- Apr 19, 2004
- Arlington, TX
In recent studies, we've determined that the squelch setting can dramatically affect P25 decode quality, especially on Simulcast (LSM) sites. For more background on Simulcast issues, see the wiki article with the caveat that it is not fully correct on the root cause of the problem. Here is a quick bullet-point summary that more correctly describes the root cause:
- Any receiver dropout during digital reception creates data loss that results in bad decoding.
- Because multiple towers are transmitting the same signal, simulcast systems can have many areas that have “nulls” where the same signal from different towers arrives at slightly different times, causing the signal strength to drop and/or vary suddenly.
- Even atmospheric changes or someone opening/closing a door can affect where the nulls are, so even optimizing antenna location cannot prevent dropouts 100%. Moving a receivers antenna even a few inches can sometimes move it out of a null zone. Vehicle and fixed-location radios typically avoid the problem by use of a diversity antenna system. Also, system engineers try to design the system so that predicted null areas occur in areas that system users are not likely to be, such as over water or unpopulated areas. Even then, nulls cannot be 100% eliminated in intended service areas. Handheld radios w/single antennas and scanners are both subject to a higher level of signal amplitude changes.
- When the signal strength drops below the squelch threshold point, the scanner’s squelch action immediately stops the receiver from providing a signal to demodulate/decode, even if the drop is very brief. This results in data loss and choppy or no decoding.
- You must have End Code detect enabled. Otherwise, when an analog comm ends on a trunked system, the scanner will remain on the voice channel and you will either hear open squelch or the next comm assigned to the voice channel.
- It will take longer for the scanner to acquire the trunked control channel (normally, it only looks on those programmed frequencies that have a signal strong enough to break squelch). You can mitigate this delay by only programming control channels into your scanner.