Utilities Professional 1-1500/Utilities Professional 1-1500 (1994)(WPD)[!].iso/07511000/var0859.dms/var0859.adf/Audiotool.doc

Text File | 1989-08-10 | 10KB | 198 lines

    It is recommended that you print this out and then run the program
and experiment as you read.
==========

    Audiotool was written to provide direct access to the Amiga's sound
generating hardware.  The Amiga has four Direct Memory Access (DMA)
channels and four Digital to Analog (D/A) Converters (also called DACs).
The DACs feed two audio line outputs (the left and right jacks on the back).

    In the normal operating mode, a DMA produces a stream of numbers
(representing an audio waveform) to a DAC which converts it (with help from
a filter stage and other hardware) to an analog signal which is then sent
to a line output (represented by a speaker).  The line output provides
a continuos analog signal varying in strength within the limits of
+/- 0.4 Volts.  If the line is connected to an amplifier and speaker,
the speaker vibrates, generating air vibrations we call sound.

    The timbre of the sound (the quality of a sound which differentiates
it from others of the same frequency and amplitude) is controlled by the
stream of numbers feeding the DMA.  These are represented by four waveform
representations in boxes (sine, square, ascending and descending sawtooth
respectively).  You may select one by clicking on a box with the left mouse
button.  The pitch or frequency of the sound is determined by two factors,
the number of samples in the waveform representation, and the sampling
frequency being used.  Each number in the waveform representation is called
a sample.  The sequence of samples (in this program) represents one waveform
cycle.  One waveform cycle is one up and down movement of a wave (as
represented by the waveform boxes).  The top row of numbered boxes
represent the number of samples in the waveform.  A different number may be
selected by clicking in a box with the left mouse button.  The more samples,
the lower the output frequency.

    The sampling frequency is the number of samples that are output from
the DMA every second.  This is controlled by the top slide pot.
This pot may be thought of as a speed control for the DMA.  The number
selected by this pot is displayed in the box just above the slide pot
on the left.  This box is also a numeric input gadget.  By clicking the
left mouse button in this box and then using the delete, backspace, and
numeric keys, you may enter a new number.  But its easier to just hold
the left mouse button down on the slide pot and move it back and forth.
As the number selected increases, the output frequency decreases.

    The output frequency is displayed above the slide gadget to the right
in Hertz.  One Hertz is one cycle of sound per second.

    The DAC also has a slide pot, it controls the volume or amplitude of
the sound (how loud it is).  It operates similar to the other slide gadget.
The bigger the number, the louder the sound.

    The outputs of the DACS are hardwired to the line outputs.  In other
words, there is no way to move a particular DAC's output to the other line
(left or right) instead of the one it is connected to.

    The DMA inputs and output are much more versatile however.  Each DMA
has an on/off switch.  It is the crude knife switch image just under the
DMA box.  Each knife switch may be toggled on/off or up/down by clicking
the left mouse button over the image.  (Yes, yes, always the left button,
the right button is useless with this program, there aren't even any menus
for Pete's sake.)  The speed of a DMA controller may be switched from the
manual pot to the output of the DMA on its left.  This means that the
frequency of the right DMA's output will vary by the waveform selected as
input by the left DMA unit.  Try this example, turn on the second DMA and
select a nice audible tone.  Now turn on the first DMA and select a
frequency of about 2 Hz.  To do this, select the 128 samples box and then
move the frequency slide to the right.  As you get below 30 Hz, the sound
will become inaudible except for a clicking sound if you have selected a
waveform with a sharp edge.  Now click the second DMA's top toggle switch
(the one that disconnects the DMA from the frequency slide gadget and
connects it to the first DMA's output).  Now select different waveforms.
Fun, no?

    In the same way, the volume of a DAC may go from a steady level under
manual control to modulation by the DMA channel on it's left.  You can
even modulate both frequency and volume at the same time.



Features:

    There is no way to quickly switch the waveform input of a DMA
channel.  One must either turn off the DMA channel for more than two
samples or let the hardware automatically reload its input address
register after it finishes its current cycle.  This program uses the
latter.  It becomes apparent when small frequencies are being used.
Waveforms will not switch immediately when alternate waveforms are
selected.  When switches are thrown changing the routing of DMA output,
some strange sounds are produced until the DMA resets itself with the
new waveform.  Sometimes, the DMA does not recover by itself.  In these
cases, the former method may be manually used by clicking off the DMA,
and then clicking it back on.

Equations:

    The frequency slide pot produces a number which is output to the
Amiga's period register.  It is a clock divider value.  It divides the
master clock rate of 3,579,545 ticks per second to produce the sampling
frequency used by the DMA unit.

Letting         P = period register value
                N = number of samples per cycle in waveform representation
                F = output frequency

  cycles             ticks               ticks                samples
F ------ = 3,579,545 ------ divided by P ------- divided by N -------
  second             second              samples              cycle


    The volume level is represented in decibels.  This is the ratio of the
output level to the maximum output level.  The volume register can take on
values from 1 to 64 (0 is off) yielding dB values according to the following
formula:

Letting         V = volume register value
                D = decibel rating

           D = 20 * common log ( V / 64 )



Technical notes:

    I have tried to produce a program that graphically interacts with
the Amiga hardware that is simple yet not limiting.

    The DMA on/off switches each represent one bit in the DMA control
register (DMACON).  The remaining three switches per channel represent
two bits in the audio control register (ADKCON).  Hence not all switch
combinations are legal.  Opening up the switch that controls the feed
into the top of a DAC for instance, must force the closure of at least
one of the pot control switches for the channel on the DAC's right.
The program arbitrarily closes the period control switch in this
case because the author feels this is the most interesting case of the
three.  Here is a chart showing the bit values and the corresponding
switch positions.

 feed     feed       "DAC feed"   "period feed"   "volume feed"
period   volume        switch        switch          switch

  0        0           closed         open            open
  1        0            open         closed           open
  0        1            open          open           closed
  1        1            open         closed          closed

    To allow a full range of frequencies, I had to split the operation
into six octaves.  The middle four allow the frequency pot to select
period values from 200 to 400.  This is to allow reasonable operation of
the slide gadget.  However, any valid period value may be input with
the numeric keys.  Valid values are 124 to 65,535 inclusive.  The outer
two octave ranges lift the normal value restrictions to allow for more
than a full octave.  Here is a chart that shows what I mean.

"samples"    Pot selectable       Frequency
 number      period values          range

   4          124 to   400    7.2 kHz to 2.2 kHz
   8          200 to   400    2.2 kHz to 1.1 kHz
  16          200 to   400    1.1 kHz to 560  Hz
  32          200 to   400    560  Hz to 280  Hz
  64          200 to   400    280  Hz to 140  Hz
 128          200 to 65535    140  Hz to 0.43 Hz

    The Amiga 1000 has a low pass filter that operates from 4 kHz to 7 kHz.
It is possible to hear antialising distortion above 5 kHz with this program
for this reason.

    Each octave range has its own set of samples for each of the four
waveforms.  This means that there are 6 * 4 or 24 sets of samples, not
just four.  But wait, its still more complicated yet.  The DAC's take
values ranging from -127 to 128, so that when a DMA is directly feeding
a DAC, the waveform samples must have values ranging from -127 to 128.
But when a DMA is switched to modulate the next channel's period register,
it must produce valid period register values.  These values range from 124
to 65,535 as we have seen.  This means another 24 sets of waveform sample
values.  When modulating periods, the samples take on values from 200 to
400, thus modulating frequencies by exactly one octave.  When modulating
the next channel's volume register, values from 10 to 64 are used, thus
modulating volume by 16 decibels.  When modulating both volume and period,
twice as many samples are required.  One for volume, then one for period,
etc.  Therefore there are:

4 (waveforms) * 6 (octave ranges) * 4 (operating modes) = 96 (sets of samples)

The program automatically switches between these as required to make
operation seem simple.

  The source code is available to answer other questions.

This program expects to run in a 640 x 200 sized workbench window.
A custom window may be opened instead by making a quick change to
window.c.

Comments, ideas, code improvements, and questions may be sent
to the author.  No reply guaranteed.

        David Reynolds
        32 Greenwood Rd.
        Burlington, Ma  01803