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304 IX. SOUND-RECORDING EQUIPMENT AND ARRANGEMENT
Some of the more common visual monitoring arrangements are :
(I) A standard electrical Volume Unit meter.
{2) A cathode-ray oscilloscope.
(■«?) A moving light beam falling upon a screen or ground glass; this light beam is usually derived from the optical system of the recording machine itself. (Oftentimes, it is the chromatic portion of the light beam that is not used for film exposure ; or it may be an auxiliary light modulator arrangement used for monitoring purposes only.)
(4) A line of gas-tube ''trigger" lamps, each of which lights at a different voltage; the length of the lighted line indicates the volume level.
The standard VU meter is described in ASA C16.5 "Volume Measurement of Electrical Speech and Program Waves. ' ' This instrument reads 99% of final deflection in 0.3 second. Being slightly less than critically
Fig. 72. American Standard electrical volume level indicator (Weston Model 862 Type 30) "A" Scale. Deflection time: 99% of its normal OVU in 0.3 sec. ±10%. Pointer overswing : 1 to 1.5%. Distortion introduced due to circuit loading: Not over 0.3% on a terminated 600 ohms circuit (under worst conditions). Frequency error: Less than 0.2 db up to 10 kcps at OVU. Temperature error: Negligible at room temperature; less than 0.2 db from 50 to 100 F at OVU. Scales: Type A — with upper figures VU emphasized and percent figures small ; Type B — with upper percent figures emphasized and VU figures small. Reference zero: 72% of scale length. Calibration voltage: OVU (100% scale marking) is indicated when 1.225-v. EMS sine-wave is applied to the instrument in series with 3600 ohms resistance; the instrument resistance including the external resistor is 7500 ohms. This calibrating voltage represents + 4db above a reference level of 1 milliwatt in 600 ohms. A standard attenuator is required with the instrument for reading different program levels; a typical attenuator is shown schematically in Fig. 72A.