CB Radio CTCSS tone code squelch
Continuously transmitted subaudible squelch systems such as CTCSS (Continuous Tone-Coded Squelch) and DCS (Digital Coded Squelch) are available on FM radios including CB radios so equipped. CTCSS on CB radio allows maximum range while virtually eliminating unwanted static and skip from other users. FCC WT Docket 10-119 permits CTCSS and DCS on CB radio, along with FM mode and other long-awaited and useful features to the American public. We will call CTCSS and DCS “coded squelch” for short. Subaudible coded squelch is reliable for communications systems that have adequately high SNR and are frequency stable. For example, satellite downlinks, broadcast radio remote control, utility control, broadcast radio identification, which are all applications that have a long time to decode repetitive signals. Mobile two-way radio has typically weak and fading signals with interference from co-channel users and general noise. The squelch should open quickly (say with half a second) and close quickly at the end of the transmission to avoid annoying bursts of noise. The coded squelch must tolerate Doppler shift and frequency drift between transmitters and receivers.
Subaudible continuous coded squelch is not available on AM modes in general. Some communications receivers have coded squelch that also seems to work in AM, whether by accident or design. I have tested such receivers with a signal generator and on AM mode, coded squelch can seem to “work” on the bench. Why then isn’t AM mode CTCSS or DCS generally available?
The following sections explain why CTCSS and DCS are only useful on FM radios. Selective calling on AM and SSB is available by on-channel signaling such as Selcall CCIR 493-4. Selcall CCIR 493-4 is a selective calling system that works on AM and SSB by using in-band signaling that takes several seconds at the beginning of a conversation to open the squelch of one, many, or all receivers in a group. Selcall is used in HF marine radio, HF land mobile radio, and HF amateur radio and is also legal for CB radio in the USA.
Threshold effect and capture effect
FM receivers have a threshold effect that makes them less susceptible to noise than AM receivers. Once the input signal exceeds the noise floor sufficiently, the output SNR increases dramatically. This is the threshold effect. The threshold effect is why FM is less susceptible to noise than AM. The FM capture effect is a similar phenomenon where the FM receiver tends to hear only the strongest signal once it’s a little bit stronger than all the other signals.
These two effects make CTCSS, DCS, and other subaudible modulation like LTR work extremely well on FM, but are completely impractical on AM for mobile two-way communications systems.
SNR compromises
Continuous subaudible coded squelch modulation must not consume too much of the available modulation bandwidth, or the output SNR (audio quality at the receiver’s speaker) will suffer. In FM systems, the deviation of the subaudible information has generally been accepted by industry and de facto standards in the range of 15% .. 35% of the deviation of the main audio signal. For example, on obsolete 5kHz deviation FM systems, the CTCSS, DCS, or trunked subaudible deviation would be 750-1000Hz. In 2.5kHz deviation FM systems, the subaudible deviation would be 375-500Hz for CTCSS or DCS, and 800 Hz for LTR trunked subaudible modulation. Practical experience with 450 MHz and 900 MHz SMR trunked systems led some operators to set the LTR subaudible deviation to 1000 Hz instead of 800 Hz despite the 2.5 kHz maximum total FM deviation, compromising the audio quality and output SNR of the main signal.
If we apply the same principles to AM, we would have to reduce the modulation depth of the main audio signal to accommodate the subaudible tone. This would reduce the output SNR of the main signal, which is already compromised by the nature of AM. The output SNR of AM is generally lower than FM, and the modulation depth of AM is already limited by the need to avoid overmodulation. Adding a subaudible tone to AM would further reduce the modulation depth of the main signal, reducing the output SNR of the main signal. This would reduce or eliminate the slight advantage AM has over FM in weak input SNR conditions.
By way of reference, in broadcast AM (530-1710 kHz) the subaudible modulation depth by 47 CFR § 73.1570 (b)1(ii) is limited to 6% of the total modulation depth. Utilities that used broadcast AM subaudible tones for remote control of transmitters with such little modulation assumed a powerful signal and consistent input SNR with fixed location receivers. Canadian patent CA1149023A discusses using angle modulation of the carrier frequency within 20 Hz of nominal channel center frequency to avoid the typical 5% modulation depth limit of AM. The broadcast AM subaudible signaling standard was field tested in about 1969 as discussed in FCC Docket 17873 that permitted AM broadcast subaudible signaling. The C-QUAM AM stereo system uses a 25 Hz phase-modulated tone at 5% deviation to indicate stereo availability. Once again, strong signals and consistent input SNR are necessary for successful detection of the pilot tone, and detection is not near-instantaneous as required for coded squelch.
Suppose a 5% subaudible modulation depth is used on CB Radio for coded squelch. The power in each sideband of the subaudible tone would be 50 milliwatts (0.05 Watts) for a 4 watt carrier. The input SNR would have to be sufficient to decode the subaudible tone. The output SNR of the main signal would be reduced by the subaudible tone.
Thus the useful communications range between stations would be so dramatically reduced to open the coded squelch that the feature would be impractical with AM mode.
Beat frequencies false squelch
When more than one carrier is above the noise level in an AM communications receiver, the carriers produce a beat frequency defined by the difference in carriers’ frequencies. The beat frequencies may typically fall within the subaudible tone range given modern transmitter frequency stability. This can cause false squelch opening on the receiver. Conversely, the beat frequencies may interfere with the subaudible tone, causing the receiver to not open squelch when it should.