> > > >

1954-55 Theatre Catalog, 12th Edition, Page 285 (249)

1954-55 Theatre Catalog, 12th Edition
1954-55 Theatre Catalog
1954-55 Theatre Catalog, 12th Edition, Page 285
Page 285

1954-55 Theatre Catalog, 12th Edition, Page 285

moves across the stage from the lobe of one miciophone into that of another, and the rear line will be shortened. At the front line the directivity effect may be so great that the sound appears to recede between microphones. Experiment has shown that with moderate directivity, and by toeing in the lobes of the side microphones somewhat, an advantageous compromise between these two effects can be made and better overall coverage of a rectangular stage obtained.

This effect may be obtained with microphones of uniform directive prop erties, such as the cardioid types, or with the directivity only at high frequencies characteristic of a relatively large condenser or dynamic microphone at normal incidence. The latter will give accentuated directional effects with less change in overall loudness. Here directional effects are really quality changes. While in monophonic reproduction these quality changes would be objectionable, in stereophonic work the listeners fused impression consists of the contribution from several sources and the source is always in the direct lobe of one microphone. If the normal incidence characteristic of the microphone is considered in overall system performance, the fidelity will remain high from all source positions.

The elimination of the pickup from behind the microphones is a definite advantage in most cases. Obviously it eliminates noise. But it also eliminates part of the reverberation, and since most stages have more than the desired reverberation ratio for the physical depth, this is an advantage.

Bridged Microphones

Since channels are expensive and the complications grow with greater numbers, it is tempting to use bridged microphones to simplify the system. If thls technique is used with restraint to galn additional realism in reproduction, it can be, extremely useful. If it is used in the hope that it will be a cheap way of duplicating the performance of a more elaborate system, the results are bound to be disappointing.

An example of a useful application of the bridged microphone is its use to emphasize a small group of instruments in orchestra, when the overall pickup is satisfactory in other respects. This was employed in the Hollywood Bowl demonstration where one extra microphone was used continuously on the right channel, and others were employed during special parts of the performance. In monophonic systems, multiple microphone pickup often leads to poor fidelity because of cancellation between the sig nals from the microphones in specific frequency regions. In stereophonic systems this effect is ameliorated because sound is fused from several sources.


Amplifiers for use in stereophonic systems do not differ from those of monophonic systems except in number. The characteristics of the amplifiers in

FIGURE 4 is a chart showing that when a source moves on a stage where multiple pickup is med. three factors vary, plus the intensity.


the various channels should be similar, and the gain should be stable so that no undesired level differences will occur. It is usually found desirable to have a ganged volume control which will adjust the overall level, and as individual control in each channel for balance or intentional unbalance settings. Similar provisions for quality-changing networks are desirable. If bridging systems are to be used proper networks and bridging amplifiers must be provided tovinsure that signals how only in the desired directions, and inadvertent gain changes are not made during switching. It is also good practice to observe a poling convention throughout all channels, including the microphones and loudspeakers, although the channels spacings are so be considered at other than random wide that only very low frequencies can phases in one channel compared to another.

As a matter of economics, it is probably true that the added complication of stereophonic reproduction will be em. ployed only for high fidelity reproduction. Consequently, the amplifier systems will require the same care that is required to secure high fidelity in monophonic systems.



Following presentation of the paper in New York, the author addressed the SMPTE in HoBlg/wood and added a fuller discussion of the use of the pseudostereophonic or bridged-microphone system. Since this is a very active subject at present and affects the pictures being made, it is of importance to exhibitors.

It is tempting to use manually controlled methods in pickup to create the stereophonic illusion because of many

3 U fl,

0 5- 10 Feel?

{27/4} 076 SIOUKLTJ. (VJ 5%:sz


practical complexities in multi-channel microphone technique, and differences from the standard monophonic techniques developed during years of sound recording in motion picture production. This takes the form of using a single microphone recording and controlling the relative recorded amplitudes in the stereophonic release tracks by means of an ganged mixer or ftpan-pot" during dubbing. However, when a source moves on a stage where multiple pickup is employed, three other factors vary in addition to intensity. This is illustrated where the source, as shown at A, moves from left to right in a straight line. The curves at B illustrate the changes of intensity Int, ratio of reverberant to direct sound intensity R/D, arrival time T, and sound quality Q which occur at the right microphone. (Effects on the center microphone are omitted to reduce complication in the curves.) It will be seen that the intensity rise 12.5 db. Reverberant energy remains constant at the microphone; consequently the ratio of reverberant to direct intensity goes through a decline of 12.5 db. The arrival time decreases by 11 milliseconds as the source comes nearer. And finally, the quality changes when a microphone not absolutely uniform in directivity vs. frequency is used. In this illustration an extreme case of a three-inch diameter dynamic microphone is used for emphasis, and the curves show that there is much more relative high frequency when the source is near the microphone. While it is true that the stereophonic ediect is an illusion, experience has shown the illusion is enhanced when all of these factors are varied properly and simultaneously by movement, compared to manual variation of a single element. It is known to be possible to produce

1954-55 Theatre Catalog, 12th Edition, Page 285