1.5
General Description
1.5.1 Overall Function Description
The CMX673 Call Progress Tone Detector uses different tone detection methods from those
commonly found with other products.
Many traditional devices from other suppliers 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, and the amplitudes of signals in this range are then checked against a level
threshold. Any signal of acceptable level in this frequency band is classed as a Call Progress
tone, including signals due to speech and noise. False outputs caused by speech are a common
feature with these products, and background noise may lead to a stuck “detect” output.
The CMX673, by contrast, uses a stochastic signal processing technique based on analysis in
both the frequency and time domains, with signal amplitude forming part of the decision process.
This analysis includes checks on whether the signal has a “profile” which matches international
standards for Call Progress tone, or a profile more likely to match that of speech, noise or no
signal.
The following Glossary, and the Decode Truth Table in Section 1.5.4 provide a simple
explanation of the decoding functions and features offered by the CMX673.
1.5.2 Glossary
Call Progress Tones: The single and dual frequency tones in the range 350Hz to 620Hz
specified widely for call progress signalling.
Call Progress Band: The nominal range 315Hz to 650Hz within which the CMX673 will detect
Call Progress tones. The detection algorithm requires that the tones have the characteristics
typical of Call Progress Tones.
No Signal:
The absence of an input signal or
A signal below 250Hz or
A signal between 750Hz and 10kHz.
Note that signals above 10kHz should be at a level below -38dBm so as to avoid
aliasing.
Nominal: Subject to dynamic tolerances within the signal analysis process. Absolute values are
not material or adverse to performance.
ã 2001 Consumer Microcircuits Limited
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D/673/5
CMLMICRO [ CML MICROCIRCUITS ]
