Topband: PSK-31

[Rick Karlquist]

Garry Shapiro said:
> Mike:
>
> You said:
>
> "I am not entirely
> familiar with the PSK31 specification, but I suspect that
> they are using root-raised cosine symbol shaping to
> minimize the occuppied bandwidth of the signal."
>
>
> Although I have not delved into PSK31 deeply, either, I have to take
> issue with your raised-cosine statement. It's mostly not about
> minimizing bandwidh.
>

You're both right.  The RRC filter does limit occupied bandwidth
at the transmitter and limits noise bandwidth at the receiver, and
does it (as you describe) in a way that does not introduce harmful
ISI.  What makes the RRC filter different from just any old filter
is the lack of harmful ISI.  But you need at least some kind of receiver
filter in any system, no matter how simple.

The raised cosine filter was superceeded 40 years ago by Lender's partial
response (duobinary) filters, although you still see RC being designed in
even today (don't ask me why).  The disk drive industry finally converted
to PR filters about 10 years ago (ie PRML).

Rick N6RK

Rick N6RK