.

by H. E. ROYS - Chief Engineer, RCA Victor Record Division

• Anmerkung : Bezunehmend auf den vorangegangen Artikel wußten die RCA-Ingenieure um Problemantik der Stereo-Quellen bei der FM-Mono Wiedergabe und um die Paralellschaltung der beiden Kanäle eines Streosystems zwecks Mono-Wiedergabe.

.

Stereophonic recordings involve two separate channels of information. In order to achieve the acoustical perspective, depth, spaciousness and other benefits that are possible, great care must be observed to maintain the proper phase relationship between the two channels of information, both in recording and reproduction.

The same care must be observed in FM stereo broadcasting. This is particularly true because the transmission may be reproduced either stereophonically or monophonically, and it is essential that the quality of reproduction be the best possible in either case.

The phonograph industry readily recognized the importance of phase relationships of the two channels and quickly established standards in order to assure a uniformity of product.

Since it is quite likely that many stereo records will be used as program material in FM broadcasting, it was thought that an article describing the record standard, giving some of the reasoning involved and including some helpful hints, would be of interest to the broadcaster.
.

The 45°-45° stereophonic system chosen by the Record Industry Association of America (RIAA) as a standard was selected after careful consideration of the available systems.

It was selected, for one reason, because of the compatibility that can be achieved with respect to mono-phonic reproduction. To achieve this, the RIAA standard states:

In 45°-45° sterephonic disc phonograph records, equal and in-phase signals in the two channels shall result in lateral modulation of the groove. *1)
*1) RIAA Engineering Bulletin E-3.

As illustrated in Fig. 1 for the 45°-45° system, the two channels of information are recorded in a single groove with the modulation axis of the two systems at right angles with respect to each other, and 45° with respect to the surface of the record.

The diagrams at the left and right, A and B, show the type of groove obtained when an identical signal is applied to the left and right coils separately and then together.

It is important to note that in Fig. 1-C vertical modulation results if the signals are equal in amplitude but out of phase. Likewise, Fig. 1-D shows that lateral modulation results if the two signals are equal and in-phase.

In terms of record reproduction, if in the case of 1-D the recording is reproduced with a suitable lateral pickup, information from both channels will be present.

The sound quality will be like that of a monophonic record that had been cut with a lateral recorder where the signals had been obtained by combining the output of the two channels electrically.

If the phase is reversed, as is the case shown in Fig. 1-C, the modulation of the groove will be vertical and the results will be poor due to cancellation (= Auslöschung) of the lateral components.
.

The problem of correct phasing is similar to that which must be observed in stereophonic broadcasting where monophonic reproduction is the sum of the left and right (L and R) channels.

The phase relationship is of importance since an incorrect phasing will result in considerable cancellations, particularly at low frequencies. Only if the proper phasing has been observed, will the sound quality be satisfactory.
.

A simple method of judging the (resulting) mono-phonic quality of a stereophonic record is to reproduce it monophonically. This may be achieved by using a suitable lateral pickup and reproducing the record over a single channel amplifier and speaker system.

By "suitable" is meant a pickup designed for monophonic record reproduction, one that has sufficient vertical compliance to properly track the vertical undulations of a stereophonic record without undue distortion or damage to the groove.

A stereophonic pickup with the output leads tied together for monophonic reproduction provides a ready means of providing such a pickup. (stimmt nur für Testzwecke und zum Abhören der Auslöschungen !!)
.

Another method and one that might offer greater appeal to the broadcaster is to combine the outputs of the two pickup channels at the inputs (not the outputs) of some of the amplifiers along the chain.

When doing this, there may be some question about the channel gains and the phase relationship. These may be easily checked by playing a lateral frequency record.

The VU meter readings for each channel should be equal. If the phase relationships are incorrect, the single VU meter that reads the combined outputs will show a drop in output as the channels are connected together.

The cancellation of signals due to improper phase relationship when reproducing music records results in a loss in the low frequencies and undesirable high frequency characteristics.

When the phase and gain relationships are "correct", a properly recorded stereo record will show nearly undetectable tonal balance differences between monophonic and stereophonic reproduction.

The stereo reproduction will, of course, exhibit acoustical perspective, depth and spaciousness due to the additional information since it is derived from two channels instead of one.
.

.

Realizing the importance of observing and maintaining the proper phase between the stereo channels, a logical question that arises is: How can one check the phase relationship.

Ideally, the two channels should be exact duplicates throughout their operating range. Frequency and phase response should match closely. In general, the phase relationship is the most difficult one to measure.

However, with the aid of an audio oscillator and an oscilloscope, simple observations can be made that will determine whether or not the connections are in-phase.

If an audio oscillator is connected to an oscilloscope, as illustrated in FIG. 2. with the high side of the oscillator connected to both high side terminals of the oscilloscope, a straight line inclined at a 45° angle to the right should be observed.

Since this is a common signal equal in amplitude that is being applied to the oscilloscope, it is obviously the in-phase condition.

If two signals of the same frequency and amplitude were applied 180° out of phase, a 45° line would be observed which would slope towards the left. A 90° phase shift would result in a circle.

The oscillator and oscilloscope provide a simple set of tools for determining the "in" or "out" of phase conditions. They may be used for microphones and loudspeakers as well as amplifiers.

The arrangements for such measurements are shown in Fig. 3, 4 and 5. When acoustic transmission is involved, a low frequency such as 200 cycles should be used to minimize phase differences due to the transmission of the signal through air.

When checking loudspeakers as illustrated in Fig. 5, a quick check of the system can be made by first placing both microphones in front of one loudspeaker and noting the trace on the oscilloscope.

The same trace should result when the microphone is shifted back to its original position. For checking the phase relationship of high frequency speakers,
.
EIA *2) recommends that direct current be used and the direction of motion of the diaphragm be observed, or a sensitive dc meter be connected across the terminals and the polarity of the voltage noted when the diaphragm is moved manually.
*2)EIA Standard RS-233 "Phasing of Receiver Loudspeakers."
.

It is quite likely that in the beginning FM Stereo broadcasts will be heard largely on monophonic receivers. In order to retain public acceptance, it is necessary that the quality of the monophonic reproduction be equivalent to that obtained from a pur monophonic transmission.

Phase relationships of the signals of the two channels are important. Stereophonic records made in accordance with RIAA standards will provide program material suitable for both monophonic and stereophonic reproduction.
.
FIG. 1. 45°-45° Stereo Disc Recorder.
FIG. 2. Phase Indicator.
FIG. 3. Checking Amplifiers.
FIG. 4. Checking Microphones.
FIG. 5. Checking Loudspeakers.
.