Get the Right Signal Tone

Radio amateurs use tone signaling for many different uses on the amateur radio bands. Tone signaling works especially well on FM because the received audio is spot-on in terms of frequency (unlike SSB, where the precision of the tuning can affect the frequency of the recovered audio). On FM VHF and higher, we use tones to perform many functions, including activating repeaters, controlling repeaters, accessing IRLP links and making autopatch calls. This can be confusing for new Technician licensees (and maybe for the old timers, too?) so this article examines the most common tone systems.

DTMF Tones

One of the most common set of signaling tones is called the Dual Tone Multi-Frequency (DTMF) tone system, often known as Touch-Tones. This system was invented for use in telephone systems by AT&T in 1963.  Today, the pleasant dual-tone sound is very familiar to most people as part of everyday telephone use.

When a key is pressed, two sine waves are produced, as defined by this matrix:

DTMF tones

Table 1: The matrix of DTMF keys and frequencies.

For example, pressing the number 6 produces these two frequencies: 770 Hz and 1477 Hz. The frequencies listed in the left hand column are called the “low group”, while the frequencies shown on the top are the “high group”. Most telephones will just have the keys for 0 through 9, * and #. The amateur radio world makes use of a fourth column of keys labeled A, B, C and D to provide some additional signaling options.

These frequencies were carefully chosen so that no frequency is a harmonic of another, which would have increased the possibility of a decoding error. The frequency accuracy is specified at 1.5%, fairly tight, to keep the tones separated.

Most modern FM radios include a DTMF encoder as part of the radio. Mobile rigs tend to have the keypad built into the microphone and handheld radios have a keypad on the front of the rig. DTMF tones are most commonly used for sending commands over the air, including repeater control, autopatch access and IRLP access. DTMF can also be used for selectively calling or alerting another station.

DTMF tones

DTMF tones can be transmitted from an HT to control repeater operations.

DTMF keys mobile

Most mobile transceivers implement the DTMF keypad on the microphone. (Image courtesy Icom America, Inc.)

Distortion is the enemy of any tone signaling system, so DTMF levels should not be set too high. Most radios are adjusted at the factory and shouldn’t require any tweaking by the user. For most amateur FM systems, we use a peak deviation of 5 KHz. One rule of thumb is to set the DTMF deviation at no higher than 2/3 of the maximum deviation (2/3 of 5 kHz equals 3.33 KHz.) Most repeater operators I know set the DTMF deviation between 2.5 to 3.0 kHz. If you are using narrowband FM, such as 2.5 KHz peak deviation, the DTMF deviation should be correspondingly smaller, 1.65 KHz or less.

DTMF is an “in band” signaling system, so we will normally hear the tones being transmitted on the air just by listening on the frequency. Be aware that it is common for repeaters to filter out DTMF tones so they don’t pass through the repeater, so you may not hear the tones on the repeater output.

CTCSS

Many FM repeaters operate using carrier squelch, which means that the repeater keys up any time it hears a signal on its input frequency. I’ve heard some radio technicians refer to this as “going naked”, since any signal that comes along can activate the repeater. In today’s world, we have many electronic devices with digital circuitry spewing out all kinds of frequencies, just waiting to trigger a receiver. (Walk around a typical office building with a handheld radio…the squelch opens up when you pass a computer or other electronic device due to the frequencies being radiated.)

A more controlled squelch system is called Continuous Tone Coded Squelch System (CTCSS). The idea is very simple…the FM transmitter includes a continuous tone on the transmitted audio. When the receiver (repeater) hears the required tone, the squelch opens. If there is no tone, the receiver stays squelched, no matter how strong the signal is at the receiver.

To make the system flexible, 50 unique tones are defined so that different systems can use their own unique tones. (Some radios do not implement all of these tones, so check your radio manual.) This tone system is sometimes used in the land mobile service to allow multiple users to share the same repeater without having to listen to the other users on the channel. Each user group is assigned its own specific CTCSS tone.

 

CTCSS Frequencies

Table 2. Fifty Standard CTCSS Frequencies in Hz (not all radios implement all of these tones)

The CTCSS frequencies are in the range of 67 Hz to 254 Hz. Unlike DTMF tones, which are sent momentarily as the key is pushed, CTCSS tones are sent any time the transmitter is on. This means that the tones will be present with the normal voice signal. To keep the transmitter from over deviating, we usually set the tone deviation at around 600 to 800 Hz. Because the tone is active the entire time the transmitter is on, we’d like to keep it from being heard in the receiver. Although we call these frequencies subaudible they are within the hearing range of most people. Voice communication systems are designed to use the frequency range of 300 Hz to 3 kHz, which allows for normal speech to be understood. As long as we filter out the frequencies below 300 Hz before the signal gets to the radio speaker, the user won’t notice the presence of the CTCSS tone, but we’ll still have the desired voice frequencies. Virtually all modern VHF/UHF FM transceivers for amateur radio use include CTCSS encode and decode as standard features.

Selective Calling

Let’s look at how we can use CTCSS tones for selective calling. Suppose an ARES group wants to monitor a particular simplex frequency on 2 Meters to always be available for a call 24 hours a day. They also don’t want to be awakened by a random call (not from the group) in the middle of the night. They could all agree to program their radios to transmit a particular CTCSS tone, for example 100 Hz. To keep their receivers from unsquelching on non-ARES signals, they would set their CTCSS squelch for 100 Hz on receive (usually referred to as Tone Squelch in the manual.) The group’s receivers will remain silent until a signal shows up with the 100 Hz tone on it.

We can expand this approach to allow another set of users to do the same thing on the same channel, but using a different CTCSS tone (for example, 123 Hz). This second group of users can listen on the same channel but never hear the ARES group.

We do have a very practical problem to deal with. If users from both groups try to transmit at the same time, they will interfere with each other. To keep this from happening and to cooperatively share the frequency, everyone needs to listen on the frequency to make sure it is not in use before transmitting. Some radios have a “monitor” button that opens the squelch to check whether anyone is on the channel. Another way to accomplish this is to temporarily turn off the tone squelch, perhaps by programming another memory channel.

Nomenclature

CTCSS is the technically correct name for this subaudible tone system. You’ll often hear it referred to as “PL” which is short for Private Line, a trademark of Motorola. I always try to remember to say CTCSS but somehow that does not slide off the tongue as easily as PL.

I already mentioned that many repeaters require a CTCSS tone on the user’s  signal to access the repeater. You need to find the correct tone in a repeater directory, or obtain it from the repeater trustee. Some repeaters also transmit a CTCSS tone on the output of the repeater. You aren’t required to use this tone but you can take advantage of it to control the squelch of your receiver. Why would we want to do that? Suppose there was another repeater on the same frequency that was strong enough to be received by your station. You really want to listen to your local machine but occasionally the distant repeater pops open your squelch. Assuming they don’t use the same CTCSS tone on the repeater output, you can mute the distant repeater using tone squelch. Tone squelch is also useful for suppressing random noise sources on your radio (computer hash, spurious signals, etc.).

Keep in mind that most repeaters do not pass CTCSS tones through the system. Common practice is to filter out anything below 300 Hz on the audio path, so attempting to use CTCSS for selective calling through a repeater does not usually work. Check with your local repeater trustee to find out how a specific repeater handles subaudible tones.

In land mobile radio, a different CTCSS frequency may be used on the input and output of a repeater. This is also possible in amateur repeaters but not commonly used (we usually have the same tone frequency on input and output). In fact, many amateur radios cannot be set up for mixed input and output tones.

We call these CTCSS tones subaudible but that are not really “inaudible”. We try to keep these low frequency tones from popping out of the audio speaker but sometimes it still happens. For example, I run a 100 Hz CTCSS tone on the output of my repeater and occasionally I get a report that the repeater transmitter has power line hum on it. In particular, I have gotten this report from hams using headphones on the speaker jack of their FM rig. What the user is really hearing is the 100 Hz tone that is supposed to be there…the use of headphones makes it more noticeable.

Audio Tone Burst

Another repeater access method is to send a short tone burst of 1750 Hz. This method is common in Europe but is rarely used in North America. If you look through your radio manual, you’ll probably find this feature is available on your equipment. This method is sometimes called “whistle up”, since a repeater user with good pitch can access the repeater by whistling a tone into the microphone.

DCS

Digital Coded Squelch (DCS) is a newer signaling system that now comes standard on most amateur FM transceivers. Although it operates differently, it can be roughly thought of as a digital form of CTCSS. Instead of sending a continuous low frequency sine wave, DCS sends a 134.4 bits per second digital signal on the transmit signal. The FM deviation for DCS is similar to CTCSS, about 600 to 800 Hz.

DCS uses a 9-bit digital number that is the basic DCS code. Rather than just sending this binary code out in raw form, DCS makes use of an error correction scheme. DCS sends out a 23-bit Golay code, a special class of binary number that allows for correction of up to 3 bit errors in the code. The 23-bit code contains 3 fields of data: 11 error correction check bits, 3 signature bits and 9 bits for the actual DCS code. The signature bits are always 1 0 0. The 23 bits are each 7.44 msec in duration, for a total time of 171 msec.

DCS

From the user’s point of view, we only care about the 9 bits of the DCS code, since the 11 check bits are prescribed by the Golay code and the Signature bits are fixed. The 9 bit DCS code is treated as 3 octal numbers, each within the range of 0 to 7. At this point, you might expect that there are 29 = 512 codes available, using the entire 9-bit range. The number of available codes is actually much less than this due to some decoding issues. The DCS system has no start or sync bits, so the DCS decoder is tasked with watching the stream of bits go by and detecting the code. With this limitation, some of the 23-bit codes are not truly unique. (Think in terms of rotating the bits around until they happen to match another code. I won’t go any deeper here but this issue is explained very well at the onfreq.com page listed below.)

The specific codes implemented vary between various models of radio, so consult the manual to be sure which codes are supported. As an example, Table 3 shows 104 DCS codes implemented in a Yaesu transceiver. Note that the 9 bit DCS code is represented as 3 octal digits.

DCS codes, tone

Table 3.  A common set of DCS codes (from the Yaesu FT-7900R manual)

Being a digital coding scheme, the polarity of the signal is important. While conventional CTCSS uses a sine wave such that phase or polarity doesn’t matter, the DCS system can become confused by a polarity inversion in the audio chain. Most transceivers provide a way to invert the DCS code to compensate for this problem and maintain the proper signal polarity. If you find that the “normal” polarity is not working, just try changing to “inverted” to see if that fixes the problem.

From the user’s point of view, DCS operates a lot like CTCSS. To access a repeater using DCS, you need to set your radio to the right DCS code (also inverting when necessary). However, don’t count on DCS codes being passed through your local repeater…just like CTCSS, they are normally filtered out. DCS can also be used on simplex frequencies to implement a selective call feature.

Summary

This has been a quick overview of the most common forms of tone signaling used on VHF/UHF FM, both simplex and repeaters.

73, Bob K0NR

References

  1. ITU-T Recommendation Q.23  General Recommendation on Telephone Switching and Signalling
  2. Excellent technical explanation of DCS http://www.onfreq.com/syntorx/dcs.html