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 6 dB 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, 6 dB 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 6 dB. 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, 6 dB higher in level than the 1200 Hz tone. The actual dB increase is closer to 5.45 dB 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 6 dB 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

W9ZGS

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