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BlNAURAL RECORD PLAY BACK FREQUENCY RESPONSE
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FREQUENCY IN CYCLES PER SECOND Fig. 2. Overall 15-in./sec tape speed, record-reproduce frequency response.
the input tubes. This is derived from a full-wave selenium rectifier.
The output of the playback system consists of two independent 10-w amplifiers with nominal output impedance of 4 and 1 6 ohms. A 600-ohm balanced connection is also provided at a line level of +4 dbm for each channel. The system is both pre and post-equalized in order to achieve a flat response at 15 in./sec recording speed of from 50 cycles to 15 kc ± 2 db (Fig. 2). Both the amplifier unit and the tape transport are provided with facilities which allow operation at 7-g in./sec with a frequency response of from 50 cycles to 7.5 kc ± 2 db.
A signal-to-noise ratio in the order of 50 db is achieved with this equipment. The residual crosstalk between channels is essentially due to low-frequency magnetic coupling below 100 cycles/sec. This crosstalk measures approximately 35 db at 50 cycles and drops with frequency increase until it goes below the tape noise level at a little over 100 cycles/sec.
Calibration Means
In order to assure accurate localizing based upon binaural amplitude com
parison, it is desirable that all possible electronic balancing between the two record-reproduce channels be carried out. To this end, a calibration button is provided which inserts a 60-cycle/sec signal simultaneously into the first stages of both amplifier inputs. The channel gain controls may then be individually adjusted to obtain equal VU meter readings. The balanced signals may then be recorded if the tape transport is turned on. When played back, the two 60-cycle signals may again be read on the VU meters and the playback gain controls may then be balanced for the optimum binaural effect.
Commercial Applications
The design of this equipment was aimed at satisfying certain specific commercial applications although it has a definite application to highfidelity music recording-reproducing, where listening pleasure is desired to be as high as possible. The majority of commercial applications lie in the field of identification of intelligence or information where it is necessary to distinguish between each of many
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August 1952 Journal of the SMPTE Vol. 59