10-octave audio generator

To speed testing, the circuit generates a composite audio signal comprising 10 sine waves of 10 different equal-amplitude frequencies across the audio band. Values stored in EPROM determine the frequencies.

Driving an audio device-under-test with this circuit allows you to test that device's responses quickly by connecting its output to an audio-spectrum analyzer. A quick check of frequency response requires only a few seconds. The output amplitude at the LINE output is 1V rms, and harmonics are all below -20 dB.

IC₁ generates a 128-kHz clock, which R₁ adjusts, to drive counter IC₂. The count from IC₂ addresses successive locations in EPROM IC₃. The EPROM contains the data values to generate the 10 octaves of equal-amplitude sine waves (see listing in EDN BBS /DI_SIG #1559). In this case, the frequencies are 1 octave apart from 31.3 Hz to 16 kHz. IC₄ and IC₅ latch the 8-bit data from IC₃. The latched values drive IC₆, an 8-bit DAC.