I am not sure about the voice part, but manchester encode and decode is
fairly straight forward.  Just the other day, someon listed a web site that
had some already done manchester encoding schemes, but I dont have the URL
here.  If you dont get it by Tuesday, I will send it then.  I did not look
at the link myself.  I have written a manchester decoder which I use
extensively.  The encoding is striaght forward, the decoding is tricker.
After many problems, I came up with a routine to sample each half cell five
times and decide if that half cell is high or low, based on a majority
count.  Then, make sure that each full cell has two complimentary bits.

Hope that helps some.

Shawn

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