GSM Intercom DTMF Decoder Update

The UK Orange network have this annoying policy of ‘squelching’ DTMF tones coming through to recipient phones.  I’m not really sure why they do it as they are the only network in the UK to do this (you can see what the issue is in the video).  A small tone burst does get through but it is very short.

 

We wanted to make the intercom compatible with this network so we had to reduce the guard time of the DTMF decoder to match the short Orange tone burst.  We tried two ways of doing this; updating the software decoder and adding a hardware decoder.

The hardware decoder was fast but we could not detect line silence with it as we can with the software decoder which meant the GSM Door Entry Intercom could end up connecting a caller to voicemail/an answerphone without it knowing.

DTMF Decoder

GSM Intercom DTMF Decoder Update

The software decoder is based on Discrete Fourier Transforms (DFT).  We realised we only need to decode one key (in this case the * key) and so could optimise the decoder for this key without any extra resources required.  We sample for approximately 25ms which is shorter than the Orange tone burst (processing time is less than 1 millisecond as we process as we sample).  We over-sample this 4 times and process each quadrant to ensure we get a good response without requiring any ‘windowing’.  The result is a very tight and rapid decoder that can also detect any general noise on the line.

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