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-   -   DX FM reception. (https://www.vintage-radio.net/forum/showthread.php?t=179271)

SteveCG 23rd Apr 2021 9:30 am

DX FM reception.
 
The high pressure over the UK has finally brought me some long distance FM today. I'm on the West side of the Malvern Hills but today I've been receiving BBC Radio 3 from the East: Wrotham (91.3 Mc/s), Oxford 91.7, and the real prize for me, Tacolneston on 91.9. All in hiss free Stereo with RDS. The aerial is a loft mounted J-Beam 'H' model SBM2 pointed ENE to the Hills. The tuner was a Teac T-H300. In addition I may have been getting a Dutch station on 87.6 Mc/s.

Yesterday evening I detected meteor pings (Lyrid) on 87.5 on a plain dipole mounted to receive from N-S.

paulsherwin 23rd Apr 2021 10:32 am

Re: Dx fm
 
I did hear some co-channel mush yesterday, a sure sign that there's a lift on here. That was with a bog standard 70s portable.

SteveCG 24th Apr 2021 8:12 am

Re: DX FM reception.
 
Some more tropo DX this morning - Wrotham & Oxford again (both BBC Radio 3) - but not quite as strong. Tacolneston was back to its usual v. weak state.

For me, on this particular receiving set-up, Wrotham is usually blotted out by Llandrindod Wells and Oxford is weakish mono.

Hartley118 24th Apr 2021 10:11 am

Re: Dx fm
 
Quote:

Originally Posted by paulsherwin (Post 1367153)
I did hear some co-channel mush yesterday, a sure sign that there's a lift on here. That was with a bog standard 70s portable.

This is bad news for those of us in the East of England who like to enjoy high quality FM stereo. Here in Cambridge, under ‘lift’ conditions, stereo FM is no longer fit for purpose because of co-channel interference. Had the band been as crowded back in the 1960s as it is today, I guess that the current stereo FM system might have been rejected as too vulnerable.

These problems are despite a 4-element external aerial and decent tuner(s), one with selectable IF bandwidth. In particular, there’s a continental station that causes annoying ‘birdies’ on the Radio 2 frequency of 88.9 MHz, forcing me to switch to DAB to silence the background. I guess that the North Sea offers rather little attenuation under these conditions.

Martin

SiriusHardware 24th Apr 2021 11:02 am

Re: DX FM reception.
 
I'm finding this interesting because I keep one eye on the Hepburn Tropo forecast here:-

https://dxinfocentre.com/tropo_nwe.html

..and at the moment the Tropo activity level is nothing special. I'm wondering if some other mode is responsible for this current activity. We've just had the Lyrids meteor shower but any effects due to that would be quite short-term (I would have thought).

SteveCG 24th Apr 2021 5:48 pm

Re: DX FM reception.
 
SiriusHardware,

Well I agree that meteor events (usually of duration about 1 second) would not explain what I've been receiving.

For what it is worth, I set up my Eastward looking system back last Sept/Oct (before it got too cold to want to go up into the loft!) and have been waiting for any settled weather to see if 'interesting' tropo propagation could happen in the Wintertime compared to the known Summertime effects. And the answer has been for me 'not a lot'. I've had Wrotham and Oxford a couple of times but not what I was after - Tacolneston - until now. Last summer I did receive Dutch FM so I knew that this was a possible path (via knife edge diffraction over the Malvern Hills).

I reckon that ducts are sometimes formed at a height where the Tx aerials and the ridge of the Malvern Hills exist in them, hence my reception. Having a very gentle wind flow direction from the Tx to the Hills seems to aid this reception process.

As for satisfactory Stereo reception in Cambridge - oh, that used to be quite a story years ago...

Hartley118 24th Apr 2021 7:04 pm

Re: DX FM reception.
 
Quote:

Originally Posted by SteveCG (Post 1367552)
SiriusHardware,
.....Last summer I did receive Dutch FM so I knew that this was a possible path (via knife edge diffraction over the Malvern Hills).
...........As for satisfactory Stereo reception in Cambridge - oh, that used to be quite a story years ago...

I guess that Dutch FM may well account for my birdie problem. I can cope with a bit of pink noise hiss, but a twittering birdie is just too distracting.

As for the story of stereo reception in Cambridge, we feel very much part of that. When we moved to this “Little place in the Fens” in the 1970s from the civilised metropolis of Manchester. FM reception seemed just as relatively primitive as the local rail service and road system.

Far from the clean stereo Holme Moss signal we’d previously enjoyed, the Cambridge FM service (from the top of a gasometer) was strictly mono. So I’ve always been something of an FM DX enthusiast because the only available stereo signal then was from Wrotham, which had to scramble over the Chiltern Hills. (That of course was a similar problem to the one that led Pye of Cambridge to develop such expertise in fringe area TV reception). Tacolneston was an alternative, but that too was only mono, and its audio bandwidth sounded to be limited to around 7kHz.

Today, everything’s better now that high tech Cambridge has arrived in the 21st century, with fast electric trains, a motorway, and a nearby international airport. We also have a local stereo FM transmitter - on the mountain top in Madingley. But it’s pretty weedy and, here in South Cambridge, it does need a 4-element roof mounted aerial array to get what I’d call noise-free stereo. But then stereo multiplex FM always did seem to me outrageously greedy for RF input volts.

The trouble is that my high gain aerial is just too good at receiving Continental signals to interfere with the local service!

Martin

G6Tanuki 24th Apr 2021 7:43 pm

Re: DX FM reception.
 
I too have noted 'burblies' this afternoon when listening to Janice Long's show on Greatest Hits Radio [107.7MHz, Swindon Tx] on my Roberts R707.

The enhancement hasn't, alas, extended as far as 144MHz.

Synchrodyne 25th Apr 2021 1:24 am

Re: Dx fm
 
Quote:

Originally Posted by Hartley118 (Post 1367426)
Had the band been as crowded back in the 1960s as it is today, I guess that the current stereo FM system might have been rejected as too vulnerable.

An intriguing thought, although back in the 1960s I don’t think that there were any alternative systems for FM stereo that would have been materially better than the chosen Zenith-GE system. Maybe the “problems” could be moved around somewhat, but not eliminated. Other approaches suggested during the 1950s and 1960s included FM and SSB subcarriers, but these voided the interleaving that Zenith-GE offered, meaning that baseband modulation depth was reduced, in turn requiring higher transmitter powers and/or more transmitters to compensate. Whether the Russian polar modulation system, developed for ±50 Hz deviation and upper Band I transmission frequencies, offered any greater resistance to co-channel and adjacent channel interference I don’t know.

I think that basic problem was that the mono FM system, with a deviation ratio of 5 referred to a 15 kHz upper modulation frequency, was quite robust, but that robustness disappeared once subcarriers with upper sideband frequencies above 30 kHz were added, the deviation ratio then dropping towards 2. Not only that, although the subcarrier modulation itself was subject to pre- and de-emphasis, the subcarrier as a whole was not. And part of the robustness of the mono system was its pre- and de-emphasis (originally 100 µs in the USA, dropping to 75 µs in 1945, with 50 µs I think developed in both Russia and by the BBC around 1944-45).

Companding of the subcarrier was a possibility, although that adversely affected interleaving. It was actually tried with the FMX system, but that was a failure. DBX companding was used with the American Zenith-DBX (MTS) TV stereo sound system, but in that case the overall deviation was increased so that the baseband was unaffected, and interleaving was not required. (As an aside, the DBX effects were not completely inaudible.)

From a reception (of the Zenith-GE system) viewpoint, using only the lower sideband of the subcarrier might help, and I recall that that was actually done by one of the Japanese hi-fi makers (although I forget which). Evidently the idea did not catch on, which suggests that the typically realizable improvement did not justify the complication, at least absent a dedicated IC. (The FMX IC, with quadrature demodulation of the subcarrier, might have been a reasonable starting point.).


Cheers,

stuie319 25th Apr 2021 2:14 am

Re: DX FM reception.
 
Hi,

Travelling back from Sheffield to Manchester on Friday afternoon over the Woodhead pass, I discovered Radio 4 was suddenly replaced by a very strong Italian station. In the old days this would have me rushing home to switch on the D100 convertor, or my old Thorn 1400 set to 625 VHF and searching for European or Russian TV signals. Alas this is no longer possible. It hadn't occurred to me to listen on 2 metres. Whilst I do possess a two echo ticket, I've never ever actually used it

Best

Stu

Radio Wrangler 25th Apr 2021 2:26 am

Re: DX FM reception.
 
FM enjoys a signal to noise advantage over AM "The FM advantage", where the occupied bandwidth of the modulated signal is unnecessarily large.

You can view this as the FM modulation process produces multiple pairs of sidebands for each component of the baseband. With wideband modulation from wide deviation, these can be seen as many duplicate transmissions of the baseband.

The FM process puts all these components back together and convolves the baseband back into existence. In this process the noise floor of the occupied bandwidth is also convolved, leading to a stronger amount of noise in the resulting baseband. The difference is that the modulation sidebands correlate with each other as they convolve, and add as voltage, while the noise components, being random, cannot correlate with each other and add as power. So although the noise gets stronger, the baseband gets increased by a greater amount. FM can be viewed as one sort of spread spectrum modulation, complete with a spreading gain.

With stereo done as a difference signal as DSB on a high frequency carrier added to the baseband, the difference signal is shifted high in the baseband spectrum. Even with the relatively small increase in channel bandwidth, the difference information gets a lower modulation index, fewer almost-duplicate FM sideband components to add together in the demod process. So the difference signal components suffer lack the same S/N ratio benefits the lower frequency baseband (sum signal) components get. This is why stereo signals, when the stereo decoder comes on-line have significantly worse S/N than you'd expect from what there was in mono. To get back to what you would have had, you need to collect a stronger signal so coverage areas shrink, or more aluminium needs to go up in the sky.

The zenith pilot-tone stereo system inevitably had to involve a compromise, and the compromise's result in terms of a fast drop in audio S/N as the decoder switches in is clearly obvious, but the mechanism for producing that loss of S/N is not obvious to people not versed in modulation theory. Most people see it as something which just 'IS', and wonder if it can be bettered, without realising that it's mathematically inevitable. To counter it we would need either more transmitter power or a lot more bandwidth. The old 'no such thing as a free lunch' business.

Of course, some skilled and dedicated firms did manage to make tuners/decoders which were very noticeably worse than theoretical....

David

SteveCG 25th Apr 2021 5:12 pm

Re: DX FM reception.
 
... not forgetting Ray Dolby's 1970's foray into Stereo FM S/N improvement using Dolby 'B', albeit not quite mono receiver compatible ...

P.S. Hartley118 - you could have added Peterborough to your list of mono transmitters back then.

bikerhifinut 25th Apr 2021 10:09 pm

Re: DX FM reception.
 
Quote:

Originally Posted by stuie319 (Post 1367709)
Hi,

Travelling back from Sheffield to Manchester on Friday afternoon over the Woodhead pass, I discovered Radio 4 was suddenly replaced by a very strong Italian station. In the old days this would have me rushing home to switch on the D100 convertor, or my old Thorn 1400 set to 625 VHF and searching for European or Russian TV signals. Alas this is no longer possible. It hadn't occurred to me to listen on 2 metres. Whilst I do possess a two echo ticket, I've never ever actually used it

Best

Stu

This morning we had our regular eden valley "lockdown" net on 2m using theGB3EV repeater and we had some pretty good propagation with one of our members working portable with a rucksack mounted set and another on an electric mountain pushbike. Both were readable in locations we'd expect difficulties in the lakes.
Myself I was having issues from a home made dipole but once a quick and dirty fix was made with 6 foot of bamboo beanpole to get the vertical dipole away from the house structure both reception and my maximum 10W emission were going 5 9 to most in the net.

So perhaps you might have some fun with a £25 baofeng handheld and a tenner or less spent on an alternative whip antenna as afetr todays net I scrounged a n aftermarket whip antenna that turned my toy baofeng into a usable device.

The amatuer bands right now are in a state of revival due mainly to the restrictions i guess but if it carries some momentum then who knows.

73

Andy M7ELN (soon to be 2E0*** I hope, and beyond if i learn enough)

SiriusHardware 26th Apr 2021 8:36 am

Re: DX FM reception.
 
Quote:

my maximum 10W emission were going 5 9 to most in the net
A minor clarification Andy - when you are working through a repeater, the signal strength reading the other users see is the strength of the repeater's transmitter, not yours, so any signal strength figures offered to you when you are working through a repeater will be a little bit misleading.

However, it sounds as though your aerial upgrade allowed you to hit the repeater input with a signal sufficiently strong as to be noise-free so it worked, and that kind of experimentation is what the hobby is all about.

Hartley118 26th Apr 2021 9:58 am

Re: DX FM reception.
 
Quote:

Originally Posted by SteveCG (Post 1367878)
... not forgetting Ray Dolby's 1970's foray into Stereo FM S/N improvement using Dolby 'B', albeit not quite mono receiver compatible ...

P.S. Hartley118 - you could have added Peterborough to your list of mono transmitters back then.

Clearly I wasn’t alone! Ray Dolby also lived in Cambridge and wanted to rectify the weaknesses of FM multiplex stereo.

As I remember it, the Peterborough transmitter in the 1970s was a rather low power relay of the Tacolneston service which was of poor quality anyway.

Today, Peterborough, which was the last transmitter to go stereo (in 1985), gives a reasonable signal here and is a useful standby when the Cambridge reception is marred by birdies. I sometimes wonder whether the birdies are actually retransmitted by the Cambridge booster as an integral part of its service.

Martin

SteveCG 26th Apr 2021 10:50 am

Re: DX FM reception.
 
Receiving satisfactory Stereo FM in the 70's occupied many an academic mind in your neck of the woods. There was also Capitol Radio to be had - provided you were a DXer!

Hartley118 26th Apr 2021 11:22 am

Re: DX FM reception.
 
Quote:

Originally Posted by SteveCG (Post 1368056)
Receiving satisfactory Stereo FM in the 70's occupied many an academic mind in your neck of the woods. There was also Capitol Radio to be had - provided you were a DXer!

Yes, we had a massive J-Beam array on the Neve building in Melbourn near Cambridge purely for the purpose of receiving Capital Radio for the factory PA. Mono only of course.

Martin

bikerhifinut 26th Apr 2021 8:25 pm

Re: DX FM reception.
 
Quote:

Originally Posted by SiriusHardware (Post 1368022)
Quote:

my maximum 10W emission were going 5 9 to most in the net
A minor clarification Andy - when you are working through a repeater, the signal strength reading the other users see is the strength of the repeater's transmitter, not yours, so any signal strength figures offered to you when you are working through a repeater will be a little bit misleading.

However, it sounds as though your aerial upgrade allowed you to hit the repeater input with a signal sufficiently strong as to be noise-free so it worked, and that kind of experimentation is what the hobby is all about.

You are quite correct and I stand corrected on my statement!
What I should have said was that one of our members makes a point of listening in on the relays receive frequency 600kHz downband and then passes on signal reports from his QTH which is about 15 miles away but over a quite steep hill from us so when I got those reports I knew I was making progress!
Andy.

SteveCG 27th Apr 2021 9:38 am

Re: DX FM reception.
 
Given the time of year, I'll be listening out more for SpE propagation - principally in the 87.5 to 87.9 Mc/s part of the spectrum which is the only segment that is normally clear for me.

BTW I wonder whether Dolby B applied only to the stereo sub-carrier component might have assisted fringe area reception?

SiriusHardware 27th Apr 2021 9:45 am

Re: DX FM reception.
 
I keep an occasional eye on 10 / 11 metres which is normally a good early warning band for Sporadic-E, then if it's very lively there I look at 6m and 4m. SP-E Activity on 10/11 normally starts to be a more or less daily occurrence from mid-May onwards and continues until late summer.

Synchrodyne 28th Apr 2021 3:12 am

Re: DX FM reception.
 
2 Attachment(s)
Quote:

Originally Posted by SteveCG (Post 1368429)
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 (*):

Attachment 232814


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

Attachment 232815


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.


Cheers,

Synchrodyne 28th Apr 2021 4:28 am

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.

SteveCG 28th Apr 2021 9:15 am

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!

colly0410 30th Apr 2021 6:32 pm

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?

G6Tanuki 30th Apr 2021 7:06 pm

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.

Radio Wrangler 30th Apr 2021 8:26 pm

Re: DX FM reception.
 
Quote:

Originally Posted by colly0410 (Post 1369701)
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.

David

Synchrodyne 1st May 2021 12:18 am

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 (Post 1369718)
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 (Post 1369718)
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.


Cheers,

Radio Wrangler 1st May 2021 6:14 am

Re: DX FM reception.
 
Quote:

Originally Posted by Synchrodyne (Post 1369777)
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.

David

Synchrodyne 1st May 2021 7:29 am

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).


Cheers,

Synchrodyne 2nd May 2021 12:28 am

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.


Cheers,

Synchrodyne 2nd May 2021 11:04 pm

Re: DX FM reception.
 
1 Attachment(s)
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.

Attachment 233105

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.


Cheers,

SteveCG 30th May 2021 6:52 am

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.

SteveCG 31st May 2021 5:03 pm

Re: DX FM reception.
 
... 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.

Jolly 7 1st Jun 2021 8:55 am

Re: DX FM reception.
 
Quote:

Originally Posted by SteveCG (Post 1379274)
... 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 ?

eddie_ce 1st Jun 2021 9:31 am

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.

SteveCG 1st Jun 2021 10:37 am

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.

Jolly 7 1st Jun 2021 10:47 am

Re: DX FM reception.
 
Quote:

Originally Posted by SteveCG (Post 1379422)
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.

SteveCG 1st Jun 2021 10:55 am

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!

Hybrid tellies 2nd Jun 2021 10:03 am

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.

SteveCG 3rd Jun 2021 5:41 pm

Re: DX FM reception.
 
... More SpE reception this afternoon ...

SteveCG 10th Jun 2021 5:45 pm

Re: DX FM reception.
 
... yet more SpE reception this afternoon: Spanish, French & Arabic voices heard, again in the 87.5 to 87.9 Mc/s part of the spectrum.

SiriusHardware 10th Jun 2021 8:44 pm

Re: DX FM reception.
 
I'm doing a moderately long car trip this weekend - may be time to dust off my rarely used 4m radio if the nearby broadcast band has been 'up'.

bikerhifinut 10th Jun 2021 10:33 pm

Re: DX FM reception.
 
I worked a Corsican station on 50.180 MHz today at teatime, SSB with 50W at my end into a vertical triband aerial of indeterminate vintage. 5-8 in both directions. I could hear stations from all over Europe.
There were dozens on today but I reckon there was a contest going on too.
10m was also lively and was still buzzing at 2100 GMT.
Andy.

SiriusHardware 10th Jun 2021 11:35 pm

Re: DX FM reception.
 
6 is one band I've never had because, historically, my first venture onto any given amateur band was always using something which could be modified to go there - usually ex-PMR in the case of 4m / 2m / 70cms and ex-CB in the case of 10 - there was never much around that would easily go onto 6 FM, and I think the majority of serious '6' operators probably use SSB anyway.

At one time I meant to look out for a second hand Kenwood TS-60 or an FT-690R but it's incredible how well the old radios have held their prices with Yaesu FT290 / 690 / 790 'Handbags' typically still going for hundreds of pounds. I can't justify that for a single band radio.

10/11 was certainly buzzing at teatime, I just hope this wave of good SP-E lasts at least through the weekend, for our broadcast DX listening colleagues as well as we comms folk.

SteveCG 11th Jun 2021 4:03 pm

Re: DX FM reception.
 
... and again this afternoon, FM, DX, SpE - Spanish, French, and Arabic in the 87.5 - 87.9 Mc/s part of Band II.

SteveCG 16th Jun 2021 7:27 am

Re: DX FM reception.
 
No SpE for me since the 11th of June. However this morning Tropo lift bringing in distant UK FM stations and NDR 2 (Germany) on 87.6 Mc/s - confirmed by listening to their web-broadcast.

SteveCG 16th Jun 2021 8:11 am

Re: DX FM reception.
 
... spoke too soon about SpE. SpE now - Italian on 87.5 Mc/s and French on 87.6 Mc/s - and as I type, lots more crowding in ...

Hybrid tellies 16th Jun 2021 9:41 am

Re: DX FM reception.
 
A noticed a small tropo lift at around 1am this morning with two weak French stations on 87.6 and 88.9. Nothing else around which was rather strange.
I checked on the car radio this morning at around 8.15am just before I cam into work and there was no evidence of any tropo or SpE.

SteveCG 16th Jun 2021 11:04 am

Re: DX FM reception.
 
I've been listening all morning and the SpE has been continuous - likewise the Tropo (which is slowly fading away now). Quite a Red letter day. Its a bit like waiting for busses - nothing for a long time then they all come along at once.

SteveCG 16th Jun 2021 4:49 pm

Re: DX FM reception.
 
The 87.5 - 87.9 Mc/s SpE faded away at about 12:30 PM BST. Essentially it had been Italian, Spanish and French languages, with a brief appearance of Arabic.


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