>From: tlindner_at_ix.netcom.com
>
>> Here's one:
>>
>> http://www.kim-1.com/usrman.htm#E
>
>Ahh, this is really diffferent!
>
>This uses:
>
>'0 bit' - 18 square cycles at 3700 Hz.
> 6 square cycles at 2400 Hz.
>
>'1 bit" - 9 square cycles at 3700 Hz.
> 12 square cycles at 2400 Hz.
>
>Both are the same length (7.452 microseconds).
>
>As you can probably tell, I am currently anaylizing this at a very low
>level.
>
>--
>tim lindner tlindner_at_ix.netcom.com
>
Hi
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.
It looks something like this:
___ _ ___ _
|___| |_| |_| |______
Depending on past history, this might be:
0 0 1 1 0 0 0
or
1 1 0 0 1 1 1
This is the type of encoding used on my Poly88 for
what they call PolyPhase. This runs at 2400 baud.
There is a standard called Byte format that uses 300 baud
and the two tones 1200 and 2400.
Besides the type of encoding, you also need to know
what the record formats looks like. Do you have one long
record for the entire file? What does the header look like?
Is it synchronous or asynchronous? Is there parity, check sum,
CRC or ECC?
There are a lot of things that need to be built up into
a complete tape decoder. A tape writer would be used
by more than a tape reader. One should only need to read
the tape once but many people would often like to create
a tape from that same data. You really want both.
One should also note that most audio tape to digital
inputs on these old computers filtered the signal some
to optimize the audio signal for decode. This was done
on both read and write of the signal.
Dwight
Received on Tue Dec 17 2002 - 17:58:00 GMT