Topband: DSP and Digital Filter Effectiveness for

[John Kaufmann]

Barry N1EU wrote:

> Just wanted to echo Bill's AGC comment.  I put a couple of back-to-back
> diodes in the headphone line as an ear saver and go AGC-less during bursty
> static with the 1000MP.  Even the fast AGC release time is too slow in
these
> condx to maintain copy of a weak signal under static bursts.


Turning off the AGC is a very good idea.  I've also done the back-to-back
diodes and it's very effective.  The diodes do introduce a lot of audio
distortion when they go into heavy clipping.  To fix this I followed the
diodes with a narrowband audio filter to clean up the distortion and it
works amazingly well.

I also agree the "fast" AGC on the 1000MP is really not fast enough to deal
with static bursts.  The IC-781 has much faster AGC release time in its
fastest setting and works well for isolated crashes.  The problem with
really fast AGC, however, is that (like the diodes) it introduces some
distortion, especially when there are multiple signals in the passband, as
there are in pileups.  There is an INRAD AGC mod to fix the problem in the
1000MP--it slows down the AGC even more, which makes it better in pileups
but less effective in bursty static.  Even in the IC-781 you can hear the
distortion increase in pileup situations as you crank down the AGC time
constant.  No one setting is optimal for everything but at least the 781
time constant can be fine-tuned for different scenarios, which is why I like
it so much.

73, John W1FV