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The basic problem in playing recorded music is how music can be reproduced which approximates the original sound field. This problem has been the basis for development of the four-channel concept.

The constant search for more realistic reproduction of sounds on records has been the prime force behind the progress from the 78 rpm to the 33-1/3 rpm monaural LP to the stereo LP (rpm = Umdreh. / min).

Now that stereo has advanced from the two-channel to the four-channel system, the prerequisite for improvement is considered higher sound quality, greater density of information, and reduction of loss.

The matrix four-channel and discrete four-channel systems have each resulted in intensive examination of reproduction problems and the result has been improved performance. In this article, problems encountered in pickup cartridges designed for four-channel use will be examined in an effort to put them in perspective as they relate to the two reproduction systems.

Problems confronting cartridges in the matrix four-channel system and in the discrete four-channel system will be treated separately.

 The following factors prevent distortion by the pickup cartridge at the time of phase shift.

• 1) The needle should not have enough resonance to cause a phase shift of more than 11° within the matrix four-channel reproduction band. With a high-grade pickup cartridge, resonance arising from the equivalent mass of the playing needle and the equivalent stiffness of the record normally exists between 12 kHz and 20 kHz. With popular cartridges like the ceramic types, resonance arises between 6 kHz and 10 kHz. For this reason, either the mass of the vibrating system should be reduced to a minimum or adequate damping should be applied to suppress the phase shift that accompanies resonance. Phase shift with a high-quality cartridge is about 15° at maximum, but with the popular type, it is about 30° at maximum.
• 2) The output balance between the left and right channels should be uniform and stable within the band.
• 3) Tracking of the playing needle should be true and amplitude mixing and modulation distortion should be low. If distortion is high, particularly in the vertical direction, the rear channelwill suffer from distortion and poor orientation.
• 4) The pickup should have a minimal lateral tracking error rate. It is desirable that the axial line of the vibration be identical with the junction line of the sound groove, since phase shift is related to the magnitude of the error.

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As the above factors are not peculiar to matrix four-channel reproduction, many pickup cartridges on the market satisfy them completely. Since these problems have been dealt with successfully in two-channel reproduction, there should be no trouble in four-channel reproduction. Some caution is advisable, however, as some OEM cartridges may exhibit the phase shift characteristics described above.
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As shown in Fig. 1, the four sound sources of the CD-4 disc are converted into the sum signal (Chl+Ch2; Ch3+Ch4) and the difference signal (Ch1—Ch2; Ch3—Ch4).

In order to improve SN, the difference signal passes through the compressor and FM-PM-FM equalizer and undergoes angular modulation to get 30 kHz as the carrier. The modulated signal and the sum signal mentioned before become the left signal of Ch1+Ch2 + (Ch1-Ch2) and right signal of Ch3+Ch4 + (Ch3-Ch4) at the next mixing circuit.

These then pass through the cutting equalizer containing reverse RIAA characteristic to undergo cutting. As with a normal stereo record, the left signal mentioned before is cut on the left-hand wall of the sound groove at a 45-degree incline, while the right signal is cut on the right-hand wall of the sound groove.

The frequency at this point, as shown in Fig. 2, is distributed over a wide band. Turntable speed at the time of cutting is reduced to 1/2,7 of 33-1/3 rpm. (also eta 1/3 der 33 1/3 Umdrehungen)

In the following, the current state of the four-channel cartridge, which made compatible 2-chan-nel and CD-4 reproduction possible, and its various characteristics will be discussed.

In two-channel stereo reproduction, problems have been caused by the relation of tracking to distortion and the relation of needle pressure to abrasion of the sound groove.

These are particularly important in the CD-4 disc. The difference signal band has higher mechanical impedance with the CD-4 disc, as shown in Fig. 2.

Standard level (OVU) used is 22.3 mm/sec and the modulated signal is amplitude recorded at constant speed at a low level of 20 dB, facilitating reproduction needle tracking on the disc side.

Mehr über die mechanische Impedanz lesen Sie hier.
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However, a survey of the market has shown that there are no more than 13% two-channel reproduction magnetic cartridges with mechanical impedance sufficient to execute tracking up to 45 kHz.

At the contact point between the sound groove wall and the tip of the stylus, the sound groove wall is subjected to concentrated pressure. The sound groove wall undergoes either flexible or plastic deformation.

Consequently, if the needle pressure is too high, high range attenuation and sound groove abrasion take place; if the needle pressure is too low, tracking of the sound groove becomes poor and results in distortion.
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Any disc using polyvinyl chloride and vinyl acetate polymer as the main materials is subject to change in the frequency response under the influence of temperature fluctuations.

The response, in other words, changes in proportion to the frequency and to the temperature. The resonance frequency arising from stiffness of the disc and the mass of the vibrating system is particularly subject to the influence of temperature.

The needle support bearing, which both supports the needle and provides the damping effect, is made of a rubber-material. Since it is damping the resonance mentioned before, its changes are marked.

A normal magnetic cartridge undergoes, at 30 kHz, a 4 to 5 dB change between 5°C and 20°C. As this impedes operation of the difference signal circuit, caution is necessary.

As the cartridge output is unaffected in the sum signal band of the CD-4 disc, minimum 0.5 mV or above (speed amplitude of 3.54 cm/sec. at 1 kHz) is appropriate. The normal magnetic pickup cartridge poses no problem as an output of 2 to 5 mV is obtained.

However, there is correlation between the high-range reproduction frequency range of the vibrating system and the output voltage. In case the output voltage is high and efficiency is good, the high-range reproduction frequency range is liable to be narrow in many cases.

As the difference signal circuit operation uses 30 kHz as carrier, the output voltage of the difference signal band at 30 kHz may well be 1 mV or higher (speed amplitude of 3.54 cm/sec at 30 kHz). It must be noted, however, that this value takes into account losses resulting from the effect of temperature, needle pressure and load impedance.

As shown in the frequency distribution (Fig. 2), a flat response between 20 Hz and 15 kHz is good enough for the sum signal, and the response as it is results in sound quality. For the difference signal, flat response between 20 kHz and 45 kHz is required as an ideal.
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1. Type A is the Shibata stylus. This consists of a combination of two-face cutting and conventional conical stylus tip grinding.
2. Type B is the extension stylus and Pathe-Marconi stylus. In this type, 75 uR and side contact curvature are uniformly provided for the 75 uR part of conventional elliptical styli.
3. Type C is the Ichikawa stylus tip. Processing similar to that of Type B is done to the tip of a quadrangular pyramid.

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