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RE: Feedback removal in Software - An Idea!

One problem I see.  You want to be able to hear yourself.  Not as important
on the FM birds, but very important on the SSB birds.  A complication (also
on the SSB birds) is the downlink audio most likely wont be EXACTLY the same
as the uplink due to doppler shift.  Makes it much harder for the DSP to
cancel out the downlink audio - not impossible, just harder.

Jim Walls - K6CCC
Ofc:  626-302-8515    FAX:  626-302-9999
AMSAT member 32537 / WSWSS member 395

From:  Willmott, Paul[SMTP:paul.willmott@omsl.bm]
Sent:  Friday, June 30, 2000 11:20 AM
To:  'amsat-bb@amsat.org'
Subject:  [amsat-bb] Feedback removal in Software - An Idea!

I've been playing with some rather cool semi-pro audio restoration software,
not only does it takes the scratches out of old LP recordings but has some
other neat stuff too.

1). You can remove the lead vocalist leaving the band intact

2). While bugging a room with a TV on in the background (so as to mask the
people speaking), you can feed the clean TV sound into the software which
can then remove it by subtraction and suddenly the voices are clearly

Now our satellite feedback could be fixed the same way, so that we will
never get feedback again and will be able to listen on a speaker at whatever
volume without using phones.

1). If the speaker is on the output end of a full-duplex PC sound card to
start with
2). The sound card is being fed by some clever dsp software
3). When transmitting: the voice into the rig is tapped off and sent into PC
sound card to be recorded as well as feeding the rig
     The audio from the RX is fed into the PC and mixed with a time delayed
playback of the recorded audio. The dsp software does the extraction of the
recorded uplink voice, the time delay is the two way doppler shift, which
any decent tracking program can calculate from the RX and TX freqs etc.
4). The DSP software then feeds the clean audio out to the speaker.

The net result is that your voice will not appear on your downlink speaker,
and hence will not get feedback. What you will hear is everything else apart
from your voice in the downlink.

Now the hardware is easy, full duplex sound cards don't cost that much more
than the cheap ones.
The interface to the tracking software is easy, .. 
The interesting bit is the DSP appication, ... any volunteers! I'll provide
the tracking software interface, if someone can code the DSP plug-in.

Naturally there will have to be a phones button so that you can setup and
find yourself <g>.

Paul, VP9MU
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