DTMF

Decoder

Purpose

The DTMF Decoder device recognizes sequences of DTMF (Dual Tone Multi-Frequency) tones from a standard Touch-Tone phone and asserts control outputs when user specified patterns are matched.

To use the device, it must be graphically connected to an external analog connection that is patched into the phone system. This connection will typically require a line interface circuit, which provides electrical compliance with the telephone system and call progress functions, such as automatic on-hook and off-hook sequencing.

Tips:

• The DTMF decoder is very tolerant of level variations. However, it will work best if signal levels are adjusted for a nominal 0dB to the device. Erratic operation is a good indication that levels may need to be adjusted.
• You can test the DTMF decoder using the DTMF Generator device.

When the decoder detects an incoming sequence that matches any of the strings, the associated LED is lit (green). The LED remains lit until the next DTMF digit is received. To control a NioNode function, control wire the LED to the appropriate NioNode device. A typical application would be recalling a subpreset, for example.

The DTMF Decoder is designed to continuously monitor the incoming audio signal for valid DTMF digits. It discriminates DTMF digits from voice or other interference audio using the duration and spacing of the DTMF tones as the main detection criteria. The DTMF specification calls for a minimum of 40 msec duration and 40 msec spacing. The DTMF decoder works with tones as short as 30 msec for 48 kHz sample rate systems. However, in 32kHz systems the 40 msec minimum timings should be considered the specification.