Monday, 22 September 2008

VIC-20 audio modification

Both models of the VIC-20 have a 1.6kHz low pass filter in the audio output stage. The above picture shows the output stage of the original model VIC-20. Resistor R16 (1KOhm) and capacitor C20 (100nF) form a single pole passive RC network with a cutoff frequency of 1.6kHz. This can be changed to 16kHz by replacing C20 with a 10nF capacitor.

The second picture shows the audio output stage of the cost reduced VIC-20. R8 (1KOhm) and C10 (100nF) form the same single pole passive RC network with cutoff frequency 1.6kHz. As with the original VIC, the cutoff frequency can be moved to 16kHz by replacing C10 with a 10nF capacitor.

The effect of this filter is to reduce the level of high frequencies in the VIC's sound output. By moving the filter cutoff to 16kHz, the high frequencies can be retained and the overall sound is brighter.

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