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Winter 2005 Issue |
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Flat Audio |
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In this article we will start off with a quick
“fly-by” description of just how audio works in FM two-way radio. We will
briefly examine pre-emphasis and de-emphasis and repeater audio, and then
we will discuss flat audio and how it differs from most repeater audio
scenarios today. Later on we will delve into the details in more depth.
When frequency modulation (FM) began to be used in the late 1930s and early 1940s, it quickly became the preferred mode of radio communications, primarily because of its far superior signal-to-noise ratio and its ability to provide noise-free communications. At that time it was a one-radio to one-radio service, the same as what we call “simplex” today. Repeaters were not yet even dreamed of. All early FM transmitters used phase modulation, which means they automatically doubled the deviation of an FM signal with every doubling of the audio frequency. This effect is called pre- emphasis and works as a 6 dB per octave increase in deviation, or a “roll-up” in deviation. It provided for a much better signal-to-noise ratio by making the higher frequencies have as much audio punch as the lower frequencies as it evened out the audio. The receivers of the day all had a de-emphasis, or a “roll-down” circuit, in them to restore the audio back to normal. This is the way we still do it today, even with FM transmitters. FM transmitters have a pre-emphasis circuit built in, to be compatible with the existing phase-modulation (PM) transmitters, because all of the receivers out there are running de-emphasis circuits. When repeaters came along, hooking up the audio between the repeater RX and TX became a hotly contested topic, with many variations on how to do it. Some repeater builders took the user’s pre-emphasized audio and de-emphasized it in the repeater RX, and then pre-emphasized it again in the repeater TX for delivery to the end user. Then the end user’s receiver de-emphasized it again, thus returning the audio to normal. Whew! This probably could work out if all of the emphasis curves in the repeater are matched (a difficult task). Also, all the processing in the TX’s speech amplifier affects the audio quality. Another scenario is created when a mismatch occurs in a repeater because only one side of the repeater processes the audio. For instance, if there is an extra pre-emphasis stage in the repeater, the audio will sound very tinny and high-pitched—all highs with the lows missing. If there is an extra de-emphasis stage in the repeater, the audio will sound very mushy and lifeless—all lows with the highs missing. |
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