

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
------------

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
-----------------

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
