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Re: Feedback removal in Software - An Idea!



Willmott, Paul <paul.willmott@omsl.bm> wrote:

>(snip...)
>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.
>(snip...)

This is adaptive echo cancellation, standard DSP stuff.

I have an example at home (it's one of the demo programs for
the Analog Devices ADSP-2181 eval board). It generates its
own echo for demonstration purposes, and then adaptively
removes it. It works fine.

Have a look at the discussion in any decent reference (e.g.
Adaptive Filter Theory by Haykin).

Terrestrial applications don't need to deal with Doppler,
but half-decent tuning and a bit of frequency-domain
messing around could solve the problem.

Laura Halliday VE7LDH     "Que les nuages soient notre
Grid: CN89mg               pied a terre..."
                                   - Hospital/Shafte
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