Computer Audio Tape Formats - test-drb@ccmp.vtda.org

17 Dec 2002

On Dec 17, 16:00, Dwight K. Elvey wrote:

...
   Here is one that is even a little different still.
 It is what is called Manchester encoding, using biphase
 clocking. You don't indicate 1 or 0, you indicate a change
 by a quick clock and no change by a long clock. 
...
   This is the type of encoding used on my Poly88 for
 what they call PolyPhase. This runs at 2400 baud. 
The Exidy Sorcerer also uses Manchester encoding.  For 1200 baud this
equates to 1 cycle of 1200 Hz for a logic 1, and a half cycle of 600 Hz for
a logic zero.  For 300 baud it becomes 8 cycles of 2400 Hz and 4 cycles of
1200 Hz.  It sounds like plain FSK, but if you draw the waveform for
"101100" at 1200, you'll see it's actually a Manchester code:

     __    _____    __    __       _____
    |  |__|     |__|  |__|  |_____|     |

     --1-- --0-- --1-- --1-- --0-- --0--

...
  There is a standard called Byte format that uses 300
baud
 and the two tones 1200 and 2400. 
Byte format is Kansas City format.  It was proposed in Byte and formalised
at the KC meeting.

I think the Commodore PET uses modified CUTS.  It records each block twice,
which is why it's relatively slow, but also pretty robust.

-- 
Pete						Peter Turnbull
						Network Manager
						University of York