German wrote:

> I want to decode radio amateur packets,they come at 1200 bps
> but my receiver does not have digital output: may I use a simple
> amplifier and signal shaper (Two anti-parallel connected diodes
> and one transistor), to convert analog (processed) output into
> square TTL level signal?

  This is called the "HamComm" interface, used with a program of that
name, so look for it on the 'Net.

  Note however that this only works for nice clear signals, for those
with any noise at all, you want to utilise all the analog nuances of the
signal using a purpose-built modem as previously suggested.
--
  Cheers,
        Paul B.