--- [EMAIL PROTECTED] wrote:
> Hi Joe,
>
> This is an interesting article:
>
>
http://www.repeater-builder.com/rbtip/fmtheorydiscussion.html
>
> He suggests that de-emph came first to get around
> the
> rising noise of an FM receiver. In any case, I think
> most people agree that it was done for noise control
> purposes, which it does do well. If not, it would
> have
> been abandoned.
>
> I disagree because there is no possible way to
> abandon it. The author says
> this:
> In the early days of FM, there were no varactor
> diodes so it was difficult,
> at best, to produce a direct FM modulator. PM
> modulation was, however, easy to
> achieve which is the reason that it was the
> "standard".
> I agree completely. So if PM is the standard, and it
> creates preemphasis on
> an FM demodulator, then you have no choice but to
> deemphasize if you want to
> recover flat audio. It's not like the designers ever
> had a choice.
>
>
> Regarding links or repeaters with flat audio: I
> think
> that the only place the audio should be pre or de
> emped is in the end users radio, or in items that
> communicate as an end user (phone patches, voice
> synthesizers, etc.) There is no need to pre and de
> emp
> at a repeater, and especially no good reason for
> doing
> it on links.
>
> I'm rather surprised, then, that you pointed out the
> article. At the end of
> the last paragraph, the author says what I have
> maintained for years:
>
> I am not a believer in pulling receiver audio off
> the discriminator and
> directly into a "flat" transmitter. That originated
> in the ham circles and I don't
> believe that it will produce audio any better than
> "doing it right the first
> time"! Let the pre-emphasis and de-emphasis circuits
> do their job (it really
> results in a fairly flat response) and pay attention
> to the clipping levels.
> You'll have great audio and you will keep your
> deviation within spec, which is
> mandatory in today's FM bands. Actually, if we could
> get everyone to adjust
> their repeater deviation to 4 kHz, a lot of adjacent
> channel problems would be
> reduced to acceptable levels. Just a thought..
>
> Thank you, Paul, K3VIX (a 26-year Motorola veteran
> designer).
>
> 73,
> Bob
>
I said it was an interesting article, I didn't say I
agree with it comepletly.
Anyone who has tried to link multiple sites together
using the stock Micor (or other) base station audio
knows it gets muddier and muddier as you go through
links. You start getting a "rounded" frequency
response. On a single repeater, this is not
necessarily bad. Thats why there are so many good
sounding MII and Micor repeaters out there. But most
of them don't have "simplex" sound, and if you go
through 2 or three of these systems it starts to sound
like you're talking inside a 55 gallon drum.
I think his comment is directed at those that are
constructing a single RX/TX combo, such as a plane
jane Micor RPT. It has been mentioned by someone else
that land mobile radios were not designed for multiple
links, and this is true. Thats why if you want to use
them for such, you have to go in and somehow correct
the deficiencies in the RX and TX audio. The easiest
way to do that is to go in and get your discriminator
audio "flat" and then get your TX modulator audio
"flat". At that point, it doesn't take a lot of rocket
surgery (or brain science) to get good link audio,
it's basically set the output the same as the input.
> I agree completely. So if PM is the standard, and it
> creates preemphasis on
> an FM demodulator, then you have no choice but to
> deemphasize if you want to
> recover flat audio. It's not like the designers ever
> had a choice.
Remember, these people were using a PM transmitter to
create "indirect FM", not PM. If you want to have a
clipper or limiter, which is required by FCC rules,
then at some point you have to have FM in your
exciter. You can't clip audio and feed it to a PM
transmitter, the results would be horrible. Again,
here's the way it works in a PM exciter. Mike audio
comes in and get's pre-empted. Pre-empted audio is
clipped and applied to a LPF. LPF audio is DE-EMPED
and then fed to a PM. What do you call de-empted audio
thats fed to a PM? It's called indirect FM, or simply
FM. In a direct FM modulator, here's what happens.
Mike audio comes in and is pre-emped. PE audio is
clipped and applied to a LPF. LPF audio is directly
applied to the FM modulator. The only difference
between the two is a single de-emp network, only
required in a PM transmitter. The only pupose of that
DE network is to turn the PM modulator into an FM
modulator, at least over the area of interest, the
voice band.
Joe
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