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1949
PORTABLE RECORDING SYSTEM
179
frequencies are attenuated to a greater extent than for 35-mm recording to obtain the proper balance with the high frequencies which are cut off at 5000 cycles. The mid-range boost and pre-emphasis are also shown. For music recording the low frequencies are attenuated to a lesser extent than for dialog. However, the extreme low frequencies are eliminated and the mid-frequency emphasis is retained to give the most desirable over-all response. Fig. 21 shows a transmission schematic and level diagram of the complete system for normal conditions of operation. Vertical lines on the chart represent fixed gains or losses. Sloping lines represent variable gains or losses.
-30
DIALOGUE
20
100
IOOO IOOOO 2O 000
FREQUENCY IN CYCLES PER SECOND
Fig. 19 — 35-mm recording — over-all-electrical-response characteristics.
Starting at the microphone, at the left end of the chart, an input level of —70 dbm is assumed. This corresponds approximately to that obtained from a Western Electric RA-1142 microphone located 3 feet from an average dialog source.
The signal level at the preamplifier outputs under these conditions is 37 decibels below their overload point, thus allowing an adequate margin for extremely high-level signals.
For this input signal level of —70 dbm the normal mixer output level of —55 dbm is obtained with 12 decibels attenuation remaining in the mixer pots. The signal-to-noise ratio at the mixer output under these conditions is 56 decibels.
For the normal mixer output level of —55 dbm, a +15 dbm level