Date   : Mon, 12 Feb 1996 19:27:38 +0100
From   : Ole Stauning <os@...>
Subject: Re: Decoding BBC tapes...

Sorry for my lousy figure, that i broadcasted to you all!

If the following data is correct (based on Roberts info)

0 ~ 2400 Hz
1 ~ 1200 Hz
Data rate 1200 baud

then things are a bit critical. You will have the following picture:


 
 [--- sample 1 ----]         [--- sample 2 ----]         [--- sample
3 ----]  .....

 
 |<----------- T1 ---------->|<----- T2 ------>| 
 
 T1 is the time between two successive sound-samples.
 T2 is the duration of a sound-sample.

(1/2400) sek < T2 < T1 = (1/1200) sec

And you have T1 - T2 secs to determine each bit (if you do it realtime)

Assume that you have just sampled sample 1. Then you intercept the first
zero in this sample, 
lets call the location t1. Then you start looking for the next zero at
t1+(0.5/2400)sek
by searching both forward and backwards in time ( eg. 0.125/2400 secs
in each direction )
If you find a zero crossing, then the signal is 2400Hz, else it must
have been a 1200Hz.

You have to do some sync. between each block of data (i remember something
about a click
just before every block)


Maby someone is kind enough to send me 2 blocks of data WAV (maby 22kHz,!!MONO!!)
gziped
or pkziped and uuencoded ( with some secret message for me to hack :-)

Regards

Ole


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