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A New Decoder for CD-4 (Quadradisc) Phonograph Records

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CD-4 (or Compatible Discrete 4 Channel) was a short-lived, four-channel, surround-sound system for phonograph records. Developed by JVC in Japan, the system was adopted in America around 1972 by RCA where it was known as RCA Quadradisc. Unlike matrix quadraphonic systems, CD-4 took a more radical approach. The baseband signals, which modulate the groove, are the sum of the front and back signals (LF + LB) and (RF + RB). The difference signals, used to separate back from front in the decoder, are FM encoded on a pair of ultrasonic (30kHz) subcarriers recorded above this baseband signal. The development of a new, software-based decoder for CD-4 phonograph records is described in this report. A relatively complete understanding of the original hardware decoders is necessary, and this analysis is new. A special phono cartridge with an extended frequency-response up to 45 kHz is required, and this must be fitted with a Shibata or line-contact stylus to track the high-frequency subcarrier modulation. In addition, wide bandwidth preamplifiers, correct cable types, and low crosstalk are all required to recover subcarrier signals of sufficient quality and amplitude so that successful decoding is possible. A different approach to the output matrix is described based on Ambisonics theory, which increases the reliability of successfully decoding worn and damaged CD-4 media.

JAES Volume 67 Issue 9 pp. 679-690; September 2019
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Scott Dorsey
Scott Dorsey

Comment posted November 25, 2019 @ 18:16:45 UTC (Comment permalink)

I found this mostly interesting because it was a good overview on reverse-engineering a piece of very finely designed hardware and trying to duplicate it.  I would like to have seen more about the FM discriminator system and some comparisons between the software and the original PLL.... as the author states, PLL unlocking is very dramatic with the original decoders and it should not be impossible to do much better in software.  I am surprised that the author did not try one of the original Philips cartridges initially intended for these records but instead employed a modern cartridge; I think it would also be very interesting to look at cartridge behaviour above 20kc since I suspect dramatic differences will be seen between current production models.

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Richard Brice

Comment posted November 27, 2019 @ 19:14:13 UTC (Comment permalink)

Mr. Dorsey's comments are most welcome. I'm glad the sofware CD-4 decoder article was interesting.

Obviously, one has to make decsions about what (and what not) to cover when writing these reports. Digital radio techniques have been such a strong component of modern DSP work that I didn't think there was much new I could add in my description of the software FM demodulator. The only difference in this application, being a file based technique, is that there was no requirement to process in real time (or even to be causal). As I said in the article,

.... the discriminator must mute and
re-lock quickly in the event of carrier drop-outs that are
frequent when playing older discs of dubious provenance.
In dealing with these issues we were greatly aided by the
fact that the software decode is file-based and non-causal
so that problems could be identified and rectified “before
they arrived.”

In this way, at least, the software approach is very different to the hardware.

The point about phono cartridges is well taken. It would have been excellent to have had a range of vintage phono cartridges from the quadraphonic period to experiment with. But they are now rare and expensive. I don't recognise the reference to Philips cartridges. I do know about the Panasonic strain-gauge types whch were developed for CD-4 quadraphonic. More information concerning our experiements with modern cartridges is given on this Stereo Lab help page:

Best wishes,

Richard Brice

Scott Dorsey
Scott Dorsey

Comment posted May 17, 2021 @ 15:57:12 UTC (Comment permalink)

It seemed every MM cartridge manufacturer at the time developed their own low-mass cartridge for the application, starting with the JVC X-1 and the Philips GP 422.  There were a lot of different ones out there.  Shure had a crosstalk test disc that was intended for evaluating discs for CD-4 systems and it would be interesting to compare performance of modern cartridges against those two standards.

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