Call Progress Tone Detector

6

MX633 - PRELIMINARY INFORMATION

© 1997 MX

•COM Inc.

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Doc. # 20480135.003

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4. General Description

The following Glossary and Decode Truth Table (found in section 4.4), describe the MX633 decoding features and

functions.

4.1 Glossary

Call Progress Tones: The single and dual frequency tones in the range 350Hz to 620Hz specified widely for call

progress signaling.

Call Progress Band: The nominal range 340Hz to 650Hz within which the MX633 will detect Call Progress tones. The

detection algorithm requires that the tones have the characteristics typical of Call Progress Tones.

Call Progress Low Band: The nominal range 340Hz to 490Hz. The MX633 will detect single or dual tones falling

entirely within this range as Call Progress Low Band tones.

Call Progress High Band: The nominal range 600Hz to 650Hz. Single tones in this range, or dual tones

(e.g. 480 + 620Hz), having a material frequency component within this range are detected as Call Progress High Band

tones.

Non Call Progress Signal: A signal falling within the nominal range (a) 200Hz to 800Hz, but NOT within the Call

Progress Band, or (b) within the nominal range 200Hz to 800Hz, but NOT meeting the detection requirements when the

signal falls in the Call Progress Band.

Subject to the duration and other characteristics of such signals, the MX633 will usually interpret this as a Non Call

Progress Signal (e.g. speech or other signal activity).

Minimum Input Signal: The minimum signal level for the specified tone decoding performance.

No Signal: A signal falling outside the nominal range 120Hz to 900Hz or the absence of an input signal. Either will be

detected as a No Signal condition.

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 MX633 Call Progress Tone Detector uses tone detection methods different from those methods commonly found in

other Call Progress Tone Detectors.

Many Call Progress Tone Detectors 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 checked

against a level threshold. Any signal of acceptable level in this frequency band is classified as a Call Progress tone,

including speech and noise signals. False outputs caused by speech are commonly indicated by such products and

background noise may lead to a stuck detect output.

In contrast, the MX633, uses a stochastic signal processing technique. This technique is based on analysis of both the

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

checks on whether the signal has a profile that matches international standards for Call Progress tones, speech, noise or

other Non-Call-Progress signals. It also checks for U.S. Busy signal tones.

4.3.1 Slicer

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

4.3.2 Signal Analyzer

The frequency range, quality and consistency of the input signal is analyzed by this functional block. To be classified as a

call progress signal, the input signal frequencies should lie between 340Hz and 650Hz. The signal to noise ratio must be

16dB or greater and the signal must be consistent over a period of about 140 ms. This decode criteria is continuously

monitored and the assessment 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.