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2021-09-19

All about tones

 Back To Basics

Tones… we all know what they are in non-radio terms, but they pose questioning glances from many Basic course students. When we explain tones during the course. Referring to our hobby, tones may be:

  • PL, CTCSS or sub-audible tones;
  • A report of CW signal quality; or
  • A tone used by repeaters to mark the end of a transmission.

PL, CTCSS and sub-audible tones

Let’s look at a typical question:

B-2-1-5 What is a CTCSS tone?

A. A special signal used for radio control of model craft

B. A sub-audible tone that activates a receiver audio output when present

C. A tone used by repeaters to mark the end of a transmission

D. A special signal used for telemetry between amateur space stations and Earth stations

In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is one type of in-band signaling that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel.  It is sometimes referred to as tone squelch. It does this by adding a low frequency audio tone to the voice. Where more than one group of users is on the same radio frequency. CTCSS circuitry mutes those users who are using a different CTCSS tone or no CTCSS.

All users with different CTCSS tones on the same channel are still transmitting on the identical radio frequency, and their transmissions interfere with each other; however; the interference is masked under most (but not all) conditions. The CTCSS feature also does not offer any security.

A receiver with just a carrier or noise squelch does not suppress any sufficiently strong signal; in CTCSS mode it unmutes only when the signal also carries the correct sub-audible audio tone. The tones are not actually below the range of human hearing, but are poorly reproduced by most communications-grade speakers and in any event are usually filtered out before being sent to the speaker or headphone.

A receiver equipped with a CTCSS decoder will not reproduce a signal unless it carries a given sub-audible tone in the background, for example a continuous 100 Hz tone.  To work with such receivers, a transmitter must be equipped with a CTCSS encoder  Standard tones are in the range of 67 to 254 Hz, below the normal speech frequencies of 300 to 3000 Hz. 

One example of the purpose for repeater tones is here in the Greater Vancouver area of SW Canada. We operate VE7RSC in Surrey on a repeater frequency of 147.36 MHz. About 65 miles south, on Camano Island in Washington State, W7PIG operates on the same repeater frequency pair. Without tones, under ideal propagation, we hear their repeater and they hear ours, in most instances it is poor copy and distorted, so it is classed as interference. By using a different tone on each repeater, only stations on our frequency with our sub-audible tone programmed open up our repeater. The repeater will not respond if no tone, or the incorrect tone is detected. The same situation occurs on Camano Island and that repeater uses a different tone as its ‘key’.

Bear in mind that if both repeaters are in use there is still activity on one frequency by both users. Therefore the disadvantage of using CTCSS in shared frequencies is that users cannot hear transmissions from other groups. They may erroneously assume that the frequency is idle and then transmit at the same time as another user, thus interfering with the other group's transmissions. In our example, the distance is sufficient that there is little objectionable co-use and it is much more tolerable than not using tones.

Tones are not only used on repeaters. Many newer transceivers allow the user to program in a sub-audible tone when you wish to only hear simplex transmissions from other users in your group. Such use may be during an event or an outdoors recreational activity.

CTCSS is an analog system. A later Digital-Coded Squelch (DCS) system was developed by Motorola under the trademarked name Digital Private Line (PL).

Many radios also have a feature typically labelled ‘Tone Squelch’. That is similar to a repeater CTCSS tone but you can program it into your transceiver receive. When enabled, and the transmitting radio sends that sub-audible tone with the audio, your receive is unmuted only when that tone is received with an incoming signal. This feature may be handy if you are with a group of Amateurs and only wish to hear the conversation when one of the group is transmitting.


Therefore, the correct answer to the question in B-2-1-5 What is a CTCSS tone? is

B. A sub-audible tone that activates a receiver audio output when present

Next…

DTMF Tones

Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed in the Bell System in the United States, and became known under the trademark Touch-Tone for use in push-button telephones supplied to telephone customers, starting in 1963. DTMF is standardized as ITU-T Recommendation Q.23.

In Amateur Radio, DTMF is enabled with a keypad on the transceiver or microphone. DTMF uses a mixture of two pure tone (pure sine wave) sounds, a set of eight audio frequencies transmitted in pairs to represent 16 signals, represented by the ten digits, the letters A to D, and the symbols # and *. As the signals are audible tones in the voice frequency range, they can be transmitted and heard through repeaters and amplifiers, and over radio and microwave links.


DTMF is commonly used to turn on a link via radio, such as may be needed to contact a station on IRLP or EchoLink. It was also heavily used before cellular phones became common, as hams could dial telephone calls via their radio using a ‘phone patch’ on a repeater. Some repeaters and clubs still offer this service.

Tones and signal reports

B-2-6-2 What does "RST" mean in a signal report?

A. Readability, signal speed, tempo

B. Readability, signal strength, tone

C. Recovery, signal strength, tempo

D. Recovery, signal speed, tone

"RST", A short way to describe signal reception (Readability: 1 to 5, Signal Strength: 1 to 9, Tone Quality (for Morse): 1 to 9).  For example, "11" unreadable, barely perceptible. "33" difficult to read, weak signal. "45" readable, fairly good. "57" perfectly readable, moderately strong.

The ‘T’ in RST is only used for CW (Morse code) reports and is a, indication by the receiver of the quality of the received tone.

Therefore, the correct answer to the question in B-2-6-2 What does "RST" mean in a signal report? is:

B. Readability, signal strength, tone

And finally…

Repeater courtesy tone

There are no questions in the Canadian Basic Amateur Radio exam question bank about courtesy tones nut the term is listed as an incorrect answer. Regardless, you should be aware of its meaning.

Most repeaters have a courtesy tone just before the transmitter stops transmitting. This is typically a ‘bee-boop’ sound.

The ones that do not will have a squelch tail that will transmit for a second or so after the input carrier or tone drops.

After the courtesy tone or carrier drops, if there is no courtesy tone, you should wait a second or two before keying up the mic. This provides space for others to break in if needed.

The courtesy tone is merely an audible indication that the repeater has finished transmitting and another station may start.

Kerchunking

While we’re on the subject, let’s also mention ‘kerchunking.

This is  one of the most annoying things for a repeater operator and people that listen to the repeater a lot.

Just in case you do not what Kerchunking is… Kerchunking is when you press the PTT and then let off without any speaking.

Some people will kerchunk the repeater but never talk. If you kerchunk the repeater to get it to wake up and ID before starting a new net or QSO that is fine because you follow up the kerchunk with your callsign and start a net or QSO.

Some people think that kerchunking the repeater is a way to test your radio but it really is not a valid test, as you have passed no audio.

In Canada, the rules state that you must ID at the beginning and end of a conversation and at least every 30 minutes (10 minutes in the United States) if your conversation lasts that long. Just a Kerchunk (or many) does not meet the regulations

Bottom line, do not kerchunk the repeater and not ID.

The ‘Alligator’

“The alligator bit you” is a term you may hear when on the air.

This is a slang term for the timer function of a repeater controller that limits the length of time the transmitter will remain keyed without a pause.   Typically the timer is set between 3 and 5 minutes. 

It is a protection device to prevent the repeater from overheating when transmitting. Aside from so-called “Long-winded operators”, I have seen instances where repeater users have accidentally sat on their mics and transmitted for some time without realizing it (when you are transmitting you do not hear other users or the repeater because your receiver is muted). If the time-out timer did not exists the repeater power amplifier would overheat causing an expensive repair.

Such times are not unique to repeaters. There is a very good chance that the transceiver you own also has protective circuitry to limit transmitting time.

~ John VE7TI

'Back To Basics' is a regular column in our newsletter, available at https://ve7sar.blogspot.ca


 

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