Squelch is the feature of a radio that turns off the speaker when there is no signal on the tuned frequency. Especially at VHF and UHF frequencies (30 MHz and above), signals stand out from the noise in an unused channel, and it is easy to open the speaker for signal and close it for noise. The IC PCR1000 makes four different kinds of squelch available — four different ways to electronically determine whether what’s on a frequency is really a signal.
- Noise Squelch is the kind of squelch found on most analog radios. The audio on the channel is analyzed for whether it is more characteristic of noise or a signal. Squelch is closed if the audio is sufficiently noisy, and opened if it is sufficiently coherent. The squelch slider adjusts the threshold separating noise from signal.
- Signal Squelch opens the squelch (turns on the speaker) when the received signal is sufficiently strong. The squelch slider adjusts the signal-strength threshold.
- CTCSS, or Tone Squelch will keep squelch closed, even if there is a signal of sufficient strength and clarity to open signal or noise squelch, unless the audio in the signal contains a specific low-frequency tone. This scheme is often used in interference-rich systems, where spurious signals are strong enough that they would be heard continuously if they were not locked out by the absence of the agreed-upon tone. The PCR–1000 can listen for any of 51 standard CTCSS tones.
- Voice Scanning Control is a sophisticated squelch method that opens the squelch only when the audio on the current channel matches the characteristics of human speech.
All controls related to squelch are located in the box labeled Squelch, on MacR–1000’s main panel.
The slider at the top of the box controls the threshold level for noise squelch (or signal squelch if selected), with the full-right setting being the most restrictive, and the full-left setting being open squelch. The usual technique for setting a threshold squelch is to start at the open (left) position, and raise the level until noise on the channel is no longer audible.
The Open button next to the squelch-level slider temporarily sets the squelch level to full-open. Use this feature to check for weak signals that may not be making it through your squelch setting; use it also on HF (below 30 MHz) or on “noisy” modes like sideband or CW, where squelch can mask usable signals.
Press Open again to restore the previous squelch threshold. Adjusting the squelch level while the Open button is depressed releases the button.
Checking the CTCSS box closes squelch to all signals that do not carry the “sub-audible” tone selected in the adjoining box. To select a tone frequency, type it into the combo box next to the CTCSS check box, or click on the down-arrow in the combo box to select from a list of available CTCSS frequencies. Press return or tab to register your choice.
Searching for CTCSS tones is covered in another section of this chapter.
Checking the S-Meter Squelch box switches the threshold squelch mode from noise squelch to signal-strength squelch. If the signal-strength meter is in S-meter mode, a triangle will appear in the meter to show the minimum signal strength required to break squelch. Consider S-meter squelch if the frequencies you are interested in won’t quiet except at very high levels of noise squelch.
Check Voice Scanning Control to turn VSC on. The green VSC indicator will light up, and squelch will open, only when the PCR1000 detects audio characteristic of a human voice on the selected channel.
Often, when CTCSS tones are used, the correct tone for a channel is not known. MacR–1000 can help you find the CTCSS tone being transmitted on an active frequency.
Tune the radio to the frequency, mode, and filter width you are interested in. With CTCSS Squelch off, verify that you can hear transmissions, and that the signal or noise squelch silences the radio when nothing is being transmitted. Press the Search button next to the CTCSS combo field in the Squelch box.
While the Search button is depressed, MacR–1000 will try all available CTCSS tones in succession until a tone is found for which a signal comes through. The search can proceed only while a signal is present on the tuned frequency; searching pauses automatically when the frequency is not in use.
If the correct CTCSS tone is found, MacR–1000 releases the Search button, leaving the CTCSS button checked, and the tone field set to the correct tone.
If MacR–1000 tries all available tones without finding one that matches the signal, it releases the Search button, and the CTCSS button will be unchecked. In most cases, this means that no tone was being transmitted in the signals heard during the search. It is possible, however, for the timing of the search and of signal availability to coincide to the effect of falsely rejecting a correct tone. Repeat the tone search if you believe a tone is in fact present.