There are several different types of busy channel detection in use by ALE systems. Some of these are hard-coded and some may be switched on/off (such as voice detection).
Depending upon the ALE system, the busy detection has names like "occupancy" "polite" "voice detect" "channel-busy" and various other nomenclature. They each may have slightly different functions. Some are the same, but different names. In order to be practical, there must be a decision-making threshold through some means. That threshold may be user-selected or hard-coded. Generally speaking, some of the various ALE busy detectors work like this: 1. Detection of relative amplitude of in-band signal vs out-of-band signal. This detects the changes in audio within the occupied part of the channel (roughly 700Hz-2800Hz) and compares it, relative to the energy above and below the occupied part of the channel (roughly 200Hz-700Hz and 2800Hz-3300Hz). 2. Voice cadence detection. This uses the relative energy change repetition rate for audio voice frequencies having the majority of the energy in the peak audio voice band of the channel (roughly 400Hz-1300Hz). 3. Similar-signal detection. This detects waveforms that have similar characteristics to the desired modem. Such as sine waves, or FSK waveforms. 4. Relative amplitude threshold detection. This detects difference in instantaneous or time-weighted amplitude relative to overall amplitude. 5. Sync-detection. This detects frequency shifting in-band signals with roughly 100symbols/sec to 150 symbols/sec over some time duration interval. 6. Frequency change detection or windowing. This detects signals that vary in frequency and change over some preset time window and/or multiple frequency windows. 7. Combination detection. This uses a combination of two or more of the above methods. 8. Proprietary mystery detectors. These detectors use methods which are not entirely disclosed by the manufacturer or software designer. Some of them are quite effective and have amazingly accurate and reasonable thresholds... they are likely some type of combination detectors. Bonnie KQ6XA
