FREQ LO/FREQ HI (Registers 00,01)
Together these registers form a 16-bit number which linearly controls the
frequncy of Osscillator 1. The frequency is determined by the following equation:
Fout = (Fn * Fclk / 16777216) Hz
Where Fn is the 16-bit number in the frequency registers and Fclk is the
system clock applied to the Phi2 input (pin 6). For a standard 1.0 MHz clock,
the frequency is given by:
Fout = (Fn * 0.059604645) Hz
It should be noted that the frequency resolution of SID is sufficient for
any tuning scale and allows sweeping from note to note (portamento) with
no dicernable frequency steps.
PW LO/PW HI (Registers 02,03)
Together these registers form a 12-bit number (bits 4-7 of PW HI are not
used) which linearly controls the pulse width (duty cycle) of the pulse
waveform on Oscillator 1. The pulse width is determined by the following
equation:
PWout = (PWn/40.95) %
Where PWn is the 12-bit number in the Pulse Width registers.
The pulse width resolution allows the width to be smoothly swept with no
discernable stepping. Note that the pulse waveform on Oscillator 1 must
be selected in order for the pulse width registers to have any audible effect.
A value of 0 or 4095 ($FFF) in the pulse width registers will poroduce a
constant DC output, while a value of 2048 ($800) will produce a square wave.
[A value of 4095 will not produce a constant DC output].
CONTROL REGISTER (Register 04)
This register contains eight control bits which select various options on Oscillator 1.
GATE (Bit 0): The GATE bit controls the envelope generator for voice 1. When this bit is set to one, the envelope generator is gated (triggered) and the ATTACK/DECAY/SUSTAIN cycle is initiated. When the bit is reset to zero, the RELEASE cycle begins. The envelope generator controls the amplitude of Oscillator 1 appearing at the audio output, therefore, the GATE bit must be set (along with suitable envelope parameters) for the selceted output of oscillator 1 to be audible.
SYNC (Bit 1): The SYNC bit, when set to one, synchronizes the fundamental frequency of oscillator 1 with the fundamental frequency of oscillator 3, producing "Hard Sync" effects. Varying the frequency of oscillator 1 with respect to oscillator 3 produces a wide range of complex harmonic structures from voice 1 at the frequency of oscillator 3. In order for sync to occur, oscillator 3 must be set to some frequency other than zero but preferable lower than the frequency of oscillator 1. No other parameters of voice 3 have any effect on sync.
RING MOD (Bit 2): The RING MOD bit, when set to a one, replaces the triangle waveform of output of oscillator 1 with a "Ring Modulated" combination of oscillators 1 and 3. Varying the frequency of oscillator 1 with respect to oscillator 3 produces a wide range ofnon-harmonic overtone structures for creating bell or gong sounds and for special effects. In order for ring modulation to be audible, the triangle waveform of oscillator 1 must be selected and oscillator 3 must be set to some frequency other than zero. No other parameters of voice 3 have any effect on ring modulation.
TEST (Bit 3): The TEST bit, when set to one, resets and locks oscillator 1 at zero until the TEST bit is cleared. The noise waveform output of oscillator 1 is also reset and the pulse waveform output is held at a DC level. Normally this bit is used for testing purposes, however, it can be used to synchronize oscillator 1 to external events, allowing the generation of highly complex waveforms under real-time software control.
/\/\ (Bit 4): When set to a one, the triangle waveform output of oscillator 1 is selected. The triangle waveform is low in harmonics and has a mellow, flute-like quality.
/// (Bit 5): When set to one, the sawtooth waveform output of oscillator 1 is selected. The sawtooth waveform is rich in even and odd harmonics and has a bright, brassy quality.
PULSE (Bit 6): When set to one, the pulse waveform output of oscillator 1 is selected. The harmonic content of this waveform can be adjusted by the pulse width registers, producing tone qualities ranging from a bright, hollow square wave to a reedy pulse. Sweeping the pulse width in real-time produces a dynamic "phasing" effect which adds a sense of motion to the sound. Rapidly jumping between different pulse widths can produce interesting harmonic sequences.
NOISE (bit 7): When set to one, the noise output waveform of oscillator 1 is selected. This output is a random signal which changes at the frequency of oscillator 1. The sound quality can be varied from a low rumbling to hissing white noise via the oscillator frequency registers. Noise is useful in creating explosions, gunshots, jet engines, wind, surf and other unpitched sounds, as well as snare drums and cymbals. Sweeping the oscillator frequency with noise selected produces a dramatic rushing effect. [The Noise output is not "true" random, as it is based on a 23 bit internal register].
NOTE: The oscillator output waveforms are NOT additive. If more than one output waveform is selected simultaneously, the result will be a logical ANDing of the waveforms. [Transcriber's note: This is most probably wrong. Some claim that it is the "min" function that is used, but my experience is that it is more complex than that].
Although this technique can be used to generate additional waveforms beyond the four listed above, it must be used with care. If any other waveform is selected while noise is on, the noise output can "lock up". If this occurs, the noise output will remain silent until reset by the TEST bit or by bringing RES (pin 5) low.
ATTACK/DECAY (Register 05)
Bits 4-7 of this registers (ATK0-ATK3) select 1 of 16 ATTACK rates for the voice 1 envelope generator. The ATTACK rate determines how rapidly the output of voice 1 rises from zero to peak amplitude when the envelope generator is gated. The 16 ATTACK rates are listed in Table 2.
Bits 0-3 (DCY0-DCY3) select 1 of 16 DECAY rates for the envelope generator. The DECAY cycle follows the ATTACK cycle and the DECAY rate determines how rapidly the output falls from the peak amplitude to the selected SUSTAIN level. The 26 DECAY rates are listed in Table 2.
SUSTAIN/RELEASE (Register 06)
Bits 4-7 of this register (STN0-STN3) select 1 of 16 SUSTAIN levels for the envelope generator. The SUSTAIN cycle follows the DECAY cycle and the output of voice 1 will remain at the selected SUSTAIN amplitude as long as the gate bit remains set. The SUSTAIN levels range form zero to peak amplitude in 16 linear steps, with a SUSTAIN value of 0 selecting zero amplitude and a SUSTAIN value of 15 ($F) selecting the peak amplitude. A SUSTAIN value of 8 would cause voice 1 to SUSTAIN at an amplitude one-half the peak amplitude reached by the ATTACK cycle.
Bits 0-3 (RLS0-RLS3) select 1 of 16 RELEASE rates for the envelope generator. The RELEASE cycle follows the SUSTAIN cycle when the gate bit is reset to zero. At this time, the output of voice 1 will fall from the SUSTAIN amplitude to zero amplitude at the selected RELEASE rate. The 16 RELEASE rates are identical to the DECAY rates.
NOTE: The cycling of the envelope generator can be altered at any point via the gate bit. The envelope generator can be gated and released without restriction. For example, if the gate bit is reset before the envelope has finished the ATTACK cycle, the RELEASE cycles will immediately begin, starting from whatever amplitude had been reached. If the envelope is then gated again (before the RELEASE cycle has reached zero amplitude), another ATTACK cycle will begin, starting from whatever amplitude had been reached. This technique can be used to generate complex amplitude envelopes via real-time software control.
Table 2. Envelope Rates
VALUE ATTACK RATE DECAY/RELEASE RATE Time/Cycle Time/Cycle - ------------------------------------------ 0 2 ms 6 ms 1 8 ms 24 ms 2 16 ms 48 ms 3 24 ms 72 ms 4 38 ms 114 ms 5 56 ms 168 ms 6 68 ms 204 ms 7 80 ms 240 ms 8 100 ms 300 ms 9 240 ms 750 ms 10 500 ms 1.5 s 11 800 ms 2.4 s 12 1 s 3 s 13 3 s 9 s 14 5 s 15 s 15 8 s 24 s
Picture 3. 6581 Envelope
NOTE: Envelope rates are based on a 1.0 MHz ø2 clock. For other ø2 frequencies, multiply the given rate by 1 MHz/ø2. The rates refer to the amount of time per cycle. For example, given an ATTACK value of 2, the ATTACK cycle would take 16 ms to rise from zero to peak amplitude. The DECAY/RELEASE rates refer to the amount of time theses cycles would take to fall from peak amplitude to zero.