. At the time, the FCC 75 kHz upper limit on modulating frequencies (later removed) meant that the Crosby system could not accommodate an SCA (subsidiary communications authorization) subcarrier. This did go against it as compared with the Zenith-GE system, which allowed for an SCA subcarrier at 67 kHz, but that aside, the Crosby system was judged to be not as good overall.

Interleaving considerations aside, a bigger deviation ratio for the subcarrier would help with its own S/N. But accommodation of a wider bandwidth subcarrier would require that it be move upward in frequency, say to 75 kHz for a ±50 kHz deviation. Then the subcarrier as a whole would have a lower deviation ratio referred to the main carrier, which could adversely affect the net S/N. So there would be a trade-off in that respect.

Nonetheless, FM subcarriers are required where a completely separate programme is to be carried, in order to minimize crosstalk from the main channel. Thus the SCA subcarriers are FM. So was the SAP (second audio programme) subcarrier of the American Zenith-DBX TV stereo sound system. It used DBX companding in order to have an acceptable signal-to-noise ratio. The Japanese (EIAJ) TV stereo sound system used an FM subcarrier, with companding. In that case the same subcarrier was used either for the stereo difference signal or the SAP signal. Its modulation of the main carrier was additional to that of the baseband, since there was plenty of bandwidth available around the sound carrier in a TV channel. A derivative of the EIAJ FM-FM system, with an additional SAP subcarrier, was proposed for use in the USA, but the Zenith-DBX system, with AM subcarrier, was judged by the FCC to be better.

There was some discussion on SCA subcarriers towards the end of this thread: https://www.vintage-radio.net/forum/...ad.php?t=56858.

Quote:
Originally Posted by G6Tanuki View Post
I spent 6 months working with a bunch of Honeywell techies in Dallas/Fort-Worth back in the early-80s and one of them had built a subcarrier-receiver using one of the LM56x PLL decoder chips to extract this.
I think that the LM565 was fairly standard for this job. Interestingly, the original Zenith decoder circuit for the Zenith-DBX TV stereo sound system used an LM565 for the SAP channel.

Quote:
Originally Posted by G6Tanuki View Post
Practically, I've always thought that FM is really just a variant of SSB with a 90-degree-shift; in times-past there were FM-detectors using strange valves [EQ90 Nonode?] that used this approach.
That was the EQ80 enneode/nonode. It was one of several valve-era quadrature-type FM demodulators, some quasi-synchronous and some fully synchronous. There was some mention of these in this post: https://www.vintage-radio.net/forum/...6&postcount=15.

Some of the valves used for quadrature FM demodulation, such as the EH90 (6CS6) were also proposed for Zenith-GE stereo decoding, e.g. by Mullard.

I suppose that one could say that FM and SSB are both forms of angle modulation, the latter involving amplitude modulation as well. Quadrature AM (QAM) is also a combination of angle and amplitude modulation. Demodulation of a QAM signal along the I-axis recovers the I signal, along the Q-axis recovers both the Q signal and any FM. And phase matrixing of the demodulated I and Q signals allows individual recovery of the LSB and USB components. The various AM stereo systems were combinations of amplitude and angular modulation. An interesting IC of the 1970s was the Plessey SL624C, which could demodulate both AM and FM, and also act as a self-oscillating SSB demodulator. It was essentially the same as a TV intercarrier FM sound IC, the kind with an electronic volume control and an audio driver (e.g. TBA120S).

Returning to the FM stereo case, I’d say that choice of system was well-researched on both sides of the Atlantic, with the Zenith-GE system receiving almost universal approval. The USSR was the only dissenter. Apparently, it found the Zenith-GE system less suitable for its ±50 kHz maximum deviation, and instead opted for its polar modulation system, which used an AM subcarrier at 32 kHz with partially suppressed carrier. I don’t know for sure, but it looks as if the objective was to get the subcarrier to as low a frequency as reasonably possible, thus minimizing the bandwidth of the composite signal. In turn this meant that there was no room for a pilot carrier, hence the use of a partially suppressed rather than a fully suppressed subcarrier. It is conceivable that an FM subcarrier (which would not require a pilot tone) at around 32 kHz was considered, but rejected in favour of the AM subcarrier. The 32 kHz subcarrier would have had sidebands covering 17 to 47 kHz, the 17 kHz number being about as close to the 0 to 15 kHz baseband as one might want to go. I understand that the thinking was that with the subcarrier adjusted to 31.25 kHz (twice the 625-line TV line frequency), the same system might also have been suitable for TV stereo sound.

In contrast to the near-unity in worldwide FM stereo system choice, there was much diversity in TV stereo sound systems. But there in any case certain parameters had to be tied to the basic TV characteristics, such as line frequency, and also to the channel characteristics. So the diversity was effectively built-in.

I still haven’t managed to find any information on the Japanese FM tuner that used on the subcarrier LSB to decode stereo. I am fairly sure that it was in one of the British magazines in the 1980s, I think not HFN. The report might have given some information as to the claimed S/N benefit.


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Old 1st May 2021, 5:14 am   #28
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Default Re: DX FM reception.

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Originally Posted by Synchrodyne View Post
I still haven’t managed to find any information on the Japanese FM tuner that used on the subcarrier LSB to decode stereo. I am fairly sure that it was in one of the British magazines in the 1980s, I think not HFN. The report might have given some information as to the claimed S/N benefit.

Lots of rather interesting stuff there.

Using only the LSB side of the difference channel, and suggesting there is an S/N benefit? That sounds to be on dodgy ground. If it's just the tuner that takes that approach and the broadcast is normal Zenith multiplex, then I'd reckon it loses S/N ratio. The USB and LSB sidebands correlate with each other, given the synchronous demod of the DSB, while the noise in the upper and lower areas are not correlated, so summing the USB and LSB sides gives an S/N advantage.

But if the broadcast system was different, transmitting only the LSB of the difference signal, then the amount of 'power' in that sideband could be increased to cover the saving on the other side, and things will come out in the wash, equality rather than a benefit to either side.... this would mean the same overall deviation of the FM transmission, and no net advantage to either approach.

I've always been wary of DBX. They had this one tool and they seemed to tout it as if it would work magic wherever it got applied. Instantaneous companding is an invitation to distortion. Slower distortion gives you the quandry between the time taken to assass the level of an AC signal, versus letting low frequency signal components pump the compression of everything. Dolby, by having his compression kept in bands, managed to disguise his artefacts a little more.

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Old 1st May 2021, 6:29 am   #29
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Default Re: DX FM reception.

With the Zenith-DBX TV stereo sound system, you could pick up some pumping artefacts when listening to stereo sound broadcast on decent equipment, including a good outboard TV tuner with split sound. But it wasn’t overwhelmingly bad, I think because only the difference channel was companded. On the mono SAP channel, the pumping was fairly obvious. But I can’t say that I’m a fan of the DBX system generally.

Re the use of the LSB only in decoding the Zenith-GE system, we’d need to find the original report about the tuner so-equipped to ascertain just what claims were made, although I am certain that lower noise was one of them.

With regular AM synchronously demodulated, it is clear that using one sideband alone confers a noise penalty as compared with using both, which is why for example decent synchronous HF programme-content listening receivers gave you a choice between LSB, USB and DSB. DSB also gave you some benefit from sideband diversity when there was selective fading. I am relying on distant recall, but I think that the argument used to support LSB-only subcarrier demodulation with the Zenith-GE system was that because of the triangular noise spectrum of FM, the USB was a lot noisier than the LSB, such that the loss of S/N due to the loss of one sideband was more than offset by the elimination of the noisy USB. Whether that proposition would hold up to quantitative analysis, I don’t know.

Using just the subcarrier LSB for transmission was proposed in the late 1950s, including by Siemens and Philco. This was mentioned in BBC Monograph #29 of 1960 April (http://downloads.bbc.co.uk/rd/pubs/a...nograph_29.pdf), with comparative S/N numbers; it was slightly better than DSB. The Siemens proposal was also mentioned in BBC RDR 1962-49 (http://downloads.bbc.co.uk/rd/pubs/reports/1962-49.pdf).


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Old 1st May 2021, 11:28 pm   #30
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Default Re: DX FM reception.

There are various other pertinent papers by the BBC, Siemens, and others, not all on the web, but going there I think would pull the thread too far off-topic. By way of an overview, one could say that adding subcarriers to a system that had been developed and optimized for high quality mono transmission and reception without increasing the allowed maximum deviation was going to involve some non-negligible loss of performance as measured along several vectors howsoever it was done. Different systems varied the performance loss distribution amongst the individual vectors, but did not obviate the overall loss. The FCC chose the Zenith-GE system as the best compromise back in 1961 (in fact it merged the separate Zenith and GE proposals into one system.) Then in the 1960s, Europe chose the same system; in that case there might have been some extra weighting given to it on the basis that it was established in the Americas and elsewhere, such that any other contender would have had to be noticeably better rather than just directionally or selectively so in order to prevail.


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Old 2nd May 2021, 10:04 pm   #31
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Default Re: DX FM reception.

Not to further digress, but to refine what has gone before, here is a much better FM stereo noise distribution chart than that which I attached to post #21. It is from the FMX patent document.

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One could also draw up a similar chart for the FM subcarrier case. This would be a case of triangles upon triangles plus pre/de-emphasis effects, whereas for the AM subcarrier case it is triangles upon rectangles, plus the pre/de-emphasis effects.


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Old 30th May 2021, 5:52 am   #32
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Default Re: DX FM reception.

... At last some FM DX for me. Tropho lift bringing NDR 2 (Hamburg) on 87.6 to me. Also UK DX in the form of BBC Radio 3 from Tacolneston on 91.9.
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Old 31st May 2021, 4:03 pm   #33
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... first SpE for me of the season; Since 4 PM BST on 87.6 what I strongly suspect is Morocco - a mixture of French and Arabic speech and music with I think the occasional mention of "Maroc". Still there as I type.

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Old 1st Jun 2021, 7:55 am   #34
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Default Re: DX FM reception.

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... first SpE for me of the season; Since 4 PM BST on 87.6 what I strongly suspect is Morocco - a mixture of French and Arabic speech and music with I think the occasional mention of "Maroc". Still there as I type.
Very interesting. What receiver and antenna are you using ?
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Old 1st Jun 2021, 8:31 am   #35
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Default Re: DX FM reception.

I posted last week in the amateur section about my SpE reception of Kroatia and Algeria. For SpE no special equipment was used, a bog standard FM tuner and a normal FM aerial.

RDS and the interweb help with identifying distant stations. The great thing about SpE is that signals sometimes romp in with surprisng strength.
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Old 1st Jun 2021, 9:37 am   #36
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Default Re: DX FM reception.

Jolly_7,

The aerial is a dipole mounted in the loft (oriented to pick up N-S) , the receiver a (nothing special) JVC tuner/amp/cd unit. As eddie_ce says, for SpE nothing special is needed.
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Old 1st Jun 2021, 9:47 am   #37
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Jolly_7,

The aerial is a dipole mounted in the loft (oriented to pick up N-S) , the receiver a (nothing special) JVC tuner/amp/cd unit. As eddie_ce says, for SpE nothing special is needed.
Thanks. I haven't really done any serious FM DXing for a long time but am enthused by your reports. I have taken out my JVC-TV3L tuner and connected it to a circular omnidirectional antenna in the loft. At the moment I don't have anything better as far as antennas go. Will need to keep my ears open.
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Old 1st Jun 2021, 9:55 am   #38
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Default Re: DX FM reception.

Jolly_7, indeed I have picked up FM SpE on a circular omni FM aerial connected to a standard Rotel Tuner.

Given the congestion of the UK FM band I listen out in the 87.5 to 87.9 Mc/s (essentially unused in the UK) part of the band. It is a bit like fishing, you have to be patient - but I've found that afternoon is the best time to listen - but not the only time!
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Old 2nd Jun 2021, 9:03 am   #39
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Default Re: DX FM reception.

I use my Grundig Satellite 700 which is a large portable radio for al my DX'ing just using its internal aerials. Its quite surprising how well both decent tropo and Sporadic E signals are received just on a telescopic rod aerial.
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Old 3rd Jun 2021, 4:41 pm   #40
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... More SpE reception this afternoon ...
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