Hybrid FM multiplex decoder?

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#21
jackinnj said:
Get yourself one of these on EBay:

Fit all the digital goodies into an early 1960's box -- (you should see my digital attenuator in my PAS preamp)

An externally hosted image should be here but it was not working when we last tested it.
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EC8010 said:
Um. What is it? (Although I recognise a KEF "Constructor Series" box in the background.)

Actually, I hoped you might respond, but with some slightly more technical thoughts.
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Dark picture but I can recognize a CROSBY MX-101 tube multiplex decoder,one of the first to appear on the market after the announcement of an approved FM stereo system by the FCC in 1961. The Crosby circuit was amongst the competing circuits but they lost the game in favour of the G.E/ZENITH system judged superior by the FCC committee. Certainly a historical piece but a less than stellar performer,soon to be replaced by the (better) CROSBY SA-40. There are many better vintage MPX decoders,the SCOTT 335 and FISHER MPX-100 being technically and sonically far superior.
 
#22
EC8010 said:
Thanks for the offer, but it seems that VHF broadcasting is under the axe in the UK (in favour of the "crystal clear digital quality" of DAB), so it doesn't look as though there's much incentive for developing a decoder.
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Yes,it's probably too late for developing a high-quality MPX decoder as the end of the analog broadcasting is written on the wall in Europe. Sooner or later (but probably sooner) all radio broadcasters will be forced to close their analog transmitters and there will be no more FM stereo multiplex broadcastings ever... DAB (Death of Analogue Broadcasting) will rule. No deadline has been decided yet but it looks that 2012/2018 will see the end of an era in the Radio broadcasting history in Europe. Sad news,as this will put a definitive end to quality radio broadcasting , as well as to your very interesting project...
 
#23
EC8010 said:
I have a Leak Troughline III with its own (horrible) decoder so I've been wondering about a better decoder...

Some background. To ensure reverse compatibility with mono receivers, the stereo difference signal (L-R) is transmitted using double sideband suppressed carrier modulation on a 38kHz subcarrier. DSBSC requires the original carrier to be regenerated for demodulation, so a 19kHz pilot tone is transmitted between the 30Hz-15kHz (L+R) audio and the DSBSC (L-R) signal. The usual way to demodulate DSBSC is to apply the regenerated carrier and the DSBSC modulated signal to a four-quadrant multiplier and low-pass filter the output to leave just the original modulating signal. Regenerating the carrier correctly is crucial, hence the preponderance of phase-locked loops.

I believe that there's another way of doing the job. One can apply the entire multiplex signal to a switch driven by regenerated carrier. One output of the switch is L and the other R. Obviously, the output needs some serious low-pass filtering.

What I'm thinking of is a CMOS logic/valve hybrid approach. I had some 4.864MHz crystals made (cheaper than you'd think) so that the crystal oscillator can be divided down by a CMOS 28 synchronous divider to give a 19kHz square wave that can be compared with the 19kHz pilot using an EXOR gate and an RC integrator driving a varicap diode across the crystal. That's the phase-locked loop oscillator.

A 38kHz square wave can be picked off the divider and phase split by an invertor to give a differential output and amplified by a 7N7 differential pair to produce a rather large 38kHz differential square wave.

The switch will be a 6AR8 sheet beam valve. Essentially, this is a pentode with two anodes and beam deflection plates. The rather large 38kHz square wave from the 7N7 will drive the beam deflection plates. The stereo multiplex signal will be applied to the control grid and the demodulated stereo will appear at the two anodes.

Now, I know (from looking at the tuner thread) that there are people out there who know more about receiver design than I do, so are there any pitfalls I ought to consider?
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Very interesting project but unfortunately it comes way too late. (see my other reply in this post)
Actually, I had exactly the same idea about 15 years ago and started to work on a hybrid tube MPX decoder. The (very clever) JLH/ETI "Low distorsion stereo decoder" (Feb 1987) was my source of inspiration but I wanted to eliminate all SS devices from the audio signal path. I built the JLH decoder but only used the 38 KHz biphase switching signals (amplified) to drive a ... 6AR8 beam switching tube ! The 6AR8 is perfect for this application and was actually used in early tube MPX decoders like the ALTEC 359A , KNIGHT KN-MX and ZENITH in the first MPX field tests in 1960. Relatively few manufacturers have chosen this method,probably because the 6AR8 tube was more expensive than two (or even four) diodes. Nevertheless,excellent L/R recovery is possible with this circuit,provided the two halves of the tube are balanced. Actually,you're doing "time-division" demultiplexing (like the highly regarded and excellent SCOTT 335) but with PLL stability. (which the SCOTT can't provide). This association worked quite well ,though I never was able to exceed a 22 dB L/R separation.(I used a slightly modified ALTEC circuit) Filtering the 38 KHz switching transients is another challenge but not insurmountable with today's CAD circuit simulation tools. The deemphasis network will already take care of most of the 38 KHz switching residues. Input filtering and phase adjustement is needed too but I remember JLH's ETI (July 1988) input filtering update didn't worked well in my application. A circuit I stole from a HK tube decoder gave much better results,still without any coils.
To keep tube count down I used two compactrons plus the 6AR8, for a total of 3 tubes. As there was no market nor interest for a tube MPX decoder then (and now!) I put this project aside and never came back to it. You just revived some memories of the past... I'm still convinced that with some time and efforts an excellent MPX decoder could be developped from this concept. Unfortunately such a project no longer has any meaning today as the ending of analog FM broadcasting is already written on the wall in Europe.
 
#24
forr said:
Hi EC8010,
I may say that I built the JLH tuner from a Hart kit around 1985 and was very disappointed by the stereo separation and the lack of treble. I spent hours trying to fix it, I've a Philips FM multiplex generator to help me, but I failed to detect what was wrong.
I may try to connect the PLL demodulator output which is very quiet to a more standard stereo decoder but I currently live very happy with a vintage lovely little Sansui TU555 which I had carfeully aligned and which is a clear winner for my ears in a comparison involving twelve tuners.
However this does not mean at all that there is something wrong with the sample and hold approach and would be glad to know the results you will arrive at, hoping they will be better than mine.
Click to expand...

For your information, JLH subsequently published an update for his MPX decoder in the July 1988 issue of ETI, pompously titled: "Top Notch Improvements" ,basically a steep cut input parallel T bootstrapped notch filter. I suspect this mod has not been implemented in your kit.
Input composite phase (separation) adjustment is very critical too. Basically a good concept, JLH's circuit could certainly be improved. (output amp's and filtering are rather crude and load dependant)
 
#25
Hello Tubologic, it's nice to hear that my idea might have worked even if it's unlikely to be carried to completion. I'll have to find another use for the 6AR8. No point in making a PAL colour decoder because that's already been shut down where I am.
 
#27
Tubologic said:
Very interesting project but unfortunately it comes way too late. (see my other reply in this post)
Actually, I had exactly the same idea about 15 years ago and started to work on a hybrid tube MPX decoder. The (very clever) JLH/ETI "Low distorsion stereo decoder" (Feb 1987) was my source of inspiration but I wanted to eliminate all SS devices from the audio signal path. I built the JLH decoder but only used the 38 KHz biphase switching signals (amplified) to drive a ... 6AR8 beam switching tube ! The 6AR8 is perfect for this application and was actually used in early tube MPX decoders like the ALTEC 359A , KNIGHT KN-MX and ZENITH in the first MPX field tests in 1960. Relatively few manufacturers have chosen this method,probably because the 6AR8 tube was more expensive than two (or even four) diodes. Nevertheless,excellent L/R recovery is possible with this circuit,provided the two halves of the tube are balanced. Actually,you're doing "time-division" demultiplexing (like the highly regarded and excellent SCOTT 335) but with PLL stability. (which the SCOTT can't provide). This association worked quite well ,though I never was able to exceed a 22 dB L/R separation.(I used a slightly modified ALTEC circuit) Filtering the 38 KHz switching transients is another challenge but not insurmountable with today's CAD circuit simulation tools. The deemphasis network will already take care of most of the 38 KHz switching residues. Input filtering and phase adjustement is needed too but I remember JLH's ETI (July 1988) input filtering update didn't worked well in my application. A circuit I stole from a HK tube decoder gave much better results,still without any coils.
To keep tube count down I used two compactrons plus the 6AR8, for a total of 3 tubes. As there was no market nor interest for a tube MPX decoder then (and now!) I put this project aside and never came back to it. You just revived some memories of the past... I'm still convinced that with some time and efforts an excellent MPX decoder could be developped from this concept. Unfortunately such a project no longer has any meaning today as the ending of analog FM broadcasting is already written on the wall in Europe.
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You're bringing back memories....

I remember Warren Lane from Australia used the same approach but I have, alas, misplaced the circuit.
He used a couple of digital chips and a comparator (can't remember which) to build the PLL driving a 6360.
He also made a simplified design (I can't remember what the simplification was).

Stelios
 
#28
Stelios, you must be in line for a prize for reviving the longest dormant thread! I still have the tuner and the crystals, but storm Eunice broke the roof aerial. And concerted political efforts have broken the BBC and R3. Which rather finishes off any chances of my actually getting anything made. But at least all the parts are still available!
 
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#30
EC8010 said:
Stelios, you must be in line for a prize for reviving the longest dormant thread! I still have the tuner and the crystals, but storm Eunice broke the roof aerial. And concerted political efforts have broken the BBC and R3. Which rather finishes off any chances of my actually getting anything made. But at least all the parts are still available!
Click to expand...
There's still hope! Remember: its only a Europe thing. And they have predicted the death of FM more than a decade ago before but it's still here. Is R3 actually no more ? Nooohh...
 
#33
Tubologic said:
Very interesting project but unfortunately it comes way too late. (see my other reply in this post)
Actually, I had exactly the same idea about 15 years ago and started to work on a hybrid tube MPX decoder. The (very clever) JLH/ETI "Low distorsion stereo decoder" (Feb 1987) was my source of inspiration but I wanted to eliminate all SS devices from the audio signal path. I built the JLH decoder but only used the 38 KHz biphase switching signals (amplified) to drive a ... 6AR8 beam switching tube ! The 6AR8 is perfect for this application and was actually used in early tube MPX decoders like the ALTEC 359A , KNIGHT KN-MX and ZENITH in the first MPX field tests in 1960. Relatively few manufacturers have chosen this method,probably because the 6AR8 tube was more expensive than two (or even four) diodes. Nevertheless,excellent L/R recovery is possible with this circuit,provided the two halves of the tube are balanced. Actually,you're doing "time-division" demultiplexing (like the highly regarded and excellent SCOTT 335) but with PLL stability. (which the SCOTT can't provide). This association worked quite well ,though I never was able to exceed a 22 dB L/R separation.(I used a slightly modified ALTEC circuit) Filtering the 38 KHz switching transients is another challenge but not insurmountable with today's CAD circuit simulation tools. The deemphasis network will already take care of most of the 38 KHz switching residues. Input filtering and phase adjustement is needed too but I remember JLH's ETI (July 1988) input filtering update didn't worked well in my application. A circuit I stole from a HK tube decoder gave much better results,still without any coils.
To keep tube count down I used two compactrons plus the 6AR8, for a total of 3 tubes. As there was no market nor interest for a tube MPX decoder then (and now!) I put this project aside and never came back to it. You just revived some memories of the past... I'm still convinced that with some time and efforts an excellent MPX decoder could be developped from this concept. Unfortunately such a project no longer has any meaning today as the ending of analog FM broadcasting is already written on the wall in Europe.
Click to expand...
There's another thread that was opened (I'm trying to get that one resurrected too!) about this.

https://www.diyaudio.com/community/threads/build-a-fm-stereo-decoder-using-chip-and-tube.348203/

It uses a very similar approach to the one used by Warren Lane. This was (if I recall correctly) shared with me via the (closed) Yahoo Gropus H.J.Leak group. It has now been ported over to ioGroups. There's no relevant information about this circuit there. If I can find anything in my basement/files/old pc I will try to share it but don't hold your breath (it was more than 20 years ago).
Btw, the other Aussie's, Patrick Turner's MPX decoder is found here:

https://www.turneraudio.com.au/am-fm-radio-tuner-multiplex-decoder.html

The previously posted URL had an error.
Unfortunately Patrick Turner passed away not too long ago. The last update on the site concerning his health status was back in 2021.
Here's another site with Fisher MPX decoders I found:

https://ronsradios.com/2021/09/06/fishers-early-fm-multiplex-decoders/

It's an interesting read.

Stelios
 
#34
EC8010 said:
Stelios, you must be in line for a prize for reviving the longest dormant thread! I still have the tuner and the crystals, but storm Eunice broke the roof aerial. And concerted political efforts have broken the BBC and R3. Which rather finishes off any chances of my actually getting anything made. But at least all the parts are still available!
Click to expand...
I'm not sure if you're still interested/have recovered the TL from the attic (what a sad fate for a TL! totally undeserved!) but in case you or anyone else is interested, I have happened across this interesting kit from Denmark:

http://electronics.dantimax.dk/Kits/Tuner_kits/12641272062.html

It looks well made and the price is perfectly reasonable (dare I say cheap?) at 60€.

There is also another, ready-assembled, kit from Italy that I think is sold exclusively on ebay.
This one looks very impressive but it comes with a price tag: 287€.
The performance specifications are stated in the description.
It usually comes under the title:

"MPX Professional Stereo Decoder for High End TUNER Ready to Install"

and is sold by the seller

teodorolivorno

Here's the link:

https://www.ebay.co.uk/itm/194273385223

but of course it may have disappeared by the time you view this, so use the other details to try to locate the kit on ebay - if it is still sold.
 
#35
Both those decoders are very nicely made. Although the Italian one must have a fierce phase response just before that very sharp 15kHz cut-off. I couldn't see which decoder IC they used. Winding all those RM inductors will contribute to the price.
 
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#36
Tempting aren't they?

It does mention 10Hz - 15kHz +/- 1dB so I guess this means it extends higher but that it's relatively flat to 15kHz - so perhaps not that gnarly a phase response in the high frequencies.

You're right, I did not think of the cost of winding the inductors-eight of them, no less. It's a substantial decoder anyway.
I suppose one could experiment with winding them on toroid cores, if it were offered as a kit, but the pre-assembled version offers product performance consistency/uniformity.

There is one these kits from Denmark currently on e-bay.co.uk, assembled and housed in an aluminium box with RCAs etc. but with the extra cost attached.

Another path in the diy way is the one on Jogis-roehrenbude site:

https://www.jogis-roehrenbude.de/UKW-Projekt/Dekoder/Dekoder.htm

No tubes again but it's all there for the diyer to start a project.
The coils for this project can be order here:

https://www.roehrentechnik.de/html/ukw-projekt.html

Roehrentechnik.de is a treasure trove for valve or transistor radio restorers or hobbyists. It has a rather funny user interface for displaying selection menus (everything is designed as buttons, which enable further selection options to appear as additional buttons) which takes some getting used to, as you can never expect in which secret corner of the site items of interest will pop up.
Note that it offers another inductor for MPX decoders (unrelated to the above project) here:

https://www.roehrentechnik.de/html/nf-spulen.html

Stelios
 
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#37
I was looking at the response graph just now, which I had forgotten about, when I realised I was typing nonsense earlier.
Yes, of course, with a response drop that sharp, the phase response can't be anything nice.
Still, one cannot be sure without knowing what all these ICs are and seeing the circuit.
 
#40
stelios said:
I have happened across this interesting kit from Denmark:
Click to expand...
I've built this. Not recommended. It is a huge and crowded PCB covered in tuneable inductors, based on the MC1310. For a better option see http://www.fmmpx.com. 1/4 the price, 1/4 the size, < 1/4 the component count, only two trimpot adjustments, and several times as good. Based on LM4500. I've built about 20 of these, and the customers rave.

I also tried the Dantimax FM tuner. No soap. PCB errors, and incorrect enclosure that could not hold the PCBs in any way shape or form. Never got it working, and the vendor was no help.

If you want to see a better way to deploy the MC1310 see the Quad FM3, later versions.
 
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