USSR FM Broadcast sets 66-74mhz info required

[glasstransistor]

Hi All,
Sorry if this post is in the wrong place or if it sounds too much trouble, but just to ask if anyone has an imported (to the UK) radio that has the VHF band as per the USSR standard of 66-74mhz? It is that I am very close to adding a Stereo Unitra set to my collection as they seem to be quite inexpensive and common online for sale due to Poland no longer using the old ussr bandwidths. I am finding it hard to find a low power stereo FM transmitter that goes below 76mhz (for Japan) without it seems using an arduino controlled transmitter module (I have found one that drops to 70mhz) but then I am unsure about doing this more complex/pricey method of making the set able to work, and would prefer a tried and tested decent audio quality FM TX but possibly modified to cover the lower ex USSR frequencies not 88-108mhz? And also if the demodulation was the same as the rest of the world FM? I have already export sets from that part of the world (spidola/vef/vega/selena/astrad) but not the former PRL yet so I am really tempted. Any ideas or advice would be most welcome, thanks for your time.
Kind Regards,
Neil.

[paulsherwin]

This is known as the OIRT band. If you search the forum, you will find several discussions on the subject.

Unitra radios were made in Poland, and were exported to the UK with standard FM coverage.

Simple FM pantry transmitters aren't difficult to build if you don't need stereo. You certainly don't need a microprocessor. Just build one of the standard designs from the web but adjust the frequency down.

[winston_1]

I read somewhere that the OIRT FM band used a 23KHz stereo subcarrier instead of 19. Maybe not right of course.

[orbanp1]

Hi Neil,

The beauty, or ease of re-tuning Unitra radios from the OIRT to the CCIR band is that in the FM tuner sections only the parallel fixed capacitors to the tuning capacitor need to be removed, the coils do not need to be modified.
That is true for the Beta (DSS-201) and the Mercury (DSH-302/303) radios, from personal experience.
Removing those caps increases the spread of the tuning as well as moves them to higher frequencies.
After that with the coils and trimmer caps the tuner could simply be adjusted to the CCIR band.
The Mercury radio also had some other issues (faults) in the FM-IF section that needed to be repaired.

I also modified a Selena B-211 radio to the CCIR band, there both the oscillator and antenna coils needed to be modified/rewound.

Regards, Peter

[Mr 1936]

Hi

From what I can glean, the OIRT system used double sideband reduced carrier (DSB-RC) modulation for the L-R "stereo difference" signal. The subcarrier frequency was 31.25 kHz rather than the "usual" 38 kHz. No pilot tone was used, instead the subcarrier wasn't completely suppressed but was transmitted at a level 14 dB down. Presumably the decoder could use a narrowband PLL to regenerate the subcarrier and use it to drive a synchronous detector.

Maybe 31.25 kHz was chosen because it is double the 15.625 kHz line rate of 625 line TV and it avoided some sort of interference ?

I have no information on maximum FM deviation or pre-emphasis, but suspect they were the same as for the CCIR system.

[colourking]

I'm assuming these sets usually have the FM local oscillator at +10.7 MHz.

if so you may find that 'useable' images are received of strong FM stations 21.4 higher than the tuned frequency.
(i.e. ones between 87.5 up to 95.4 or thereabouts ) especially if hooked up to a outdoor aerial.
(mono only of course)

this depends on the set's front-end RF selectivity, the better it is, the less these images will be, if at all.

With a mini-transmitter for the usual band, placing it right next to the set's aerial it may be receivable clearly on an image frequency.


(of course its preferable to try modifying the radio for the usual band, as per orbanp1, if you feel competent to do it)

[glasstransistor]

Thanks so much for the replies so far, all really helpful and it's now on for my mate to bid and arrange getting the set over to me in England from Poland. I'm happy with either modification of an existing tx or the reciever itself. The reason I mentioned stereo would be nice, is because the set I am looking to buy is optionally stereo but I will put on pics of the set from the auction to give an idea of this, the add on stereo unit is the receiver case repurposed and just like the look of the pair together also was curious if FM stereo was broadcast over the oirc bands or if the stereo or if the stereo extension was made just for aux in? Thanks again.
Kind Regards,
Neil.

[Paul_RK]

Originally Posted by glasstransistor

...also was curious if FM stereo was broadcast over the oirc bands or if the stereo or if the stereo extension was made just for aux in?

I'm just about sure stereo broadcasts were made - the Leningrad line of high-end world band portables came to include models to receive them,

https://www.radiomuseum.org/r/lenin_...10_stereo.html
https://www.radiomuseum.org/r/lenin_...15_stereo.html

They're just the examples I happen to be aware of through an interest in that end of things, no doubt other stereo receivers will have been more widely known than these!

Paul

[Synchrodyne]

Originally Posted by Mr 1936

Hi

From what I can glean, the OIRT system used double sideband reduced carrier (DSB-RC) modulation for the L-R "stereo difference" signal. The subcarrier frequency was 31.25 kHz rather than the "usual" 38 kHz. No pilot tone was used, instead the subcarrier wasn't completely suppressed but was transmitted at a level 14 dB down. Presumably the decoder could use a narrowband PLL to regenerate the subcarrier and use it to drive a synchronous detector.

Maybe 31.25 kHz was chosen because it is double the 15.625 kHz line rate of 625 line TV and it avoided some sort of interference ?

I have no information on maximum FM deviation or pre-emphasis, but suspect they were the same as for the CCIR system.

Indeed so. This was the so-called “Polar modulation” system, dating from c.1959. The subcarrier frequency was determined with possible TV sound applications in mind, although as far as I know, never so used. Nor should I expect that it would have been all that satisfactory without serious companding on the subcarrier.

Deviation was ±50 kHz and pre-emphasis was 50 µs. These were the same as adopted for Russian TV sound, possibly derived around 1944. That was also around the time that the BBC developed 50 µs as the “right” number. At the time, the American number was 100 µs, dropped to 75 µs in mid-1945.

One aspect of polar modulation was that it made for quite simple decoding, as shown in this item from “Electronics World’ 1966 May.

EW 196605 p.66 Soviet Polar Modulation Stereo.pdf


There is some indication, although unconfirmed, that the original standard IF for Russian FM (66 to 73 MHz) receivers was 8.4 MHz. But imagine that even if that were the case, there would have been a drift to the 1945 RMA number of 10.7 MHz once exporting became important.

I’d guess that the Russian industry would have developed polar system decoding ICs, but I have never seen anything to that effect. In the West the timeline was 1969 for the non-PLL type (MC1304/5) and 1971 for the PLL type (CA3090 and MC1310).

Some more detailed information on the Polar modulation system is provided in these CCIR documents:


CCIR 1966 Oslo Rep. 300-1 Stereo.pdf

Cheers,

[paulsherwin]

The Soviets developed silicon technology relatively late - they only launched mass market silicon planar technology in the early 70s, 15 years after the Americans and a decade after the Europeans. Even then their silicon transistors had rather low specs.

[German Dalek]

Hi,
My collection cover a Polish Stereo Tuner and an old Russian tube radio from the 50's with their FM band, too.

If you want a solution to use the old FM-band try this:

Find an low price Nordmende Stereo coder like this one:
https://www.radiomuseum.org/r/nordme...3841sc384.html

Discuss here the changes which have to be done to let it work on the old eastern Stereo standard.

Good luck!

German Dalek

[marceljack]

Originally Posted by Synchrodyne

There is some indication, although unconfirmed, that the original standard IF for Russian FM (66 to 73 MHz) receivers was 8.4 MHz.

Given the identical characteristics of their FM TV and radio signals (deviation and preemphasis), it's surprising that they did not use an IF value of 6.5 MHz for radio as it is the value of the intercarrier in the OIRT TV system. Given the small width of their FM band, this low value would not have given significant image rejection problems.
At the beginning of FM in Europe with a frequency band from 87.5 to 100 MHz, some receivers used an IF value of 6.75 MHz.

[Synchrodyne]

Quite possibly 6.5 MHz was used as an FM Rx IF in Russia and the other OIRT countries.

Interestingly though, the single datapoint that I have found in respect of 8.4 MHz comes from a Wireless World 1961 August article on Russian Radio, TV, and audio equipment. To quote the pertinent part, said in respect of TV receivers:

“…and some employ ingenious means for f.m. broadcast reception: for instance, the "Rubin 104" and "Almaz 105 " have a second frequency changer to convert the 8.4 Mc/s f.m i.f. to the 6.5 Mc/s sound i.f. so that the set's intercarrier circuits can be used.”

One – but not the only - interpretation of this is that these TV-FM receivers employed fairly standard Russian front ends that delivered an 8.4 MHZ IF.

One possible rationale for using 8.4 MHz might have been, assuming supradyne conversion, to keep the local oscillator above 75.2 MHz. 75.0 MHz was a protected aviation beacon frequency with a guard band of ± 0.2 MHz. The 8.4 MHz IF would imply that the lowest usable OIRT FM transmitter frequency was 66.8 MHz; whether that was actually the case I don’t know. Also, with 6.5 MHz for both TV and FM receivers, there might have been mutual interference with two receivers in the same area, perhaps more so from TV to radio given that intercarrier radiation from valve era receivers could be quite strong.

Be that as it may, the issue here seems to be less that of modifying receivers, but finding a way to feed appropriate signals to unmodified OIRT band receivers, both mono and stereo. As already said, modification of an existing FM mini-transmitter to cover the 66 to 73 MHz band would be the easier part. But a coder that provided a Polar modulation stereo feed would be more difficult, although an existing pilot tone encoder circuit might be adaptable. On its face, that would require changing the subcarrier frequency to 31.25 kHz, adding back the requisite amount of 31.25 kHz carrier to the subcarrier mix, then getting the right injection level for that mix, and blocking the pilot to feed to the output. Getting the right subcarrier delay relative to baseband might be an issue. But there does seem to be some chance that someone, somewhere has already done this.


Cheers,

[Julesomega]

Technics and Sony made tuners for the OIRT countries. My Technics tuner is single-band FM (and AM) and tunes in 10kHz steps with a high-side LO and 10.7MHz IF. There is only a hint of an image from Holme Moss! Being a late entry to this market it has the familiar Zenith multiplex stereo with the European 50kHz pre-empahsis. You can start by making a transmitter with a free-running oscillator, and look for a stereo encoder circuit when you have it working in mono.

[colourking]

Originally Posted by orbanp1

Hi Neil,

The beauty, or ease of re-tuning Unitra radios from the OIRT to the CCIR band is that in the FM tuner sections only the parallel fixed capacitors to the tuning capacitor need to be removed, the coils do not need to be modified.
That is true for the Beta (DSS-201) and the Mercury (DSH-302/303) radios, from personal experience.
Removing those caps increases the spread of the tuning as well as moves them to higher frequencies.
After that with the coils and trimmer caps the tuner could simply be adjusted to the CCIR band.
The Mercury radio also had some other issues (faults) in the FM-IF section that needed to be repaired.

I also modified a Selena B-211 radio to the CCIR band, there both the oscillator and antenna coils needed to be modified/rewound.

Regards, Peter

Presumably some distortion is present on the modified CCIR reception, since reading above that the OIRT band uses less deviation (50khz Versus 75 kHz) ?
this would vary from set to set (and also station to station), depending on how tight the IF selectivity is, and maybe not much of an issue usually ?

Originally Posted by Julesomega

You can start by making a transmitter with a free-running oscillator, and look for a stereo encoder circuit when you have it working in mono.

At least some of these sets are likely easily re-align-able to go up to 76MHz without any component change.
That would allow the use of cheap modulators/mini-transmitters for the Japanese/Brazil markets which should be easy to get online and more stable than a typical homebuilt one.
Doesn't solve providing Polar stereo though, and an internal stereo encoder of a Japanese mini-transmitter would of course have to be bypass-able if attempting to build an encoder for Polar Stereo .

[glasstransistor]

Hi All,
Thanks for all of the info and advice, the radio and it's stereo extension are all paid for and on the way via my mates house from Polska. Super interesting learning the theory of FM Broadcast as before my limit was years back reading of Armstrong's format possibly in a BVWS journal. I have for the meanwhile invested in a tx from China that goes down to the oirt frequencies. It seems however that software into an fm mpx transmitter seems to be the way to go in regards to polar stereo, I found a program called 'StereoTools' and off the back of their website's faq/help forum they had via a request from someone in the former USSR wanting to broadcast stereo in the OIRT polar stereo method, and the developers wrote it in to their software this seems to be licensed for private hobbyists and up to commercial radio stations with ta/rds, but the oirt stereo demo using this software on youtube looks like someone with a USSR hifi separate tuner illuminating the stereo 'Beacon' and displaying the stereo waveform. Thanks again for all the info and replies may 'retune' it eventually up to 108mhz via recapping the fm module and an alignment as pointed out earlier in the thread as I have found a few polish articles on doing this for my exact radio/fm module, now just hoping it turns up intact via courier!
Kind Regards,
Neil.

[webber77777]

Originally Posted by Synchrodyne

Hi

Attachment 268643


There is some indication, although unconfirmed, that the original standard IF for Russian FM (66 to 73 MHz) receivers was 8.4 MHz. But imagine that even if that were the case, there would have been a drift to the 1945 RMA number of 10.7 MHz once exporting became important.

I’d guess that the Russian industry would have developed polar system decoding ICs, but I have never seen anything to that effect. In the West the timeline was 1969 for the non-PLL type (MC1304/5) and 1971 for the PLL type (CA3090 and MC1310).

Some more detailed information on the Polar modulation system is provided in these CCIR documents:


Attachment 268645

Cheers,

The intermediate frequency was also 8.4 MHz, then 6.5 was used, 6.8 rarely, and 10.7.
Chips of offhand stereo decoders K174XA14 and XA51, these are analogues of TDA4500 and TDA1591 /

[Synchrodyne]

Thanks for those IF numbers. It looks then as if 8.4 MHz was the original norm, 6.5 MHz was taken from the TV system D/K intercarrier frequency, and that then there was a move to the international 10.7 MHz norm. FM IFs in the 6 MHz range were also found in a minority of Western European FM equipments, in some cases perhaps because of gain considerations with early transistors. The 6.8 MHz number would just about ensure that local oscillator frequencies fell outside of the OIRT band. And it does appear that standard PLL-type decoder ICs developed for the Zenith-GE stereo system were readily adapted to work with the polar system.


Cheers,