```ATR: chpt.17: Sound

From: Doug Wokoun (aa384)
Date: 01/02/90-07:59:54 PM Z

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From: aa384 (Doug Wokoun)
Subject: ATR: chpt.17: Sound
Date: Tue Jan  2 19:59:54 1990

CHAPTER 17

SOUND

Generating sound can be very simple.  For simple sounds there are four
audio channels, each controlled by two control registers.

GENERATING SOUNDS

To generate a sound in channel 1, put the frequency and volume codes
into the frequency and control registers.  The frequency register for
channel 1, AUDF1 [\$D200 (53760)] can have any number from 0 to \$FF
(255).  0 causes the highest frequency; 255 causes the lowest.  The
volume/noise (control) register for channel 1, AUDC1 [\$D201 (53761)]
is more complicated.

Audio channel control (volume/noise) register

7 6 5 4 3 2 1 0
-----------------
AUDCx  | noise | volume|
-----------------
1 6 3 1 8 4 2 1
2 4 2 6
8

The noise bits can have various values.  The best way to learn to use
them is by experimentation.  The technical details of the polynomial
counters which generate the noise has little bearing on what is heard.
The two special values of interest are: \$1 (volume+16 in decimal),
which causes a DC voltage proportional to the volume bits and; \$A
(volume+160), which causes a pure tone (square wave).  The volume bits
select the relative volume, 0=off.  Therefore, the number, \$A8 (168
[8+160]) in AUDC1, will cause the frequency selected by AUDF1 to be a
pure tone of medium volume.

In BASIC the dirty work is done fore you.  The SOUND command will do
all the calculations for you.  The Sound command format is shown
below.

The BASIC sound command format

SOUND channel,frequency,noise,volume

The channel numbers is 0 to 3 instead of 1 to 4. The frequency, 0 to
255, is put into the frequency register.  The noise is put into the
high bits of the channel control register with volume in the low bits.
Therefore...

SOUND 0,125,10,8

will produce a pure tone of medium frequency and volume in channel 0
(called channel 1 in assembly language).

The Audio Control register, AUDCTL [\$D208 (53768)], (not to be
confused with the four audio channel control registers), adds more
control for assembly language programmers.  Again, to go into
technical details will be less productive than experimentation.

The audio control register. (AUDCTL)

7 6 5 4 3 2 1 0
-----------------
AUDCTL | | | | | | | | |
-----------------
1 6 3 1 8 4 2 1
2 4 2 6
8

7    0 = 17 bit polynomial noise
1 =  9 bit below polynomial noise
6    0 =  clock channel 1 with 64 KHz
1 =  clock channel 1 with 1.79 MHz
5    0 =  clock channel 3 with 64 KHz
1 =  clock channel 3 with 1.79 MHz
4    0 =  clock channel 2 with 64 KHz
1 =  clock channel 2 with channel 1
3    0 =  clock channel 4 with 64 KHz
1 =  clock channel 4 with channel 3
2    1 =  insert logical high-pass filter in
channel 1, clocked by channel 3
1    1 =  insert logical high-pass filter in
channel 2, clocked by channel 4
0    0 =  64 KHz main clock
1 =  16 KHz main clock

All bits of AUDCTL are normally zero.  The BASIC sound command causes
it to be reset to zero.

By clocking one channel with another, the range can be increased.
This essentially allows two channels with twice the range as each of
the four normal channels.  This is called 16 bit sound.

To calculate exact frequencies, use the following formulas.  The exact
clock frequencies are also given if more accuracy is needed.  The
clock frequencies are acquired by dividing the signal from the TV
color-burst crystal.  This crystal has a frequency of 3.579545 MHz.

Clock frequencies:

1.7897725   MHz    (color-burst/2)

63.920446 Khz    (color-burst/56)

15.699759 KHz    (color-burst/228)

Formulas:

For 1.79 MHz

clock                    clock
f = ------------         f = ------------
2(AUDFn + 7)             2(AUDFn + 4)

16 bit                    8 bit

AUDFn is the number in the audio frequency register.

For 16 KHz and 64 KHz

clock
f = ------------
2(AUDFn + 1)

AUDIO TIMER INTERRUPTS

When the audio timers count down to zero they generate IRQ interrupts
(if enabled).  The timers can be reset by writing any number to STIMER
[D209 (53769)].

THE CONSOLE SPEAKER

The console speaker is where key clicks and the cassette signals come
from.  On XL and XE models this speaker is heard through the TV
speaker.  It is operated by toggling bit 3 of CONSOL [\$D01F (53279).
This bit always reads 0 but it is actually set to 1 during vertical
blank.

Useful data base variables and OS equates

CONSOL \$D01F          (53279): bit 3 controls console speaker
AUDF1  \$D200          (53760): Audio frequency 1
AUDC1  \$D201          (53761): audio control 1
AUDF2  \$D202          (53762): Audio frequency 2
AUDC2  \$D203          (53763): audio control 2
AUDF3  \$D204          (53764): Audio frequency 3
AUDC3  \$D205          (53765): audio control 3
AUDF4  \$D206          (53766): Audio frequency 4
AUDC4  \$D207          (53767): audio control 4
AUDCTL \$D208          (53768): general audio control
STIMER \$D209          (53769): audio timer reset

--
--

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