spandsp 0.0.6

The 2280/2400/2600Hz signalling tone processor

# What does it do?

The signalling tone processor handles the 2280Hz, 2400Hz and 2600Hz tones, used in many analogue signalling procotols, and digital ones derived from them.

# How does it work?

Most single and two voice frequency signalling systems share many features, as these features have developed in similar ways over time, to address the limitations of early tone signalling systems.

The usual practice is to start the generation of tone at a high energy level, so a strong signal is available at the receiver, for crisp tone detection. If the tone remains on for a significant period, the energy level is reduced, to minimise crosstalk. During the signalling transitions, only the tone is sent through the channel, and the media signal is suppressed. This means the signalling receiver has a very clean signal to work with, allowing for crisp detection of the signalling tone. However, when the signalling tone is on for extended periods, there may be supervisory information in the media signal, such as voice announcements. To allow these to pass through the system, the signalling tone is mixed with the media signal. It is the job of the signalling receiver to separate the signalling tone and the media. The necessary filtering may degrade the quality of the voice signal, but at least supervisory information may be heard.