> [EMAIL PROTECTED] wrote:
>
> Hi Joe,
Hi Bob.
> 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.
OK. I'll accept that. However, let me add that if you consider the
average stock exciter, is the same still true? I mean, not everyone is
an 'audio god' like you are. Most of us have to deal with pretty much
what is offered by the manufacturers with few intensive mods. Yes, we
may choose a better tap point, and some have designed their own
circuits, as the 'other' Joe M. has done, but most can't so that for
various reasons.
So, given the poor designs that come with most (if not all) PM exciters
(the preemph/limiter/clipper/de-emph), does my statement hold up better?
> 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.
Ahhh ahhh... 'my' mobile rig does DPL, and that requires FM (true,
direct, whatever you want to call it). True, the voice may be AC
coupled, but there has to be some DC coupling. I will admit that I am
most likely in the minority. All my mobile equipment is commercial due
to the need for LEGAL commercial and ham frequency coverage. :-)
> 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.
I was one of those people.
> That was an excellent point!
Thank you. :-)
> 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.
Well, it all depends on the situation. Again, some people have more
experience at interfacing controllers and repeaters. for some people,
the best answer is to use de-emph/pre-emph - especially if they don't
want to dig into the guts of their radio. There was recently a guy who
posted a query about how to get a COS signal without opening up the
radio. I'm certain he would not have wanted to hear about Unprocessed
Audio mods if he was scared to find a COS point.
> 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.
Well, pushing out the corners usually means redesigning the audio stages
in the exciter, and many people simply don't want to do that. Others,
like myself, don't have the experience with audio design to do it, even
though the desire to learn and do is there (but frequently lack the
time). Still others can and do design their entire control system (that
would be the 'Bob S-Com' type). :-)
Yes, you may have made the PM exciter essentially sound and perform like
an FM exciter, for most practical purposes, but how intensive was it to
do that? You do have to admit that it's beyond practicality for most
people, right?
On the other hand, a proper 'flat audio system' is relatively easy to
do, compared with redesigning the audio stages (of either FM or PM
exciters). I think you would agree with that, too.
So, and let me add this point, FM is less expensive and less time
consuming to obtain similar results compared with PM. And again, I say
that based on the comparison of 'flat audio' vs redesign of the audio
stages of either exciter. But, PM having more stages, naturally requires
more redesign.
I always enjoy and learn from our exchanges, Bob. :-))
Joe M.
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/Repeater-Builder/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
