Jim Main wrote:

> (It's a bi-phase mark waveform for those that've never heard of it - a
> '0' is represented by a full pulse width (either positive or negative)
> and a '1' has an extra transition in the middle giving two pulses in
> the same pulse width as the '0' - again, either polarity.

  Technically, that's called Manchester type 1 encoding.

  Data is recoverable most simply by using (or emulating) a
retriggerable monostable with a period of 3/4 of the clock period.
Trouble is, you have to lock in the clock.  Best done by a software PLL
which synchronizes on transitions when they exist, but doesn't alter its
timing when they don't.
--
  Cheers,
        Paul B.