October 2004, Issue 171

Telephone Message Watchdog
An Intelligent Call-Forwarding System

DTMF DETECTION

The telephone tone dialing system is called dual-tone multi-frequency (DTMF) signaling. It’s a standard telecommunication system developed by Bell Laboratories. In this system, a matrix is used to compose a signal, which consists of a lower frequency group, containing four distinct frequencies that are below 1 kHz, and a high-frequency group, containing four distinct frequencies above 1 kHz (see Table 1). Each telephone key is represented by a pair of simultaneous low- and high-frequency tones.

To detect DTMF signals by software in the digital domain, many algorithms have been proposed. The modified Goertzel algorithm is one of the most efficient computing techniques for detecting a limited number of frequencies. In the case of DTMF tone detection, the Goertzel algorithm analyzes only eight frequencies instead of performing an entire transform using something like FFT. This saves a lot on computational resources, which are critical for lower-power processors. Its noncomplexity is easy to adapt into small MCUs and DSP. Therefore, we selected the modified Goertzel DFT for our design.

Like an infinite impulse response (IIR) filter, a Goertzel algorithm contains a recursive feedback path. For a DTMF tone filter, the following calculation step is performed on incoming data:

where n = 0, 1, …, N – 1. N is the block size, and k = fDTMF/fSAMPLING. (fDTMF is a DTMF tone frequency. fSAMPLING is the ADC sampling frequency.) x[n] is the newly arrived sampled data.

Initially, Sk[n – 1] = 0 and Sk[n – 2] = 0 when n equals zero. In the last step, the modified Goertzel algorithm calculates the power of the frequency:

For computation efficiency, DTMF detection is implemented using the H8S assembler language (see Listing 1 and Listing 2). Each filter uses precalculated coefficients from a different K value corresponding to the DTMF basic frequency. The input audio signal is digitized by the on-chip ADC at an 8-kHz sampling rate. The 20-MHz H8S MCU computation speed is adequate to handle the input data at this sampling rate. The A/D sampling clock arrives from an on-chip 16-bit timer (see Figure 2). A DMA channel is used to feed the converted data to a 115-word buffer. A double-buffered memory storage scheme is used. When the DTMF filtering process is working on a data-filled buffer (first buffer), the DMA controller is feeding data to a second buffer from the ADC. The interrupt service routine then sets the DMA pointing to the first buffer and instructs the DTMF filtering process to start working on the second buffer. This ping-pong buffering is repeated every 14 ms.