spandsp 0.0.6

Bell MF tone receiver

# What does it do?

The Bell MF tone receiver module provides for the detection of the repertoire of 15 dual tones needs for various Bell MF signalling protocols. It is compliant with ITU-T Q.320, ITU-T Q.322, ITU-T Q.323B.

# How does it work?

Basic Bell MF tone detection specs:

- Frequency tolerance +- 1.5% +-10Hz
- Signal level -14dBm to 0dBm
- Perform a "two and only two tones present" test.
- Twist <= 6dB accepted
- Receiver sensitive to signals above -22dBm per frequency
- Test for a minimum of 55ms if KP, or 30ms of other signals.
- Signals to be recognised if the two tones arrive within 8ms of each other.
- Invalid signals result in the return of the re-order tone.

Note: Above -3dBm the signal starts to clip. We can detect with a little clipping, but not up to 0dBm, which the above spec seems to require. There isn't a lot we can do about that. Is the spec. incorrectly worded about the dBm0 reference point, or have I misunderstood it?