Answer Posted / devi
The concept of Pre emphasis and De emphasis is a broad
subject, however I will try to give you the basic concept
behind the advantages, and necessities of using it in FM.
In common narrowband two way fm communications, Pre
emphasis follows a 6 dB per octave rate. This means that as
the frequency doubles, the amplitude increases 6 dB. This
is usually done between 300 - 3000 cycles. Pre emphasis is
needed in FM to maintain good signal to noise ratio. Why is
it necessary? Common voice characteristics emit low
frequencies higher in amplitude than high frequencies. The
limiter circuits that clip the voice to allow protection of
over deviation are usually not frequency sensitive, and are
fixed in level, so they will clip or limit the lows before
the highs. This results in added distortion because of the
lows overdriving the limiter. Pre emphasis is used to shape
the voice signals to create a more equal amplitude of lows
and highs before their application to the limiter. The
result is that the signal received is perceived louder due
to more equal clipping or limiting of the signal, but
probably more important is the increased level of the
higher frequencies being applied to the modulator results
in a better transmitted audio signal to noise ratio due to
the highs being above the noise as much or more than the
lows. So what is the original reason for Pre Emphasis? Read
on and skip back to this explanation of the original
reason.
Transmitters that employ a true FM modulator require a pre
emphasis circuit before the modulator fore the true FM
modulator doesn't automatically pre emphasize the audio
like a transmitter that uses a phase modulator. A separate
circuit is not necessary for pre emphasis in a transmitter
that has a phase modulator because the phase modulator
applies pre emphasis to the transmitted audio as a function
of the phase modulator.
The receivers De emphasis circuitry takes the unnatural
sounding pre emphasized audio and turns it back into its
original response. Pre emphasized (discriminator) audio is
however available directly from the audio demodulation
(discriminator) circuitry.
In linking systems, many choose to eliminate the emphasis
circuitry to allow better representation of retransmitted
signals. Since the signal has already been pre emphasized
(by the user that is transmitting,) and since the receiver
you are listening to takes care of the de emphasis.... it
doesn't need to be done over and over again.
Some loss of quality does exist, but quality is better
maintained by a flat system. A flat audio response system
is one which has equal output deviation for the same input
deviation, no matter what the applied audio frequency is
within reason. Reasonable audio frequency response would be
from 50 cycles to about 3500 to 5000 cycles in a system not
filtering PL. Audio response in a system filtering PL would
be around 250 to about 3500 to 5000 cycles. Upper cut off
frequency would be determined mainly on acceptable use of
available bandwidth.
Available Bandwidth.
Injecting discriminator audio back into an FM modulator
without any limiting or low pass filtering is bad news,
plain and simple. On UHF, you've probably been able to get
away with it without excessively bothering either of your
adjacent channel neighbors, but on 2m, especially with 15
kHz spacing, you'd be asking for a lynching.
Without low pass filtering, all of the high frequency
energy (hiss) that comes from the discriminator from a
noisy user, if not low pass filtered, will deviate your
transmitter in excess of 5 kHz, in addition to pushing the
sidebands out further than they would be if the AF was cut
off at/about 3 kHz. Do this to see what I mean. Set your
repeater up for 1:1 input to output ratio (like, put in a
signal that is deviated 3 kHz by a 1 kHz tone, and set your
Tx audio gain to get 3 kHz out of the transmitter). Now
open your repeater receiver squelch wide open. You should
see your transmitter is now deviating somewhere around 8 or
9 kHz (presuming you have enough audio headroom through the
controller). Under this test condition, the combination of
the excessive deviation and the lack of high frequency
filtering will make your signal somewhere around 30 kHz
wide instead of 16 as it should be. The only
thing "limiting" the occupied bandwidth at that point is
the dynamic range of the audio circuits and the natural
high frequency rolloff of the discriminator's output
noise. Look at it on a spectrum analyzer if you don't
believe me.
Obviously that's a worst case scenario, but the fact
remains that you should have brick wall limiting at 5 kHz
(a little lower at 15 kHz channel spacing), and low pass
filtering at 3 kHz (a little higher if you want on 25 kHz
channels).
A repeater can be built to utilize a flat audio response to
maintain quality through the system. This is fairly easy in
a system using a true FM modulator. Usually modification to
the controller is necessary to allow it, especially ones
that have speech or a phone patch. Systems using a phase
modulator require de emphasis before the modulator because
of the automatic pre emphasis of the phase modulator, and
for this reason... it is easier to utilize flat audio
modifications and maintain quality audio in a system
employing a true FM modulator in the transmitter.
Kevin K. Custer W3KKC January 19 1999
| Is This Answer Correct ? | 6 Yes | 1 No |
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