GB1518299A - Audio monitor - Google Patents

Abstract

1518299 Monitoring stereophonic sound J BENTLEY 17 Nov 1976 [20 Nov 1975] 47884/75 Heading H4J Correct balance of a stereophonic or quadraphonic sound system is tested by generating sounds in loudspeakers corresponding to particular tones and levels and monitoring the sounds using a microphone arrangement. An indicator shows the relation between the sound levels received from respective loudspeakers. The system amplifiers and loudspeakers are then adjusted to balance in accordance with the indicator. A monitor is mounted in a head-like housing 1 (Fig. 1). Two directional microphones 2, 3 have a response pattern corresponding in directivity to two human ears. A cassette deck 4 generates predetermined tones in loudspeakers 6, 7. Microphones 2, 3 feed respective battery powered I.C. amplifiers 8, 9 and meters 10,11. The speaker balance is then adjusted by a balance control 12. A common meter 15 may give a difference output. In an alternative arrangement (Fig. 2, not shown) two sound channels (16, 17) feed a single microphone (18). The microphone output is split into two channels (19, 20) having respective meters (21, 22). Switches (23, 24) are operated by a switch control 25 at 10 Hz in synchronism with a tone applied to loudspeakers 6, 7 in turn. Thus one meter (21) records the output from loudspeaker 6 and the other meter (22) records the output from loudspeaker 7. Alternatively there may be two microphones and isolated electrical channels. A difference meter 15 may be provided. Alternatively the monitor may have a single channel. For example, a single microphone 18 (Fig. 3) feeds on amplifier 26 the output of which is capacitively coupled to an envelope detector 28 whose output is a series of adjacent pulses which alternately have the amplitude level of the sound received by the microphone 18 from loupeaker 6 and loudspeaker 7. The indicator 33 (see Fig. 4, not shown) includes a light beam which displays a band of light, representing the difference between the two amplitude levels. Alternatively the output of the envelope detector is applied to a coupling capacitor (37) Fig. 5 (not shown) and to a further rectifier (38). Its output is a unidirectional square wave and is applied to a coupling capacitor (37) Fig. 5 (not shown) and to a further rectifier (38). The output is a unidirectional square wave and is applied to a damped moving coil meter (39). Its output is proportional to the difference between the amplitude levels received from the loudspeaker. The amplifier 26 may have A.L.C. The indicator may comprise lamps, L.E.Ds, L.C.Ds or a pair of headphones receiving the outputs of the two channels out of phase. When the amplitudes of the signals in the two channels are equal there will be total cancellation and no sound will be heard from the headphones.