Should Stereo Lab support software decoding of CD-4 quadraphonic? As with SQ and QS, a software decoder could offer the linearity and stability of processing which eluded the discrete transistor implementations of the 1970s.
JVC's CD-4 relied on encoding front-back difference signals on a high frequency (30kHz) subcarrier, as illustrated in the above diagram. Audio files with 96kHz sampling could certainly contain all the sideband information around the subcarrier required for CD-4 demodulation in software; although limiting and FM demodulation would demand oversampling the 96kHz data to avoid aliasing.
In order to judge the feasibility of CD-4 decoding, we performed the following experiment.
A CD-4 quadraphonic LP (the immortal SUSUMU ARIMA's BRIDGE OVER TROUBLED WATER) was played using a moving-coil cartridge feeding the Groove Sleuth preamplifier.
Reasonable amounts of 30kHz carrier were immediately apparent on the 'scope - even in the run-in groove! A needle-drop was made at 96kHz sampling (24 bit resolution) and this files was over-sampled four times in software. The carrier signal was extracted (with a very steep high pass at 15kHz turnover) and the resulting signal was limited (about 30dB gain being required to do this). The resulting FM signal was slope detected, because this is a simple technique. Finally, the audio was filtered and equalised. An mp3 file of the resulting audio is available here.
From this test (and others like it) it is clear that software detection of CD-4 difference channels is possible.
But the tests also demonstrate that - at least on the discs we have tried - copious amounts of distortion and noise are present. Furthermore, scratches proved to create a nasty grumbling dropout type noise (16 seconds into demodulated and equalised example).
So the question is:
Are there enough recordings in this format in a playable condition for software decoding of CD-4 LPs to be of much interest? Email: email@example.com if you have a strong opinion about this.
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