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Six-Bit DAC, A 6-bit audio DAC for any Apple II
by
Greg Templeman
Copyright (c) 1993
FREEWARE
INSTRUCTIONS
Run the SOUND.EDITOR.6 program to play sounds. If you have an accelerated
Apple (faster than 1 MHz), you will have to slow the speed down to 1 MHz
("Normal" on a GS) to have the sounds play properly.
SIX.BIT.DAC implements a 6-bit DAC in software, with a sample rate of 11
kHz. It uses ALL of the $300-$3CF space. You can use it separately to
play back sound files in your own programs, but you should note that it
DOES NOT use the same entry points as M. Mahon's 5-bit DAC, SOFT.SOUND.
Also included by permission is a modified version of Mahon's SOUND.EDITOR
Version 1.1/6, changed only to use my 6-bit DAC instead of the previous
5-bit player.
Contrary to previous belief, 5-bit sound is not the best your "stock"
Apple II can play: 6-bits is the TRUE limit. No one can make a 1MHz Apple II
play 11kHz sounds with any more bit-resolution than this one (i.e. this is
as good as it gets!)
INFORMATION
For an informative an interesting history of sound on the Apple II, see
Michael Mahon's documentation for his 5-bit DAC. I am indebted to Mr. Mahon's
clear explanation of how multi-bit sound is produced on a "stock" Apple II.
Please note that SIX.BIT.DAC is a totally different program than the 5-bit
SOFT.SOUND. Mr. Mahon believed that 6-bit sound could not be done, and it was
only by using a timing technique radically different than his that I have been
able to produce a 6-bit sound player.
Previous 3- and 4-bit sound players for the Apple II cannot compare in quality
to SIX.BIT.DAC. The amount of distortion in the sound is noticeable compared
to the 6-bit. Mr. Mahon's 5-bit comes close, but doesn't have constant digital
"steps" in the waveform (because of limitations of his timing approach). Thus,
SIX.BIT.DAC reduces sound noise by more than the 50% one would expect from
adding an additional bit of resolution. This means noise should now be about
1%. Note, however, that the 11kHz "carrier" tone is a given of this method of
sound reproduction; if you don't have some external means of filtering it out,
you may not be able to hear the improvement in the 6-bit player. By using
synthesized sine waves, I was able to hear the difference between 5-bit and
6-bit players even with the carrier noise.
Whether or not you notice the sound improvement with the sounds you play,
however, you will still get the other advantage of increased bit-resolution:
a greater dynamic range than other sound players, even with smaller digital
steps. That is, my 6-bit DAC plays sounds louder than 5-bit (or fewer) players.
TECHNICAL INFO
My software DAC loads at $300 (and takes all the space from $300 to $3CF),
and is called at $32E after setting up the start and end addresses at 6/7 and
8/9, respectively. Note that, although the start and end are set up the same
as Mahon's player, THE ENTRY POINT IS DIFFERENT. Thus, if you are converting
programs over to run with my sound player, you will need to change your call
address to $32E (814). Also, if you are toggling the speaker/cassette outputs,
you will need to use different addresses than those in the 5-bit; I have 4
sound switch toggles instead of Mahon's 2. See the code for specifics or send
me mail.
Also, my sound player has the same requirement as Mahon's, that the sound
sample end with a $00 byte. This is because, as in his program, there is not
enough time in the loop to check if the end address has been reached for every
address. You can include 0's in the sound, as the program won't stop before
the end even if a sound is encountered.
FOR MORE INFORMATION
Download Michael Mahon's 5-bit sound player for comprehensive background,
theory, and documentation of Apple II DAC's, as well as the editor docs.
Get in touch with me for more information:
Greg Templeman
833 Hawthorne Drive
Walnut Creek, CA 94596
America Online: Barnabas
InterNet: barnabas@aol.com