Call Progress Tone Detector

MX643 - Preliminary Information

© 1996 MX•COM, INC.

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Doc. #20480148.001

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Nominal: Subject to dynamic tolerances within the signal analysis process. Absolute values are not material or

adverse to performance.

4.2 Overall Function Description

The MX643 Call Progress Tone Detector uses different tone detection methods from those commonly found with

other Call Progress Tone Detectors.

Many traditional devices use a bandpass filter followed by an energy detector. The filter is usually designed to

pass input signals with a frequency between about 300Hz and 700Hz. Each signal amplitude in this range is then

checked against a level threshold. Any signal of acceptable level in this frequency band is classed as a Call

Progress tone, including speech and noise. False outputs caused by speech are commonly indicated by such

products and background noise may lead to a stuck detect output.

In contrast, the MX643 uses a stochastic signal processing technique. This technique is based on analysis of both

the frequency and time domains, with signal amplitude forming a minor part in the decision process. This analysis

includes, checks on whether the signal has a profile which matches international standards for Call Progress

Tones, Special Information Tones, or a profile more likely to match that of speech, noise or other Non-Call

Progress signals. Checks are also performed to see if the signal is a single tone, and whether the tones which

include frequencies corresponding with the ‘U.S. Busy’ signals have been detected.

5. Block Diagram Description

(Reference Figure 1)

Slicer

The input signal to the slicer is amplified by a self-biased inverting amplifier. The dc bias of this input is internally

Signal Analyzer

The frequency range, quality, and consistency of the input signal is analyzed by this function block. To be

classified as a Call Progress signal the input signal frequencies should lie between 340Hz and 650Hz. To be

classified as a Special Information Tone, the input signal frequency should lie between 900Hz and 1000Hz or

1350Hz and 1450Hz or 1750Hz and 1850Hz. The signal to noise ratio must be 16dB or greater. The signal must

be consistent over a period of about 145 ms. These decode criteria are continuously monitored and the

assessment is updated every 7 ms, reference Figure 3.

The Signal Analyzer samples the call progress signal at 9.3kHz. Care should be taken to avoid high frequency

signals (

≥8.4kHz) aliasing into the call progress band.

Control Logic

This block categorizes the nature of the signal into various decoded output states and controls the three output

pins. See the Decode Output Truth Table in section 5.1

Clock

An external clock source is to be connected to the CLOCK input pin when using the 8-pin device. If using the on-

chip oscillator (16-pin device only) you will need to use the external components shown in Figure 2. If using an

external clock source then XTAL should be left unconnected.