Classic Computer Magazine Archive COMPUTE! ISSUE 54 / NOVEMBER 1984 / PAGE 10

Commodore 64 Audio Input

I own a Commodore 64 and have had no problems with it at all. Documentation of all its features is another story. I know that the 64 has an audio input located on the audio/video port on the back of the unit. However, I have not been able to find any literature on how to access this feature. Could you please tell me how to use it? What memory locations are affected?

Kevin Caylor

The audio input pin is used to mix in an external sound source. You can test this by feeding the sound output of another 64 into the audio input. When mixing in another audio source, be sure it's at the same low level as SID chip output. (Feeding in an amplified signal could destroy your SID chip.) Intended for chaining SID chips together, the audio input becomes a kind of fourth voice, and is affected by the SID chip's volume and filter settings. Bit 3 of location 54295 enables the filtering of external audio. You cannot process sound per se, but you can use the SID chip's filter as a simple, programmable equalizer which will emphasize or reduce various frequencies.