DTMF Detector

Currently, there is no single standard that explicitly and exhaustively covers all issues connected to the detection of DTMF digits.

• There are several ITU documents (Q.23, Q.24), TIA/EIA-464B recommendations, Mitel and BellCore tests tape, as well as other sources, but even if taken together these do not provide complete coverage.
• Different applications, such as CO, IVR, PBX, VoX, and voice mail, have different requirements for DTMF detectors.
• Different countries have different views regarding what is an acceptable DTMF digit and what is not; they have different dial tones and modifications (like stutter dial), which have to be accounted for. In some countries, PBXs use one kind of dial tone, while PSTN uses another.
• DTMF signaling was proposed more than 30 years ago simply to replace slower pulse dialing, and since then it spread well beyond initial scope.

Proposed DTMF detector accounts for all these complications. It is based on multi-rate signal processing and advanced short-term spectral analysis methods (a practical version of theoretically optimal Maximum Likelihood Estimator [MLE]), to ensure superior precision and robustness.

DTMF detector provides "Early Detection Event" within 13...16 ms from the digit start which has the talk-off immunity in the range, allowed for complete DTMF digit detection (exact numbers depend on the configuration thresholds' settings). That allows significant shortening of the system delay in VoIP applications (low-latency DTMF relay).

DTMF detector does not use notch filters to suppress dial tone, so it tolerates relatively high-level echo dial tone 10...20 dB above DTMF level) even if its frequencies are quite different from the nominal, exceeding standard +/-0.5% tolerance. Moreover, different dial tones, such as single 400 Hz, are suppressed as North American ones are. The DTMF detector works in the presence of stutter dial tone (but less hot one).

DTMF detector provides estimates on the frequency and energy of detected digits, which may help carriers identify faulty CPE if customer complains arise. Typical error of frequency estimation is 0.15% RMS. Typical error of energy estimation is 0.35 dB RMS.

DTMF detector is capable of satisfying requirements of any particular carrier in any country. DTMF detector can be configured on the fly, and such parameters as twists, frequency acceptance, spectrum cleanness, and signal duration thresholds can be altered during a call. DTMF detector is designed as a system component rather than a stand-alone algorithm; provides statistics and is capable of non-intrusive monitoring.

DTMF detector fully exploits advantages of modern DSP architectures and uses very low MIPS.

The archive with ANSI C, 'C54xx, 'C55xx Source code is available from download page.

    Specs

• 'C55x Low 0.25/0.35 MIPS
  (10/5ms frame sizes)
  
  
• Fully Run-Time Configurable
  
  
• Dynamic range
  from -55 dBm
  
  
• Talk-off hits (BellCore):
  0...5
  
  
• Early detection Latency
  13...16 ms
  
  
• Early detection talk-off hits (BellCore)
  200...500
  
  
• Noise Immunity typical
  15 dB
  
  
• Noise Immunity (special config)
  7 dB
  
  
• Dial tone immunity
  -15 dB
  
  
• Stutter Dial tone immunity
  -5 dB
  
  
• Full Specs (233K)