This is an interesting topic that I deal with regularly in broadcast
engineering. The goals are quite a bit different but the way of getting
there are similar. You may be surprised to know that many of the top-rated
FM stations in the country use substantial amounts of clipping of both the
discrete channel audio and the composite signal fed to the exciter. This is
in addition to broadband compression, multi-band spectral compression and
multi-band spectral limiting.
Distortion-free "fast limiting" can be employed to cause some apparent
loudness increase, but the real gains don't come until you shave off all the
useless high peaks and dramatically increase the average power levels which
include the most important speech formants, such as vowel sounds. Also,
clipping causes harmonics that tend to fall into the higher speech ranges
and add to the power without increasing the peak to average ratio. Since
the human ear is more sensitive to sounds in the high voice range (around
2.4 kHz) the audio tends to stand out, which is exactly what you want to do
in a pile-up.
That was the secret of the well-known processors of the past such as the DX
Engineering, the Alpha Vomax and the Datong.
Ron
N6AHA
----- Original Message -----
From: "Ken Brown" <ken.d.brown@verizon.net>
To: <tjednacz@ieee.org>; "Discussion of Ten-Tec Equipment"
<tentec@contesting.com>
Sent: Thursday, February 23, 2006 8:24 PM
Subject: Re: [TenTec] Orion Speech Compressors
Depends how you define distortion. A typical way to define distortion is
"output that differs in any way other than a fixed gain or loss compared
to the input." If a speech processor had no distortion by this
definition, it would serve no purpose at all. In order for a speech
processor to be useful, it has to do something to the speech waveform,
and that something that it does IS distortion by some definitions of
distortion.
There is NO distortion even at higher levels of compression. That is
because
the algorithm used in the DSP does the compression correctly. Same in my
hearing aids. The compression does not add ANY distortion.
DE N6KB
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