Karl Larsen <[EMAIL PROTECTED]> wrote:
Hi Geoff, cut your weeds and we can both think hard on what "Speech
Compression" really is. I got to thinking and about all it can be is
clipping of the high voice peaks, and increasing audio drive. I will look
at the ARRL Hand Book and see what they have.
----------------------------------------------------------------
Hi Karl,
My apology for my delay in getting back to you earlier, as a Scottish poet
wrote -'The best laid plans of mice .........
The end result of compression or clipping is essentially the same, which is
that the peak to average power ratio of the output signal is less than the
peak to average power ratio of the input signal, assuming that 'compression'
*is* taking place likewise clipping (slicing). This obviously means that the
average power of the signal is in both cases increased.
Compressors, frequently appearing under different names, achieve the result
more 'gently' than clippers, and there is some delay before they react in
part due to the time constants within the gain control loop. Whether or not
this delay results in any performance problem is something that the designer
will have checked, and in the K2 there does not appear to be a problem.
You will have seen that in the SSM2165 chip the sample of the signal used to
control gain is taken from a point ahead of the gain controlled amplifier,
the VCA. This method of 'forward control' makes it much easier to have the
controlled amplifier's gain set suitably for a signal as it arrives. The
picture is of course more complex in real life because many signals of
various amplitudes and frequency occupy the same time frame.
Those systems using 'backward' control, such as a typical ALC where the
control sample is taken from the output, by their very nature will allow
some output, perhaps distorted, before the gain of the controlled amplifier
is reduced - closing the barn's door after the horse has bolted. Thus in my
view, ALC is useful if used carefully and as the last line of defence.
Clippers, assuming proper design, react very quickly slicing the signal at
the required amplitude. If used directly at audio the result can be a mess
of harmonics and intermod products. For this reason an audio signal is
upconverted to become a SSB RF signal, symmetrically clipped, filtered and
downconverted back to audio. With a reasonable RF SSB filter before the
clipper to obtain unwanted sideband suppression and another after the
clipper to attenuate harmonics and to limit bandwidth , some intermodulation
products within this second filter's passband will remain. In practice their
effect is acceptable provided that a low pass audio filter with a cutoff
frequency around 3 kHz is added to the output. One function of this audio
filter is to attenuate any intermod products resulting from the RF to audio
downconversion which are outside of the required audio passband.
Of course the clipping and filtering can be applied to a transmitter's IF
SSB signal instead.
I admit to oversimplifying the picture.
73,
Geoff
GM4ESD
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