Joe --
The compander in the DSP is a replacement for compression.
The process is calld compansion because it will also do
expansion, therefore com-pression + ex-pansion.
Whether it compresses or expands, and how much of each, are
all controlled by a single parameter.
When the compander is working as a compressor, it simply
makes all quiet sounds louder -- it "compresses" all signals
*up* into a narrow dynamic range. The way it does this is by
nudging any sinusoidal frequency component in the direction
of a square wave. If you imagine a square wave, you'll see
that it spends more of its time near peak values than a sine
wave does. That's exactly what you want your compressed
sinusoids to do.
However, it comes at a price. Square waves (or
approximations of them) have much broader frequency content
than sinusoids. So in compressing your signal, you've
actually added harmonic distortion. Up to a point this is a
good thing. The additional harmonic content can add clarity
and presence to a signal, and make a voice signal more
intelligible. That's in very small doses, however. The trick
is in finding how much compression you can employ without
the ensuing harmonic distortion becoming a problem.
One reason compression is useful with voice is because the
harmonic distortion tends to reinforce overtones in the
signal itself. It's only a serious problem when the
harmonics spill out of the voice band as splatter.
Compression should be avoided entirely for any kinds of
digital signals, in any case, because the digital modes
depend on exact specification and control of frequency
components, and you don't want to be adding any spurious
content.
If you have complementary expansion at the receiving end,
you can largely undo the effects of the compression during
reception. Basically you can restore the original broader --
expanded -- dynamic range. We've put compansion into the RX
audio chain as well as TX in order to implement this tx/rx
compansion pairing. The compression is meant to be useful
alone, however.
73
Frank
AB2KT
Joe - AB1DO wrote:
Hi all,
As someone totally new to compansion, I looked at many sources on the
internet and found discriptions of the compander always requiring
involvement from both the transmitter and receiver, i.e. the transmitter
compresses and the receiver expands. However, the SDR compander involves
the transmitter only, so I'm at a bit of a loss regarding its workings.
Without diving into deep math, can someone describe what it does?
Also, should dompression and compansion always be used separately, or
may they be combined?
Thanks,
Joe
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