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Oversampler v1.0 April, 23. 1992
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The Oversampler isn't anything special at all. It's just a small program demonstrating the oversampling technique on the Apple IIGS. The other major feature of Oversampler is the ability to play digitized sound directly from disk without loading them into memory. The advantage is that you can instantly play long long sounds without waiting to load them from disk and you don't need to have 8 MB ram to play sounds of 8 MB - in fact Oversampler only requires 16 KB of free space!
Oversampling
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If you own a CD player you may have read in the manual that your CD player uses four or eight times oversampling to improve sound quality. Oversampler is doing the same. With 2-times oversampling the program loads 8 KB of sound into memory, doubles the wave to 16 KB in sound ram by adding an intermediate value between every two values (e.g. 10, 20, 40, 25, 15, ... becomes to 10, 15, 20, 30, 40, 32, 25, 20, 15, ...) and doubles the frequency in order to play the sound at the correct speed. 4-times oversampling quadruples wave size and frequency by inserting 3 intermediate bytes and so on...
50
45
40 x x
35 ===>
30 2-times * *
25 x oversampling x
20 x ===> x *
15 x * x
10 x x *
0 ... ...
-10
-15
Using the program
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Open first a sound file by selecting 'Open' from the 'File' menu. You can select files of any type but Oversampler treats every file as raw data. Zeros in the data are converted to $01 so you're able to play Macintosh sounds as well. (Otherwise the oscillator would stop when detecting a zero.)
A dialog box appears with following options:
Volume: Set the volume of the selected output channel.
Explained in section 'Output'
Frequency: With this scroll bar you can set the playback
rate. When you open a file the auxtype is taken
as default frequency (if 10 < auxtype < 900 else
freq = 200.) To get the frequency in Hertz just
multiply the shown rate by 51.406.
Echo Delay: Use this scroll bar to set the echo to off
(default) or to the delay you desire. A delay of
16 is about 1/3 second (16/60) so the echo
source would be approximately 55 m (180 ft)
away. Echo and oversampling mode won't work
together (yet)! The echoed channel (right) has
about 3/4 the volume of the original channel
(left).
Output: Stereo (default): Select this if you own a
stereo card. The volume control will affect the
oscillators volumes.
Left & Right: Use these options to test you
stereo card...(well, actually I don't know why I
added these options.)
Internal: If you use your internal speaker or if
you have connected your GS to an external
speaker via phone plug select 'Internal'. The
volume scroll bar will only change the system
volume.
Oversampling: Off: (default) No oversampling.
2-times/ 4-times: Enables the oversampling mode
as explained earlier. Please remind that
Oversampler internally doubles/ quadruples the
frequency rate and disk access will be twice/
four times as fast as at normal playback. Your
disk might be too slow then. (-> Play)
Close: Closes the file. (Shortcut: Command-W)
Play: Starts playing the sound from disk. Press the
ESC key to stop the playback. If your disk
device is to slow to keep up with the playback
rate you'll get a message. In this case try
decreasing the frequency or disabling the
oversampling mode. The highest rate for the
Apple 3.5" drive is about 375 (with Transwarp
and w/o oversampling). My Vulcan - and most
harddisks I think - won't have any problems at
rates above 900.
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Oversampler was written in 100% assembly language on Merlin 16+ (the best programming environment!) Resources were created on Genesys (the best resource editor!).
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Write to:
Andre Horstmann GEnie A.HORSTMANN
Hoehenweg 3d Internet shadow@beiz.mediatex.ch
CH-6300 Zug Fax +41-42-22 45 72
Switzerland