Signal Reporting
When we are engaged in voice communication it is often necessary to give the other station a report on the quality of their signal. The R-S-T system of reporting signal quality dates from the early days of radio. It gives voice operators only a vague idea of what their signal is really like at the receiving end. RST was originally intended for Morse code use and was later adapted for use with voice signals.
The problem with the RST system is that it is usually inaccurate. This is evident from listening in on any contest, where everyone has a signal report of “5-9”, regardless of how they actually sound. The fact is, very few people know what “5-9” means – on either end!
The CM (“Circuit Merit”) system was devised by HF radiotelephone professionals to better quantify the average quality of a VOICE signal. In telecomm engineering specifications the letters “CM” are followed by a figure from 0 to 5 – to indicate the readability and quality of the VOICE.
Charlie Mike Five: No noise, full quieting. Broadcast quality.
Charlie Mike Four: Slight noise, fully readable.
Charlie Mike Three: Marginal voice communications. Occasional unintelligible words, noisy; weak audio level.
Charlie Mike Two: Unreliable, difficult copy, frequent fills needed.
Charlie Mike One: Unintelligible, signal barely evident.
Charlie Mike Zero: No audio signal is detectable.
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