Monday, 20 October 2008

Fourier series of VIC-20 tone generator waveforms

Waveform 0000000011111111 (square wave)

Waveform 0000001011111101

Waveforms 0000010011111011 and 0000011011111001

Waveforms 0000100011110111 and 0000111011110001

Waveform 0000101011110101

Waveforms 0001001011101101 and 0001011011101001

Waveform 0000110011110011

Waveform 0010010011011011

Waveforms 0001010011101011 and 0001101011100101

Waveform 0001100011100111

Waveform 0010101011010101

Waveform 0010110011010011

The tone generators of the 6560/1 VIC chip are capable of producing 15 extra waveforms, in addition to the square waves they were intended to produce. The above charts show the amplitude of the harmonics of these waveforms, using a Fourier series expansion, up to the 17th harmonic.

4 of the waveforms turn out to be duplicates, so there are in fact 11 new sounds in total. All of the sounds have odd-numbered harmonics only. The waveforms are listed in decreasing order of first harmonic amplitude.

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.