Wednesday, April 15, 2009

Squelch and Tone Squelch, Bob, no. 47

Tonight's topic is squelch, including the related topic of tone squelch.

Generally speaking, in the radio world, squelch is a feature that silences unwanted sounds. What we refer to as just squelch is more properly called carrier squelch. Carrier squelch will silence the audio of your receiver when the signal strength is below a given threshold. This threshold is set by the radio operator. Nearly all FM radios have some kind of squelch level adjustment. It used to be standard to have a knob for continuous adjustment of the squelch threshold. Back then it was pretty common that whenever you had a group of hams with their HT's on, at least one would be adjusting the squelch and you would hear the intermittent sound of background noise hiss as the squelch level was adjusted down into the noise then backed off just above it. Some radios still have this knob but it is becoming common now to have the carrier squelch set to one of 3 or so discrete levels from a menu. This is a compromise to allow for smaller and less expensive radios by having one fewer knob.

The reason to have squelch on an FM radio, as opposed to AM and SSB is that an FM radio will "detect" noise at high audio levels even when there is no signal present. With AM and SSB there is a "noise floor" but because of the way this mode is detected the low amplitude noise creates a small sound compared to a stronger signal. In AM and SSB, it is the amplitude of the noise signal that determines the amplitude of the audio signal. In FM this is not the case. The random frequencies detected by an FM radio look like a strong audio signal and create a loud white noise hiss. Squelch is the answer to this problem. By setting the squelch threshold above the noise level, the radio effectively silences it. When a signal is detected above the squelch threshold the squelch circuit is deactivated and the signal is passed through. This is what is known as "breaking squelch".

Squelch circuitry is not magic. It works pretty well to allow the signal through and not the noise but it is not perfect. Some problems you might have heard are someone that has a signal just strong enough to break squelch but not strong enough to be heard above the noise, or someone that breaks squelch and the signal can be heard but it is "scratchy" or has "popcorn" or some other noise component, or someone that has a signal that is so variable and so close to the squelch threshold that he is "in and out", which is to say the signal doesn't consistently break the squelch. Another way to say that last bit is that the person does not "hold" the repeater.

Still, even with these problems, squelch is a necessary function of FM radio and it serves well to allow us to hear the signals we want and only the signals we want.

One case where you can still hear unwanted signals even with carrier squelch is when someone is transmitting on the frequency but not intending to transmit to you. The common reason for this is when you have two repeaters that share the same frequency pair and that are close enough that it is possible for someone intending to break carrier squelch on one repeater will also do so on the other. Sometimes the repeaters don't even have to be all that close. Occasionally atmospheric conditions will carry VHF and UHF signals hundreds of miles further than they would normally go. The solution for this kind of interference is tone squelch. The standard tone squelch that is built into FM radios is called CTCSS which stands for Continuous Tone-Coded Squelch System. You will also hear them called PL tones (Motorola's trademarked name for it), sub-audible tones, or just tones. The way they work is that the entire time you are transmitting you send a single audio frequency tone along with your voice. The repeater is configured to detect the specific frequency you are sending and will only re-transmit the signal if the tone is present. In turn, the repeater transmits the same tone so that your radio will only break squelch if the signal you are receiving is coming from your repeater. The frequency is one of 38 specified frequencies ranging from 67 Hz to 250.3 Hz. The frequency range of these tones was chosen to be low enough as to normally not be heard. Occasionally, especially with the higher frequencies in this range the tone can be heard but it usually doesn't interfere with the voice audio.

Tone squelch detection and transmission can be turned on and off at the repeater just as it can be at your radio. PSRG usually does not require a tone on incoming signals but usually does transmit a tone on outgoing ones. PSRG's output frequency is 146.96 MHz with a CTCSS tone frequency of 103.5 Hz. There is a repeater in Portland that is also on 146.96 MHz. I didn't find a CTCSS tone listed in the online repeater database I checked. On rare occasions we sometimes get complaints about some of our members breaking the carrier squelch on the Portland repeater. One way to prevent this would be to enable CTCSS detection on both repeaters and require all hams to transmit a tone for their own repeater.

There is a catch with tone squelch. You need to remember that even with tone squelch, you are still transmitting and receiving on the same frequencies. So if our Portland repeater were instead located in Bellevue this would not work. Here's why. You are all familiar with the problem of doubling, that is two people transmitting at the same time. It creates a distorted mess most of the time and at most you will understand only one of the two. If we had a repeater on 146.96 MHz in Bellevue with a different CTCSS tone and tones fully enabled for both repeaters here is what would happen. If only one person was transmitting and using the PSRG tone, then only the PSRG repeater would transmit and only those listening to the PSRG repeater would hear the transmission. Same would be true if there was only one person transmitting but with the Bellevue tone. So far this is working just as we want. But what happens when two people transmit at the same time, one sending the PSRG tone and one sending the Bellevue tone? Both repeaters will "break squelch" and transmit because each one has received the tone it was expecting but both repeaters will receive both transmitted signals and they will send out the signal as a double. This is made worse by the fact that someone could be talking on the PSRG repeater and because of the CTCSS the Bellevue repeater won't transmit anything and the folks listening to the Bellevue repeater will not hear anything and will think the frequency is clear. This makes it even more likely to have doubles.

There are situations where you may want to have a group of radios all on a single frequency but with different tones for different sub-groups. You have to make sure it is well coordinated and that everyone listens with the tone squelch off first before transmitting to be sure the frequency is not in use. This is the reason to have that Monitor button on your radio. On my HT and on most radios it disables both the tone squelch and the carrier squelch. This way you can also find out if the frequency is in use even if you are not using tones but the carriers of the parties that are talking are not high enough to break the carrier squelch.

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