On Sat, 13 Jan 2007 22:13:25 -0500, "Jimmie D"
wrote:

+++
+++"james" wrote in message
m...
+++ On Sat, 13 Jan 2007 17:43:52 -0500, "Jimmie D"
+++ wrote:
+++
++++++
++++++"Jimmie D" wrote in message
et...
++++++
++++++ "Telstar Electronics" wrote in message
++++++ ups.com...
++++++ Jimmie D wrote:
++++++One thing did get me thinking. I
++++++ have a nice scope to look at the output of my TX and to lookat the
++++++ output of
++++++ the processor , for me its a piece of cake to setup. How can you do
+++this
++++++ without test equipment?
++++++
++++++ Absolutely, I have a nice Tektronix scope I use...
++++++
++++++ Don't miss seeing the new CB Radio Speech Processor Prototype at
++++++ http://www.telstar-electronics.com/d...s/WhatsNew.htm
++++++
++++++ But how does most of the people who buy them do it?
++++++
++++++Even a good quality limiter/compressor is just anotheer splatter box if
+++its
++++++not set up right.
++++++
+++ ************
+++
+++ The theory behind a compressor amp is to provide a more constant power
+++ signal to the modulator. There have been many schemes in the past 70
+++ yrs or so to do that. Even to the point of what was once called Super
+++ Modulation.
+++
+++ A properly setup with compression will not have excessive compression
+++ and also not use the compression stage as the major gain stage. Where
+++ most compression schemes come into dislike is when the person using it
+++ feels he needs to get far more gain from the circuit than is really
+++ necessary and so much compression that you can hear a roach fart 20
+++ feet away.
+++
+++ The idea behind audio compression and other audio/rf processing
+++ schemes is to increase signal to noise ratios. When using AM and the
+++ carrier is already 30 dB above the ambient atmospheric noise then any
+++ processing is typically useless. The gain in signal to noise ratio is
+++ not worth the effort or even the cost of adding such a circuit. Audio
+++ compression is totally useless on FM unless you want a constant
+++ deviated signal. For what ever reason that one would want this is
+++ beyond all logic.
+++
+++ Even when the AM signal is marginal to the atmospheric noise, audio
+++ compression alone yields between 1 and 2 dB improvement in the
+++ received signal to noise ratio. On AM, when you are at 6 to 8 dB
+++ signal to noise ratio, improving one to two dB is generally worthless
+++ improvement.
+++
+++ james
+++
+++
+++Clippers are much more useful. Ususally you can clip off the top 8 or 9 db
+++without substantial effecting intelligebility of the voice. This does make
+++for a louder signal without overmoduation. Compression works best when used
+++with an expander on the rx end. But how often are you going to see that..
+++
**********

The goal of both compression and clipping of the audio signal is
reduce the peak to average power of the human voice. Typical Human
voice peak to average power range is about 3:1.

Clipping can be as high as 30 dB as long as there is sufficient
filtering after the clipping stage to restore the audio bandwidth to
its original bandwidth. Otherwise you have splatter or increased
modualted bandwidth.

Compression is somewhat more effective in reducing the peak to averge
power in the voice. It can be implemented with better results. A major
drawback to compression over clipping is increased cost with a more
complex circuit and more difficult adjustments. All to often in CB
work the compression stage also doubles up to be mic amp and a large
gain stage at the very low level stages of the audio stream. I
personally don't like this as it requires to many adjustments for one
stage. Audio compression is better done, in my opinion, in or near the
final stage, high level, of the audio stream. This too is not without
its drawbacks either.

Neither circuit requires the receiver to have any special ciruitry to
enhance intelligibility.

james