TTY Tones
TTY characters consist of a mix of low and high pitched tones. In the language of bits, the low frequency stands for '1' and the high frequency stands for '0'. Each pattern of 5 bits stands for a letter or number. For example, this image shows the bit pattern '10110', or the letter 'P'. When you press 'P' on a TTY, this pattern of tones is what comes out. When the TTY at the other end of the phone line hears this pattern of tones, it decodes it as the letter 'P'.
Digitizing TTY Tones
Notice that the TTY waveform looks different from the voice waveform:
- TTY is a "pure" tone signal of one frequency, not a lot of mingled frequencies
- The TTY's top amplitude is always the same
Keep in mind that the digitizing method used for telephones is based on what is needed to process speech. Since the phone system is not expecting TTY tones, it does not do as good a job digitizing TTY.