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DTMF Receiver DSP C Code
Description:
detects digits dialled by standard Dual Tone Multi-Frequency (DTMF) telephones.
With DTMF, the 16 possible digits are each signified by pairs of tones,
one from a "low" group - 697Hz, 770Hz, 852Hz, 941Hz - and the
other from a "high" group - 1209Hz, 1336Hz, 1477Hz, 1633Hz.
Requirements on a DTMF receiver design
are conflicting. Detection parameters - tone absolute and relative (twist)
levels, tone frequencies, digit-on and -off timings - have specified tolerance
ranges, either side of which the DTMF receiver must definitely register
or not register the digit conditions. There may be interference from noise
or dial-tone. False digits must not be simulated by speech. Administration
specifications differ. This DTMF receiver combines highly selective digit
detection with very efficient DSP resource use.
The C code is suitable for any processor
with an ANSI-compliant C compiler. It is designed especially for efficient
operation on low-cost fixed-point DSP- and general-purpose micro-processors.
All data memory is specified as 16-bit integer words. Multiplies are integer
16x16-bit with 32-bit products.
The software is in the form of 4 callable
subroutines: two are for initialising per-channel memory & loading
default parameters, one is for accessing any newly detected digit, and
the main one is for per-sample processing. To suit applications, some
digit detection parameters are made user-programmable. Operation can be
at 8.0kHz or 9.6kHz sample rates. A user's guide is supplied and full
test results are available.
Specification:
the conditions under which DTMF digit states are registered either a new
digit when in the "pause" state or else a pause(non-digit) when
in the "digit" state are as follows. The symbols Pt/f/r and
* signify that the parameter is user-programmable. All levels are per
DTMF tone.
| Parameter: | Digit detected | Digit NOT detd. | Notes |
| Frequency offset: | <= 2% | >= 3% | |
| Signal level threshold*: | >= Pt (Pt >= -45dBm) | < Pt & >= 0dBm | Clipping @ -3dBm/tone |
| Forward Twist*: | <= Pf (Pf <= 10dB) | > Pf | |
| Reverse Twist*: | <= Pr (Pr <= 10dB) | > Pr | |
| Signal duration: | >= 40ms | <= 20ms | |
| Inter-digit pause*: | >= 40(30)ms | <= 20(10)ms | ie 40<>20ms (OR 30<>10ms) |
| False detection in speech: | < 1.0 digits (avg.) | using Mitel CM7291 CD | |
| Noise interference: | 1000/1000 digits | +12dB Signal-Noise-Ratio | |
| Dialtone interference: | 1000/1000 digits | -12dB Signal-Dialtone-Ratio |
The code meets all known specifications eg ITU Q.24 Table A-1 and the Mitel CM7291 data sheet.
Processor Load:
| MIPS# | Data memory (words) | Prog memory (bytes)# |
| 1.3 | 4(common)+27(/channel) | 1.4k |
-
#example figures from compiling for the TI TMS320C5000(C55x) DSP processor
at 8.0kHz sampling
Digit analysis option: this function
can be added to the basic DTMF receiver. On each digit detected, reported
are: the digit-on and -off times to 2ms accuracy, the per-tone levels
to 0.5dBm accuracies, and the per-tone frequencies to 1.5Hz accuracies.
Availability: NOW - sale is under licence - integration support offered
