Fortunately, the volume level could be mostly restored with either a hardware modification (biasing the audio-in pin), or more commonly a software trick involving using the Pulse waveform to intentionally recreate the required bias. The better manufacturing technology in the 8580 used in the later revisions of Commodore 64C and the Commodore 128DCR caused the bias to almost entirely disappear, causing the digitized sound samples to become very quiet. Also, it was hugely CPU intensive - one had to output the samples very fast (in comparison to the speed of the 6510 CPU). The amount of sampled sound possible to store on a fraction of 64 kilobytes was very limited. The first instance of samples being used in actual musical compositions was by Martin Galway in Arkanoid (1987, Imagine), although he had copied the idea from an earlier drum synthesizer package called Digidrums. The glitch was known and used from an early point on, first by Electronic Speech Systems to produce sampled speech in games such as Impossible Mission (1983, Epyx) and Ghostbusters (1984, Activision). By adjusting the amplifier's gain through the main 4-bit volume register, this bias could be modulated as PCM, resulting in a "virtual" fourth channel allowing 4-bit digital sample playback. The ring modulation, filter, and programming techniques such as arpeggio (rapid cycling between 2 or more frequencies to make chord-like sounds) together produce the characteristic feel of SID music.ĭue to imperfect manufacturing technologies of the time and poor separation between the analog and digital parts of the chip, the 6581's output (before the amplifier stage) was always slightly biased from the zero level. An external audio-in port enables external audio to be passed through the filter. The programmer may vary the filter's cut-off frequency and resonance. Using a combined state of lowpass and highpass results in a notch (or inverted bandpass) output. The filter has lowpass, bandpass and highpass outputs, which can be individually selected for final output amplification via master volume register. Oscillators may also be hard-synced to each other, where the synced oscillator is reset whenever the syncing oscillator's msb raises.Įach voice may be routed into a common, digitally controlled analog 12dB multistate filter, which is constructed with aid of external capacitors to the chip. A voice playing Triangle waveform may be ring-modulated with one of the other voices, where the triangle waveform's bits are inverted when the modulating voice's msb is set, producing a discontinuity and change of direction with the Triangle's ramp. The SID features three-voice synthesis, where each voice may use one of at least five different waveforms: square wave (with variable duty cycle), triangle wave, sawtooth wave, pseudo-random (but not white) noise, and certain complex/combined waveforms when multiple waveforms are selected simultaneously. All control ports are digital, while the output ports are analog. The SID is a mixed-signal integrated circuit, featuring both digital and analog circuitry. external audio input (for sound mixing with external signal sources).two 8-bit A/D converters (typically used for game control paddles, but later also used for a mouse).oscillator sync for each audio oscillator.three attack/decay/sustain/release (ADSR) volume controls, one for each audio oscillator.The different filter-modes are sometimes combined to produce additional timbres, for instance a notch-reject filter. one multi mode filter featuring low-pass, high-pass and band-pass outputs with 6 dB/oct (bandpass) or 12 dB/octave (lowpass/highpass) rolloff. four different waveforms per audio oscillator (sawtooth, triangle, pulse, noise).three separately programmable independent audio oscillators (8 octave range, approximately 16 - 4000 Hz).4 C64 SID on Amstrad CPC, software or hardware ? || SID file & AY+ file.
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