Voice Disguiser Circuit

Posted on Apr 3, 2012

A complete schematic diagram of the voice disguiser is shown. Microphone MIC1 picks up the voice signal and feeds it to an audio amplifier, consisting of Ql and Q2, and a few support components. The amplifier has a low-pass gain response that limits the voice frequencies to 5 kHz or lower. The voice signal is then fed to the input of the first, balanced modulator, which is comprised of Ul-a, Ul-b, U2-a, and U3-a. The output of the first 4-kHz oscillator, built around U3-f and U3-e, is fed to the carrier input of the first, modulator. The frequency of the first oscillator is controlled by the setting of potentiometer R13. The modulator outputa double-sideband suppressed-carrier signal centered on 4 kHzis then filtered by the first 5-kHz low-pass filter, formed by U2-b, which eliminates the upper-sideband signals.

Voice Disguiser Circuit
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At this point, the voice frequency spectrum is inverted (e.g., the frequencies that were low now become high, and vice versa), making the voice signal completely unintelligible. The output of the first low-pass filter is fed to a second modulator formed by Ul-c, Ul-d, and U3-b, where it is frequency modulated with the output of the second carrier oscillator, comprised of U3-c and U3-d; the frequency of the second oscillator is controlled by potentiometer R36. The output of the second modulator is filtered by the second low-pass filter, which consists of U2-d and few support components, and amplified by Q3. The voice output signal from Q3 is fed to U4 (an LM386 low-voltage, audio-power amplifier) through an impedance-matching transformer, Tl. The output of U4 is then used to drive SPKR1 (ari 8- speaker). In operation, if both carrier oscillators are set to the same frequency, the voice signal from the speaker will be an exact duplicate of the input signal from the microphone. However, if the frequency of the second oscillator is varied (via R36), the output voice signal also shifts in frequency. That makes the voice reproduced by the speaker sound higher- or lower-pitched than normal.

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