Touch-Tone Telephones When a key is pressed on a touch- tone telephone, the keypad generates two pure tones, which combine to produce a sound that uniquely identifies the key. The figure shows the low frequency f, and the high fre- quency f, associated with each key. Pressing a key produces the sound wave y = sin(27f) + sin(27ff).

Touch-Tone Telephones When a key is pressed on a touch- tone telephone, the keypad generates two pure tones, which combine to produce a sound that uniquely identifies the key. The figure shows the low frequency f, and the high fre- quency f, associated with each key. Pressing a key produces the sound wave y = sin(27f) + sin(27ff).