Z89340 datasheet(15/30 Pages) ZILOG

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Z89340

Digital Wavetable Engine

DS96DSP0201

P R E L I M I N A R Y

1-15

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

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.)

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 21/12 frequency

multiples; a cent is equivalent to 21/1200 frequency

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

π/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

π/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.

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