Dual-Tone Multi-Frequency (DTMF) Signaling DHRA Training Item – 2/5/2015 Dave Smith, N4DSZ
DTMF Signaling
● DTMF Use in Amateur Radio
● History of DTMF
● DTMF Technical Characteristics – Full DTMF Keypad Layout and Tone Frequencies – Touch Tone Telephone Keypad Layout – The Meaning of “Dual-Tone” and “Multi-Frequency” – DTMF Keypad Layouts for Amateur Radio ● Questions?
DTMF Use in Amateur Radio
● DTMF signaling has several uses on amateur radios: – Entering a key sequence to access a closed repeater – Entering a key sequence to access and control features such as IRLP – Dialing a telephone number for an autopatch ● To transmit DTMF, press the push-to-talk switch and then press the desired numbers and symbols
● DTMF tones will be audible to the receiving station as long as keys are being pressed (for example, here is what “0” sounds like)
● As you can hear, DTMF tones are distinct from sub-audible repeater access or squelch tones, such as the 100 Hz tone needed for the Draper repeater
History of DTMF
● “Early” telephones used a rotary dial to generate pulses on the telephone line that corresponded to each digit being dialed
● For anyone who has not seen one, YouTube has a number of videos that show “rotary dial” telephones – Rotary dialing was better than having to ask the operator to connect your call – But rotary dialing was still mechanical and slow ● American Telephone & Telegraph Company (which was known back then as “Ma Bell”) wanted a better dialing solution
● Around 1960, “Ma Bell” introduced “Dual-Tone Multi-Frequency” (DTMF) signaling, which they trademarked as “Touch Tone”
● The Touch Tone trademark was canceled in 1984 after “Ma Bell” was broken up by the U.S. government, but the phrases “touch tone” and “tone dialing” have lived on
● ITU Recommendation Q.23 describes DTMF
Full DTMF Keypad Layout and Tone Frequencies
Tone 1209 Hz 1336 Hz 1477 Hz 1633 Hz Freq.
697 Hz 1 2 3 A
770 Hz 4 5 6 B
852 Hz 7 8 9 C
941 Hz * 0 # D
Touch Tone Telephone Keypad Layout
● The typical touch tone keypad has 4 rows and 3 columns of digits and symbols: – 1,2,3 – 4,5,6 – 7,8,9 – *,0,# ● The DTMF standard specifies a 4th column (A,B,C,D) that is rarely seen on telephones
The Meaning of “Dual-Tone” and “Multi-Frequency”
● Multi-Frequency: DTMF uses a total of 8 frequencies – Four row frequencies (“Low Group” < 1000 Hz): 697 Hz, 770 Hz, 852 Hz, 941 Hz – Four column frequencies (“High Group” > 1000 Hz): 1209 Hz, 1336 Hz, 1477 Hz, 1633 Hz ● Dual-Tone: DTMF simultaneously transmits two tones when a key is pressed – One tone from the “Low Group” corresponds to the key's row – One tone from the “High Group” corresponds to the key's column – Using one tone from each group separates the tone frequencies and makes them easier to detect
DTMF Keypad Layouts for Amateur Radio
● Unlike telephone keypads, amateur radio DTMF keypads typically include all 16 digits and symbols: 0-9, *, #, and A-D
● However, the keypad layout varies from radio to radio: – Some radio keypads follow the standard layout for all 16 keys – Some radio keypads follow the standard layout for the 12 telephone keys, but use non-standard layout for A-D – Some radio keypads do not label A-D even though they support them – HOMEWORK: Look at your radio and consult your radio's manual ● Regardless of a radio's keypad layout, the transmitted tones should follow the standard