Re: [Repeater-Builder] Audio Reproduction

7 Apr 2004 23:16:29 -0000

Hi Joe,
 

I think this is where you go astray. FM doesn't
require pre-emp, it will work fine without it. Pre-emp
has been added to make the FM sound better by
redistributing the  voice energy.
 
I disagree. As far as I know, preemphasis was not "added" by some standards committee that said, "Let's make narrowband communications sound better by adding preemphasis just like we do in phonograph records". PM predated FM by a very long time, and PM became the standard. Since PM has natural preemphasis, FM transmitters had to have preemphasis added to make them compatible with that standard. Yes, FM with its flat audio would work fine without it, but that's not the system we inherited.
 
Virgil and I have copies of studies dating all the way from 1944 on such topics as speech clipping and intelligibility. One study even talks about adding "tilting", which was their term for what we would call preemphasis and deemphasis, and the effects of tilting on the listener. But nowhere have we found any kind of historical document to support the intentional adding of preemphasis. If you have access to this kind of info, we'd be very interested in seeing it.
 
 
 
 
You can have a "true
FM" exciter that runs or doesn't run pre-emp. In
digital paging, we don't run pre-emp, but we still use
true FM exciters. We end up with FSK, which is
basically the data input modulated onto the carrier
frequency.
 
I agree, but digital paging is a different animal from voice repeaters.
 
 
 
A "true FM" modulator or exciter is denoted
by it's ability to shift a certain amount of deviation
for each volt of signal applied to the modulator, and
that doesn't change with frequency. A PM exciter
doesn't do this.
 
True.
 
> If we had an FM system, we'd
> all be transmitting flat audio. Since we don't
> transmit flat audio and we do
> transmit preemphasized audio, an engineer outside of
> our circle would say we
> have a PM system.

Or an FM system with pre-emphasis. This would be more
correct, since the PE only covers a certain portion of
the bandwidth.
 
In narrowband FM communications, what we call "preemphasis" and "deemphasis" covers the entire band of interest, i.e., 300-3000 Hz. Therefore, our system is not technically a preemphasis/deemphasis system, it's an integration/differentiation system. The broadcast FM system, on the other hand, is correctly called a preemphasis/deemphasis system because the preemphasis starts at a point well into the band of interest.
 
 



So, if I build a repeater using "flat audio"
techniques, is that a PM or FM repeater?
 
The exciter will be either PM or FM as chosen by the builder. Since it transmits preemphasized audio, it is part of a PM "system". That's all I'm saying - - we all live within the same system, and that system can be described as a PM system.
 
 

If someone talking through it has bypassed their pre-emp, would
it be FM for that user, and the next guy has got
regular pre-emp, so would it be PM for him?
 
Good point, thank you! Why would someone do that? If a user eliminates his preemphasis circuit, then yes, it is FM for that user. It wouldn't matter if he had put deemphasis ahead of a PM exciter, or if he had deleted the preemphasis circuit of an FM exciter. His signal would sound terrible, right? He is outside the "PM system". He is using an "FM system" (a flat audio system), and since nobody else is using that system, he's incompatible.
 
The next guy who uses preemphasis would be participating in the PM system and he would sound okay.
 
 
 
 
 


I think when we talk on this board, and FM or PM is
mentioned, we're usually talking about specific
hardware that has these specs, not talking about a
"system". A Micor mobile may be PM and a Micor base
may be FM, not because they will sound significantly
different, but due to the way their exciters were
built.
 
I agree.
 



> I will readily admit that available PM exciters can
> have all sorts of
> shortcomings. They distort when you ask for a lot of
> deviation at very low
> frequencies like CTCSS. There are inherent
> nonlinearities in the varactor tank circuit
> that create a little distortion. Tuning the
> multipliers correctly is important
> for low distortion. And we've seen some sloppy
> engineering in the design of
> the audio and limiter circuits. But if a guy wanted
> to, he could design one heck
> of a great PM exciter using, say, DSP, and it would
> work exactly as well as
> any FM modulator.
>

This is probably true, but no-one I know of has done
this. Reason why? It's much easier to build/buy a
"true FM" exciter that has the characteristics people
want and need, without going through the trouble of
re-inventing PM to do the same thing.
 
Sure. This is an academic exercise intended to open up folks' thinking, not push PM. We don't make exciters, we make controllers. We gain or lose nothing with either modulation method.
 


 


I don't dislike PM modulators, I have used them on a
number of repeaters (and they sound good). But as you
have ponted out, there are certain limitations to
them, especially at low modulation freqs. There are
certain people out there (KC comes to mind) that like
their repeaters to faithfully reproduce sound down to
40 or 50 Hz. This is very difficult for currently used
PM exciters (ala Micor or MII) to do. I prefer
filtered audio on my repeaters, everything below 300
hz is thrown out. Does this make it sound different
(ref "simplex")? Yes, it would have to. Does this make
it sound worse? That depends on the individual. I like
the filtered HPF sound. And in so filtering, I can
make use of the PM equipment, since there are no
really low freqs to worry about. In this instance, it
is nearly impossible to tell the difference between a
PM and an FM exciter, since over the area being used
(300-3000 Hz) the exciters will have virtually
identical freq responses. 
Agree with all of the above. It's not that PM itself is incapable of modulating down to low frequencies, but some radios are. It's their particular design.
 
73,
Bob


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