### Pre and De-Emphasis

#### The use of pre and de-emphasis in FM broadcasting (Entertainment and
Communications) gives a decided improvement in received signal to noise
ratio. The higher audio frequencies are boosted in level at the transmitter and
are reduced in level at the receiver. Any higher frequency noise that was
picked up along the way also gets reduced at the receiver. Entertainment
broadcasting usually has the high frequency boost begin around 1500 Hz to
2000 Hz and continue upward at an approximate 6db per octave rate. This
translates to a 75 microsecond pre-emphasis. Communications equipment
seems to have pre-emphasis start at a lower frequency and is different with
various manufacturers. Simple RC circuits can be used although other
methods are popular. Resistance and capacity work because the AC
resistance of the capacitor (impedance) becomes lower as the frequency goes
higher. At the "crossover" frequency the AC resistance of the cap equals the
resistor value. In pre-emphasis the 2 elements are in parallel. In de-emphasis
they are effectively in series, an AC voltage divider. If you do any transceiver
modifying for packet use, you'll eventually be faced with having to
accommodate "emphasis." You may wonder, "What does that mean, 6db
per octave?" First you need to know what an octave is. Every time a
frequency doubles, it has increased by an octave. Assume you are measuring a
1200 Hz tone from an oscillator across a 600 ohm load. It measures .775 volts
RMS. The tone is increased to 2400 Hz. The frequency has now increased one
octave - it has doubled in frequency. The level of this 2400 Hz tone is now
increased by 6db. The voltage would now measure 1.545 volts RMS - about a
1.99 increase. In 1200 baud VHF packet, we use tones of 1200 Hz and 2200
Hz. Not quite an octave but very close to it. The 2200 Hz tone should be about,
but not quite, 6dB higher in level than the 1200 Hz tone. The actual db
increase is closer to 5.45dB but 6 is close enough. Figuring RC time constants
is an exercise in great pain. At the end of this piece is a chart showing
standard resistance values versus capacity to obtain a 75 microsecond
"emphasis." For pre-emphasis, the resistor and capacitor are placed in
parallel and then this combination is placed in series with the audio to the
transmitter at an appropriate point. For de-emphasis, the resistor is placed
in series with the receive audio, at an appropriate point, and then the end of
the resistor closest to the last device in the chain (TNC, speaker amplifier, etc.)
is bypassed with the capacitor. You must give some thought to the resistance
value selected. Input capacity of the port involved will affect your results.
Watch out for that input capacity. If the input capacity is already .01 Mfd, a
series resistance of 10,000 ohms would put you at greater than 6db per octave
and give you too much high frequency roll-off without the use of an additional
capacitor! "Purists" will object to this but my rule of the thumb allows me to
use a resistor value roughly equal to the impedance it is FACING. If you were
going to place a de-emphasis resistor in series with an audio signal feeding a
device with a 600 ohm input, you probably wouldn't want to use a 22,000 ohm
resistor - unless you had a large surplus of signal and a low input capacity.
Simple math indicates that you would approach a 40 to 1 reduction in signal
voltage. For a 600 ohm input you'd probably want to use a 470 ohm resistor -
this would cut your audio by something less than half. The same holds true on
the pre-emphasis side. I will admit that I have been known to "split the
difference" on a resistor value if I am feeding a low impedance from a high
impedance and have a surplus of signal voltage and a low input capacity. Try
it, if you're not satisfied, try another combination.

__RESISTANCE__ __CAPACITY__
100 ohms .75 Mf

220 ohms .34 Mf

470 ohms .16 Mf

1,000 ohms .075 Mf

2,200 ohms .034 Mf

4,700 ohms .016 Mf

10,000 ohms 7500 Pf

22,000 ohms 3400 Pf

47,000 ohms 1600 Pf

100,000 ohms 750 Pf

220,000 ohms 340 Pf

470,000 ohms 160 Pf

Verne

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