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Old 30th January 2007, 02:22 PM   #1
EC8010 is offline EC8010  United Kingdom
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Default Hybrid FM multiplex decoder?

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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Old 30th January 2007, 05:13 PM   #2
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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)

Click the image to open in full size.
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Old 30th January 2007, 05:24 PM   #3
EC8010 is offline EC8010  United Kingdom
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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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Old 30th January 2007, 06:54 PM   #4
forr is offline forr  France
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HI EC8010,
Aren't you thinking of a sample and hold decoder ?
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Old 30th January 2007, 07:15 PM   #5
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EC8010 Great minds think alike see this:
http://www.hi-fiworld.co.uk/hfw/feat...radecoder.html
The trend to phase locked decoders was started in 1969 ish with the publication in Wireless World of the Portus and Heywood decoder
This prompted the bods at Motorola to come up with the MC1310 integrated circuit decoder
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Old 30th January 2007, 07:25 PM   #6
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Scientific American carried a 12AX7 based PLL back in 1968 or 1969 -- I recall using the device (which I had assembled as a Lab-RAT) in the Physics Lab at the Universitatis Bronxiensis.
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Old 30th January 2007, 08:25 PM   #7
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Quote:
Originally posted by forr
Hi EC8010,
Aren't you thinking of a sample and hold decoder ?
I don't know. Am I? Obviously, I could use a pair of 6AR8 and modify the duty cycle of the signal feeding the deflection plates so that the signal is sampled. That might slightly ease the filtering at the expense of reducing output.

The Tim de P decoder is completely different (although rather elegant).
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Old 30th January 2007, 10:29 PM   #8
forr is offline forr  France
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Hi EC8010
Email me if you would like to have details about a sample and hold decoder.
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Old 30th January 2007, 11:37 PM   #9
kevinkr is offline kevinkr  United States
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I can't see why this wouldn't work. Upon rethinking this I'm assuming that both the L+R baseband and the double side band supressed carrior L-R are fed to the grid of the 6AR8 demodulator? I suspect delays in the IF strip would have to be consistent across the IF bandwidth for this to work ok though. I'd say its worth trying..

IIRC Isn't this time domain de-multiplexing? If so this is the basis of all Scott decoders in existance during the tube era, and they worked quite well. They used (double) balanced diode demodulators to do the job. My favorite outboard decoder was the Scott 335..
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Old 31st January 2007, 12:46 AM   #10
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what's wonderful about this is that you could make a metaphysical hybrid FM multiplex decoder with the toolbox available in most SIM programs -- why bother to solder?
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