z8934050fsc ZiLOG Semiconductor, z8934050fsc Datasheet - Page 15

• Manufacturer Part Number: z8934050fsc
• Description: Digital Wavetable Engine
• Manufacturer: ZiLOG Semiconductor

Wavetable Synthesis
Another method of synthesizing sound is sometimes
called wavetable synthesis. The Wavetable Mode Oscilla-
tor has some wavetable-synthesis extensions that set it
apart from the Sample Loop Oscillator. With wavetable
synthesis, one or more complete periods of a waveform
are recorded and stored in a wavetable. The wavetable is
then played at the desired frequency. This is similar to the
loop described earlier except that the wavetable is often
not a recording, but a single period of a sound created
through additive synthesis. The Z89340 can move to other
wavetables or stay on the same wavetable during the life
of a note.
To play a note or sample sequence from waveform ROM,
you would set the following: desired frequency, wave be-
gin and end addresses, wave loop length, the initial ampli-
tude envelope begin and end values, envelope rate, Am-
plitude/Tremolo/Filter/Pan (ATFP) envelope type to
amplitude, output channel(s), effects send(s), pan loca-
tion, and filter tuning values. All of these settings can be
changed during the life a note as desired. All oscillators are
completely independent of each other, even for features
such as vibrato rate and filter cutoff points.
The Z89340 assumes that the amplitude envelope will oc-
cur in multiple segments-attack segment, several initial de-
cay segments, possibly a sustained segment, and several
final decay segments. On-chip support is given for one en-
velope segment at a time. An interrupt is generated when
the segment end is reached, at which time the host CPU
will set up the next amplitude segment, supplying a new
amplitude end value and envelope rate (the slope that de-
fines how long it will take to reach the end amplitude). It is
not critical that the interrupt be serviced immediately; a de-
lay of 10–20 milliseconds (ms) normally is not noticed; the
amplitude merely remains stationary until the new seg-
ment is initiated. The Z89340 has amplitude steps well be-
low the threshold of perceptibility, so there is no zipper
noise. If there is a sustained segment (one where the am-
plitude does not change), the Envelope-type ATFP con-
trols can be used to define the envelope rate parameter as
tremolo rate; tremolo depth can then be set. If tremolo is
not needed, the ATFP envelope system can also be used
for variable pan or swept filter. (Refer to the Oscillator Pa-
rameter Block section for a detailed description of each
control bit.)
DS96DSP0201
P R E L I M I N A R Y
Vibrato is independent of the ATFP system. There are 16
settings of vibrato rate ranging from 0.1–10 Hz, and 16 dif-
ferent settings of vibrato depth ranging from plus or minus
a few cents to two semitones.
Note: A semitone is equivalent to 2
multiples; a cent is equivalent to 2
multiples.
The vibrato value, which is derived from a sinewave in-
dexed by the vibrato phase accumulator, is added to or
subtracted from the frequency. The initial vibrato phase
can be set by writing a value to the 8-bit vibrato phase ac-
cumulator-value of 64 is /2 radians, which would start the
vibrato at maximum positive swing.
Each oscillator has its own second-order (two-pole, two-
zero) digital filter. At the initialization of an oscillator, the
two delays should be given values of 0 unless a click is de-
sired. The filter Q (in the Control Byte) and the filter tuning
value are used together to set the desired characteristics
of the filter. Low-pass filters with varying amounts of Q are
available. A few useful high-pass and band-pass filter set-
tings are also provided. The ATFP envelope system can
be used to create a variable or swept low-pass filter. (Refer
to the tables in the Oscillator Parameter Block section for
details.)
The Polar Pan Control provides selection of output chan-
nels and pan between or among up to four output chan-
nels. When four channel quadraphonic output is selected,
the spatial location is specified with a modified polar coor-
dinate-a value of 16 is /2 radians. The radius select is a
two-bit number with 2 at the edge of the circle and 0 near
the center of the circle. A radius of 3 is reserved for stereo
panning when only two output channels are needed. (Oth-
er items such as effects channels and submix channels
are covered in sections that follow.) All parameters in the
Oscillator Parameter Block can be modified by the CPU
during the life of a note.
Digital Wavetable Engine
1
/1200 frequency
1
/12 frequency
Z89340
1-15
1