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CAN I CONNECT YOU?

Talk Back includes a Dual Tone Multi Frequency feature so that it can generate telephone dialling tones for the keypad characters 0 to 9, A to F, star, and hash. Just like the tones you hear as you key-in a phone number. Here they are:

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Of course, technology changes, and you'll probably have noticed that your mobile phone generates the same beep for every key. Unless it's set up like a pocket piano.

DUAL TONE MULTI FREQUENCY
PART OF A ROTARY DIAL

If you listen to most land-line phones as you dial, you'll probably notice that some of the tones sound similar. Surely this must make it difficult for the technology at the telephone exchange to recognise the tones and route calls correctly? So, here's a quick explanation:

Old phones, with a rotary dial, generated sequences of on-off pulses. However, it was easy to fake the pulses and hack into the system.

I can remember, as a child, tapping on the handset cradle, to make and break the circuit, and create pulses which, sometimes, connected me. I told the lucky people that they had won 2000, and should look out for a taxi with two passengers, tap on the driver's window, and hold up two fingers, ready to claim their prize.

Electronic Touchtone phones generate DTMF tones which are sent to the telephone exchange as you press the keys. A human voice, or a simple beeper, can't copy them because each key generates two specific frequencies: one from a low frequency group of four and one from a higher frequency group of four, making a total of sixteen possible combinations. The two combined frequencies produce a single tone. Although electronically different, some of the tones do sound similar.

DTMF
KEY FREQUENCY HZ FREQUENCY HZ
1 697 1209
2 697 1336
3 697 1477
4 770 1209
5 770 1336
6 770 1477
7 852 1209
8 852 1336
9 852 1477
* 941 1209
0 941 1336
# 941 1477

Other tones are not normally available from the keypad:

A is a mix of 697 Hz and 1633 Hz
B is a mix of 770 Hz and 1633 Hz
C is a mix of 852 Hz and 1633 Hz
D is a mix of 941 Hz and 1633 Hz

E is the same as *, usually called star, and F is the same as #, usually called hash. Sometimes, in the telephony industry, A, B, C, and D are known as F0, F, I, and P. Touchtone is an AT+T registered trademark.

Microchips generate, send, receive, and identify the tones. Telephony Application Program Interface, abbreviated to TAPI, software recognises DTMF tones so that you can use phone applications on your computer.

CULTURE TIP FOR ENGLISH LEARNERS
GREG USING HIS MOBILE

If you're not a natural English speaker, or are learning English as a second language, you might be interested in learning English on line. This section is taken from Learning English: there's a link at the top of the page.


Over the years, the word telephone has been abbreviated to 'phone, and now to phone. In the UK, mobile phone is usually shortened to mobile. In the US, they say cell phone.

Most people don't say that they'll telephone you. They say that they'll call you. This is a US use of the verb to call which has become international. It also has a noun form, as in to make a call. Here are some examples:

I'll call you as soon as I get to the airport.
Give me a call after 11:00 on Wednesday.
Don't call me at work.
If anyone calls, ask them to ring back tomorrow.
Call the cinema and see if that Bruce Willis film is on tonight.
Before we leave work, I have to make a quick call home.

Old telephones had an electro-mechanical bell which would ring when someone called you. We still use the verb to ring, for calling someone. It also has a noun form, as in give me a ring. Here are some examples:

I'll ring you in my lunch break.
Give me a ring if you want to meet next week.
Don't ring me too early.
If anyone rings, I've gone to the post office.
Ring the theatre and see if there are seats for tomorrow.
Before we leave work, I need to ring home.

Also, old telephones had a circular number mechansism, called a dial. We still use the noun dial, as the verb to dial, for pressing buttons. Here are some examples:

To call Technical Support, dial 020 8847 2525.
In an emergency, dial 999.

In the Four Pillars Hotel in Witney, a sign in the Health Club said Emergency! To summon attention, dial zero, even though it was a push-button phone.