DTMF Decoder

Purpose

The DTMF decoder recognizes sequences of DTMF (Dual Tone Multi-Frequency) tones from a standard touch tone phone. The basic device simply activates an LED, but with control grouping, this basic functionality can be extended to allow your phone to directly control many functions in a Media Matrix system.

To use the device, the input must be graphically connected to an external analog input 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. 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.

When the decoder detects an incoming sequence that matches any of the strings, the associated LED is activated. To control a MediaMatrix function, simply “control group” the LED to the appropriate control. Typical applications might be recalling a subpreset or bypassing a function. This is where the View designer’s creativity can really come out. The LED will remain active until the next incoming DTMF digit.

The DTMF decoder is designed to continuously monitor the incoming audio signal for valid DTMF digits. Its most difficult task is not decoding the digits, but discriminating DTMF digits from voice or other interference audio. One of the key elements of the decision making is the duration and spacing of the DTMF tones. 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.

Controls

PATTERN

This is a script string to be recognized. It may only contain the digits 0-9,A,B,C,D,*,and #. It is suggested that the “#” key be reserved for an enter function. This is an effective way to signal the end of the string and also helps prevent detection of partial strings. It is further recommended that command strings be of the same length. This is not a requirement, but the designer must be aware that if string1 is set for 4321# and string2 is set for 54321#, when the input sequence 654321# arrives, both LEDs will become active.

Please note that although the decoder will decode A,B,C, and D, these keys are not present on a standard phone. These are available on military phones and certain custom dialing devices. These 4 digits can also be used to further secure command sequences.

The standard device is configured to recognize up to 8 patterns, each of length 1 to 16 characters. The standard device can be easily extended to a max of 50 strings by editing the Algorithm string, duplicating the pattern and LED fields, and renumbering the control id’s of the new controls.

Match LED

Each Input String has an associated LED that is active only when the incoming DTMF digit sequence matches the input string. The LED will remain active until the next incoming digit invalidates the string match.

Input

As each DTMF digit is received and decoded, it is shifted into the rightmost position of the Input field. You can use this to verify that the decoder is correctly wired into the system.