On 7/20/2025 3:12 PM, QRP Labs via Elecraft wrote:
This is
important because often people are using compression and without realizing
that due to the SSB overshoot problem, the ALC now has to reduce the gain
even more to avoid the splatter. In other words some of the dB you got by
using compression, you give back because the ALC had to more aggressively
reduce it due to higher envelope overshoots; but with CESSB you get to keep
all the compression dB advantages (plus the regular advantage of CESSB).
Good points, Hans. But the important thing to realize, though, is that
audio is a very complex signal, whose spectral distribution, is
approximately logarithmic, as is the human ear/brain hearing system. And
as more than a century of studies of how we hear, engineers learned that
components of speech below about 500 Hz contribute almost nothing to
speech intelligibility, and while components above about 3 kHz do help
some, we can do quite well without them and save a lot of bandwidth (and
transmitter power).
Bottom line is that steady state measurements of anything involving
audio are pretty meaningless. We can define system clip and assess the
noise floor, but that's about it. Two-tone tests for IMD tell us next to
nothing about what happens with program material.
I determine the cleanliness of transmitters by accumulating peaks on a
system like the P3 that shows the spectrum, and by looking for
horizontal overshoots on a waterfall. I study key clicks in the same
way, not by looking at square waves on a scope. I learned most of what I
know about audio measurements under Dick Heyser, the inventor of TDS
(Time Delay Spectrometry), and used it extensively in my consulting
practice.
73, Jim K9YC
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