Re: [Repeater-Builder] Re: Audio Reproduction

9 Apr 2004 02:01:08 -0000

Hi Joe,
 
Fact: PM does not reproduce DC well. Actually, it doesn't reproduce DC
at all (beyond a spike) because in DC there is no change in the phase of
the signal. The closer you get to DC, the worse PM will perform. FM, on
the other hand, has no such limitation.
 
As a blanket statement, the above is not true. I can generate a beautiful 10 Hz or 1 Hz or 0.01 Hz phase modulated output at any deviation you want using one of the newer function generators. It's done with digital techniques and not with a varicap and LC tank because the deviation simply isn't obtainable with the latter. But nevertheless, the theory contains no such low frequency limitation and there is equipment on the market that performs the function just fine.
 
BTW, you won't get your mobile rig or HT to modulate with DC regardless of whether it's PM or FM because the voice circuitry is AC coupled. The only way to transmit really slow digital data is if the exciter has a special DC-coupled input. And again, there is nothing in theory that says PM can't handle really slow digital data. A frequency discriminator would not be the best detector to use, you'd use something called a phase detector, which as far as I know is not generally used in narrowband radio today. While theoretical does not always equal practical, that doesn't mean it can't be done.
 
 

Now, the best way to match the 'curves' is to eliminate any changes. If
you run the discriminator signal directly to the FM modulator (commonly
called Flat Audio repeaters, or a term I coined... Unprocessed Audio
repeaters), through the limiting/clipping circuitry, without any
de-emphasis or pre-emphasis, you are transmitting a signal that is about
as close to the original as you can possibly get.

 
 
You know, the other day a fellow named Eric asked this list how he could improve the audio on his repeater. He cited his outboard de-emphasis circuit as a possible culprit, and I picked up on that and questioned where he had his breakpoint. Then, folks with more radio experience stepped in and said that that particular radio had both highpass and lowpass filters on the mic circuit that would have a big effect on the repeat audio.
 
That was an excellent point! And when another gent later asked a similar question about his repeater, and one of the responses was that he should do the Unprocessed Audio thing.
 
My opinion is that if he did a careful sweep of all the circuitry his audio was going through - - RX, line cards, TX, whatever - - he was going to find a 'constriction' somewhere. Somewhere there is going to be a filter, or a transformer terminated in the wrong impedance, or a bad component, or something that results in bad repeated audio. I maintain that there is nothing wrong with one or more preemphasis or deemphasis steps provided that the corners are sufficiently far out, which is not difficult to do. There are numerous preemphasis and deemphasis steps already in our radios, and those are not responsible for poor audio complaints. There are plenty of other culprits.
 
73,
Bob


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