Re: VIC-20 audio modification

From: Richard Atkinson (
Date: 2008-09-24 01:06:55

I put up some information about the mod online:

On Aug 6 2008, Richard Atkinson wrote:

> I was listening to some VIC-20 demos and it occurred to me that the VIC 
> sound output always sounds rather muffled, as if a treble tone control 
> had been turned down. So I had a look at the schematics of the new and 
> old VICs, and found that both have a 1.6kHz low pass filter (single stage 
> RC network, 6dB/octave) in the audio output stage. On the old VIC 
> (324001) it is R16 (1K) and C20 (100nF) and on the new VIC (251027) it is 
> R8 (also 1K) and C10 (also 100nF).
> So I tried replacing C10 on a new VIC with a 10nF capacitor, to bring the 
> cutoff frequency of the filter up to 16kHz, and the result was 
> substantially brighter sound. The effect is most noticeable on demos such 
> as pwp's Robotic Liberation, where the new waveforms (viznut waveforms) 
> have a lot of high frequency content.
> What's also interesting as an aside is that on the new VIC (the cost 
> reduced model) there is an external audio input from pin 2 of the 
> cartridge port, connected via another 1K resistor R40, which is not 
> present on the old VIC.
>       Message was sent through the cbm-hackers mailing list


Richard Atkinson MA (Cantab) MEng
Director, Atkinsoft Ltd
33 Brimley Road, Cambridge CB4 2DQ. Office 01223 353449 Mobile 07951349449
Company No. 06411520 VAT No. 922271741

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