Message from Mr. K. Suzuki, the Director in charge of JVC's Stereo Division.

This booklet has been specially written to give people concerned with sound an understanding of JVC's CD-4 discrete 4-channel system. It is addressed to dealers in and salesmen of both hardware and software and all audio writers and critics who, in the course of their work, will have to deal with the CD-4 system.

Audio realism is the watchword behind JVC's progress. Ever since JVC emerged as a company in its own right there has been a continuous quest for perfection. The aim has been to give the consumer the wherewithal to recreate in his living room exactly the conditions that existed where the recording was made.

As you may or may not know the 45/45 stereo system was independently developed in Japan by JVC. This was merely a step in the direction of reaching "audio realism". CD-4, compatible discrete 4-channel, is yet another step. I do not claim that it is the ultimate step, but I think I am justified in claiming that it is the most important step to have been taken in audio for many years and will establish the recording standard for many years to come.

From Edison to the 1950s played back sound was monaural. The quality of the sound improved over the years but the sound remained monaural. A sound field which originates from one source cannot sound like anything other than it is. And when you're in a live listening environment the sound doesn't appear to come from just one place.

The late 1950s brought 2-channel stereo. This was a big advance over mono; with the sound coming from two speakers instead of one the apparent source could be anywhere between the two speakers. You could actually hear the singer move and the violinists were on the left and the brass was on the right just like in the live performance.

The major drawback, which exists in these matrix systems is, that interchannel crosstalk cannot be eliminated. This is inherent in the system. And as crosstalk increases, channel separation deteriorates. With conventional stereo a channel separation of around 20dB can easily be achieved, but with matrix 4-channel manufacturers are asking the public to be satisfied with 3dB. Very few people can distinguish sound with such a low channel separation from monaural sound.

There have been improvements in matrix systems, but they all add to the cost of the decoder needed to play the records back. Phase shift circuits and logic components have added to channel separation, but generally, it seems that, as one problem has been eliminated, another has arisen, especially the loss of original sound material due to phase shifts.

The approach, that JVC engineers decided on was completely different. They did not consider themselves to be restricted by current recording techniques and set about their research along another path entirely. Their initial experiments showed them, that matrix systems had inherent weaknesses and that the only system which could satisfactorily reproduce the 3-dimensional sound field of a recording studio was a fully discrete system.

Their system requires a more accurate record cutting system than was previously available and a stylus/cartridge with a wider range. The only constraints, that they imposed on themselves was, that it should be compatible with existing playback equipment and be fully discrete 4-channel. These two principles - Compatibility and Discreteness - give the system its name - the CD-4 system.

Now the breakthrough occurred when it was realized that each wall could carry two signals if the latest technological advances were used. One signal can be held as a regular audio signal and the other as a frequency modulated signal - like in FM radio - with a base frequency so high that the FM signal doesn't interfere with the audio signal at all.

So that the groove can be tracked by a conventional stylus and to minimize any chance of interference by the FM signal with the audio signal - the FM signal will have to be treated specially anyway - the FM signal's level can be reduced below that of the audio signal. A level difference of 19dB was decided on as an optimum.

The most critical element of the system is the demodulation of the frequency and phase modulated carrier, which holds the difference signals. There are several ways, in which this demodulation can be done, but the Phase Locked Loop (PLL) IC used in the CD-4 demodulator is most appropriate in this application.

The theory of PLL was discovered in the 1930s and has been used for detecting cosmic rays, measuring signals masked by noise and in other fields of telecommunication measurement. Its uses, however, were limited because of its size and complexity and it wasn't till the 1960s, that the PLL was perfected and compacted by NASA for use in space telecommunications.

The Phase Locked Loop is a feedback system consisting of a phase comparator (PC), a low pass filter (LPF) and a voltage controlled oscillator (VCO). The frequency of the VCO is synchronized with the incoming signal.

When the phase comparator detects a phase difference indicating a change in the frequency of the input signal, its output increases or decreases just enough to keep the VCO's frequency the same as the incoming frequency. This means that the voltage applied to the VCO is a function of the incoming frequency. If the VCO has a linear voltage to frequency relationship, and if the input to the PLL is a frequency modulated signal, then the voltage which must be applied to the VCO to correct its output to the phase comparator is the demodulated output required.

When there is no input signal to the PLL the VCO operates at 30kHz and there is no demodulator output. When an input signal with a frequency close to 30kHz is applied to the PLL, the VCO's frequency will follow the input signal and the signal which controls the VCO will be the demodulated signal.

The benefits of PLL demodulation are independent center frequency adjustment, independent bandwidth adjustment, high noise immunity, high selectivity and high frequency operation. Perhaps the most important in its application to CD-4 demodulation is the bandwidth adjustment which enables the carrier based difference signal to be detected without shattering distortion when the CD-4 record has been played back many times.

The groove in the CD-4 system is complex and this presents a serious obstacle to the accurate tracing of the groove by the stylus as it must track both, the audio and modulated signals. If there was no kind of compensation, the stylus would tend to skate along the groove, tracking the imaginary waveform resulting from the mixture of audio and modulated signals, tracking neither accurately. The system developed by JVC is called Neutrex. It modifies both, the cutter waveform and the modulated signals, so that the correct signals are picked up in playback.

The effect of Neutrex is to improve channel separation by reducing interference between the sum and difference signals and the resultant distortion of the sum signals by minimizing fluctuations in the carrier level.
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