On Thu, 13 Sep 2007 05:24:23 +0100, " Peter"
wrote:

|What you have to remember is that other FM users have much
|more bandwidth to play with than CB users. The broadcast
|stations have shed-loads of bandwidth to play with.
|
|-------------------

Actually they don't have loads of bandwidth to play with. In most
cases 15KHz is the highest frequency respone that they can have
without exceeding the peak diviation with Stereo FM. About 10% of the
modulating power is reserved for the 19KHz pilot carrier for stereo
FM. Commercial FM broadcast stations need the bandwidth for
transmitting higher fidelity music. Here in the US they are limited to
75KHz peak deviation. Besides tranmitting stereo music with FM is far
more complex than that of voice with FM.


james

"james" wrote...

Actually they don't have loads of bandwidth to play
with. In most cases 15KHz

James,

That is AUDIO bandwidth of the input signal, not the
bandwidth allowed for the transmitted radio signal.
For CB, the audio bandwidth is just 3KHz.

I'll bet you would consider that ratio significant if
it was applied to a pay-rise... and I bet you would find a
way to spend 5-times the income :~)
We are greedy creatures. Give us more and we will manage
to use it, wonder how we ever managed, and demand
another increase.

Here in the US they are limited to 75KHz peak
deviation.

Now we are getting closer... that is much better than the
2KHz deviation now allowed for UK FM CB.

Broadcast station deviation is the same here, 75KHz.
With an audio bandwidth of 15KHz, the total bandwidth
required for a basic mono signal of that quality is around
120KHz.

12 CB channels in each direction just to match the quality
of the mono signal.

The CB channels, and the 10KHz spacing, were set back in
the days of AM. But then we in Europe took this spacing
and decided to put an FM signal in there.
It took some squeezing and some careful balancing tricks,
we even had to reduce the deviation from the original
2.5KHz down to 2KHz.


Regards,
Peter.

"TelSpam Electronics" The Griffter Man wrote...
Why Do You Need a Speech Processor for Your CB Radio?

The simple answer is that you don't. You need air, you
need water and you need food... but you will survive
without a speech processor.

The word "need" is used by salesmen to create a feeling of
urgency, so you rush out and buy their product without too
much thought or product comparison.

A speech processor is *useful* for AM and SSB CB, but
you really can take your time and shop around for the
best product for your needs.
Here are just a few alternative products:

Lou Franklin's DSP:
http://www.cbcintl.com/dsp.htm
Bobs CB speech processor - kit, pre-built or boxed:
http://bobscb.com/kits/tr_kits.htm
Spectrum Communications speech processor:
http://www.spectrumcomms.co.uk/cbkits.htm#SP1000
Build your own:
http://www.rason.org/Projects/speech/speech.htm

Maintaining a high modulation level is crucial in providing
the highest possible efficiency from any transmitter

Transmitter efficiency is set by the output biasing, not
the audio levels.

operating on AM, FM, or SSB modes.

Speech compression is NOT suitable for FM CB - there are
only 10KHz between channels.
Compression works on broadcast FM because they do have
the bandwidth - equal to 30 CB channels for one transmission.

If you want broadcast quality FM, you require broadcast
quality bandwidth. This would make the whole Citizens'
Band just one single channel.

You cannot put a Pint in a 1/2 Pint jug. Those who tell you
otherwise are selling something, and really don't care how
they make the sale.

When deciding which product to buy or use, trust is
important. In deciding trust, people may wish to take notice
that the above linked pages make no reference to use
on FM CB.
Do you think they just don't want to sell their product, or
that they are simply not willing to mislead people just to
make a sale?

The average modulation of a typical voice signal is only
about 40%. This low percentage applied to the transmitter,
results in less than optimal transmission range.

Except in the case of FM (NBFM) CB, where it makes it
possible to fit your transmission on a single channel.
Increasing average audio on an FM CB transmission increases
the average adjacent channel power. This is then picked up
by people on the next channel, but clipped by those trying
to listen to you on your channel.

clipping distorts the original signal and sounds fuzzy
on the air.

Fuzzy at best, large amounts of clipping cause severe
distortion and difficulty undestanding the person.
This is a common problem when people try to increase
their peak or average deviation on FM CB.

VoiceMax holds your transceiver at 100% modulation
allowing you to punch through heavy channel traffic

This does not apply to FM.
The output amplitude of an FM transmitter is constant,
extra deviation will NOT boost your signal above
the "traffic".

VoiceMax works with your non-amplified dynamic microphone

Another issue with FM CB, is that they use very little audio
power to modulate the transmitter. There is not a UK FM CB
made that does not have all it needs to give far more audio
than required... without additional add-on devices.


Regards,
Peter.