Register 24 determines which type filter you want to use. This is in addition to register 24's function as the overall volume control. Bit 6 controls the high-pass filter (0 = off, 1 = on), bit 5 is the bandpass filter, and bit 4 is the low-pass filter. The low 3 bits of the cutoff frequency are determined by register 21 (Lcf) (Lcf = 0 through 7). While the 8 bits of the high cutoff frequency are determined by register 22 (Hcf) (Hcf = 0 through 255).

Through careful use of filtering, you can change the harmonic structure of any waveform to get just the sound you want. In addition, changing the filtering of a sound as it goes through the ADSR phases of its life can produce interesting effects.

ADVANCED TECHNIQUES

The SID chip's parameters can be changed dynamically during a note or sound to create many interesting and fun effects. In order to make this easy to do, digitized outputs from oscillator three and envelope generator three are available for you in registers 27 and 28, respec- tively.

The output of oscillator 3 (register 27) is directly related to the waveform selected. If you choose the sawtooth waveform of oscillator 3, this register will present a series of numbers incremented (increased step by step) from 0 to 255 at a rate determined by the frequency of oscillator 3. If you choose the triangle waveform, the output will incre- ment from 0 up to 255, then decrement (decrease step by step) back down to 0. If you choose the pulse wave, the output will jump back-and-forth between 0 and 255. Finally, choosing the noise waveform will give you a series of random numbers. When oscillator 3 is used for modulation, you usually do NOT want to hear its output. Setting bit 7 of register 24 turns the audio output of voice 3 off. Register 27 always reflects the changing output of the oscillator and is not affected in any way by the envelope (ADSR) generator.