In 3f14f684.0@entanet, "Brainbuster" wrote:

Frank Gilliland wrote in message ...
In ,
(Richard Cranium) wrote:

I'd like to be there when you tell that to Avanti, makers of the
PDL-II and Moonraker series of antennas, which feature selectable
horizontal and vertical polarization! I think they would laugh at you,
Frankie, as will the many owners of those antennas.

I would like you to be there, too.


When one of you has a time machine, let me know.
I was under the impression that Avanti was no more.
:~)

Beats me. I don't keep up on antenna manufacturers or their products because
most of them only sell hype.

the Digital Speech Processor kit. I have the skills required to
construct either, but not the knowledge to know if either project is
worth the effort.

A speech processor is an excellent idea, but I have no idea if Lou's
even works.
I certainly have some misgivings about 90% average modulation!

What? A "commercial broadcast engineer" that can't look at a schematic
and know whether the circuit works as advertised or not?

Not when you have four high-gain, high input-Z OP-amps on the same chip,
all inverting, all DC isolated on both the inputs and outputs, and all
outputs
are unloaded.


Digital speech processor?
I have details of his "DSP" - Dynamic Speech Processor, which makes claims
about a 90% average... so seems to be what is being referring to above.

I'm sure it was just an honest mistake.

Looking at the diagram, the first amp is a buffer stage, the second has a
gain of about 45 and the third a gain of about 21.

Look again -- the first stage is a mic preamp with no input impedance matching
(IOW, it has a very high input impedance and probably picks up a lot of hash).
The second stage has variable gain, which is how he adjusts the clipping level.
It would be much easier to eliminate that stage completely, and adjust the
clipping by varying the voltage on the tail of the clipper.

From his figures for a
standard mic output, a loud whistle should produce about 6 volts into the
clipper... which is down to 500mV from the clipper into the output amp.

Hmmm... "a loud whistle"... gee, wouldn't it be great if all technical
specifications could be that simple...

IOW, any one of them could start oscillating at any time.

I assume that you do not have a copy of Lou's guide to the DSP...

quote
you may get self oscillation (squealing) due to the overall high gain of the
combined DSP, power mike (if used), and radio circuits
/quote

He obviously hasn't had much experience with OP-amps. The hardest part about
working with OP-amps is making them NOT oscillate. When you have four inverting
amplifiers with high gain (open loop gain for the LM-324 is around 100 dB) mixed
with very high input impedance (we're talking giga-ohms) and no load on the
output, and when the inverting input pins are directly adjacent to the output
pins (as is the case with all four sections of the LM-324), that puppy can
probably be coaxed into oscillation by nothing more than an abstract thought.

And...

quote
The squeal problem is very common in newer Uniden SSB rigs
/quote

His "cop-out" is that with "correct adjustment", this should not happen.

Sure, just switch the first stage to low-gain, and turn down the clipper
adjustment (the gain of the second stage). But that defeats the purpose of the
circuit.

But, he is putting this adjustment in the hands of people who he has said
cannot even solder correctly. And how can anyone adjust it correctly with,
as he suggests, an SWR/power meter or someone listening to your signal?

A dual-trace scope would be nice, and an audio signal generator would help. I
wonder if Heathkit ever sold kits for "whistle-generators"....

Or, like I said, the thing may not even work at
all because it might lock up on power up.


More likely, it will fail due to poor building of the circuit.
I had someone ask me to fit one for him, which he supplied as supplied by
the shop, ready made. There were all kinds of problems, including the
screened wires not being correctly connected - they were shorted between
screen and inner wire.

Also, the version I have the details for had the LM324 fitted in place of
the LM3900. It should be noted that the LM324 is a "domestic" spec chip...

....that's "commercial" grade...

and may fail in temperatures below Zero or above +70° centigrade.
The LM2902 (-40° to +85°) or LM124 (-55 to +125°) are better.

I never found the temperature grade to be critical. Those are the guaranteed
specs, but even in extreme conditions most commercial grade chips will still
work fine.

"90% average
modulation" would be great on AM (ask a real commercial radio
engineer, Frankie; they use 'em all the time),

Considering that the circuit is nothing more than a preamp and clipper, I
have no idea where he gets the idea that it will achieve 90% average
modulation.

Does he claim that *the DSP* will manage that magical 90%, or just that the
figure is possible?

quote
With correct speech processing, it is possible to increase the average
modulation to about 90%!
/quote

Unless someone has seen a statement claiming that his product does that
"90%", or that the "90%" can be done without unacceptable distortion or
harmonic content... then it is nothing but sales bumf- careful wording.
Also, it says, "about 90%".

Nice catch!

However, here is another of quote from Lou:
quote
Up to 15dB of clipping is quite acceptable to the ear
/quote

The average peak to average ratio figure for the human voice is 14dB,
although some people may be higher or lower. The 90% quoted may
be for some people with a lower than average ratio.

There is no absolute value to the amount of clipping that is "acceptable", let
alone intelligible. The peak-to-average volume ratio varies greatly from person
to person because people have different voice properties -- differences in
harmonics, pitch, articulation, etc. There are also differences in people's
hearing. Then there are other factors such as background noise, band noise, the
type of speaker and radio used, etc. There are WAY too many variables to make
such a broad statement like "15dB of clipping is quite acceptable".

Even then, to get such a high average, without an unacceptable level of
distortion, would be pretty tight... and certainly not possible without
using proper test equipment to adjust the device exactly.

And at least a 3rd or 4th order low-pass filter after all that clipping.

in case you missed class, any form of compression and/or
clipping is a form of
distortion; i.e, more compression equals more distortion.

Certainly with clipping, but how about a VOGAD?

Some of us old farts still call that AGC.

If it is set to reasonable attack and decay times, there should be no
problem. However, the time delay means that it would not work the same as
clipping.

Which is better because you don't have all the harmonics. But the use of both is
best, since a clipper will catch any fast-rising signals that slip past the AGC.

Years ago, some AM broadcast stations
would crank up the compression to the point where the sound
quality was offensive, but that's rarely done anymore.

In the UK, Radio1 manages to create "offensive" audio without adding
distortion... they just play crap "music".
;~)

but when the operator isn't saying anything
there'd be a rather obnoxious noise transmitted as the
circuit searches for anything to keep the modulation at 90%. As
witness the BBC some years ago, until they learned to turn
down the processing.


The "searching for noise" will probably be associated with something
more complicated than the simple amp and clipper used in the
DSP... possibly a combination of VOGAD and clipping.

The noise he is describing is ambient and/or preamp noise. It can be heard when
the maximum gain is set too high or there is no noise gating.





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