Call Progress Tone Detector

FX643

© 1996 Consumer Microcircuits Limited

6

D/643/4

1.5

General Description

1.5.1

Overall Function Description

The FX643 Call Progress Tone Detector uses different tone detection methods from those commonly

found with other products.

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, 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 FX643, by contrast, uses a stochastic signal processing technique based on analysis in both the

frequency and time domain, 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 or Special Information Tones, or a profile more likely to match that of speech,

noise or other non-call-progress signals. It checks on whether the signal is a single tone. It also adds

checks on whether tones which include frequencies corresponding with the "US Busy" signals have

been detected.

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 FX643.

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 340Hz to 650Hz within which the FX643 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 FX643 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 having a material frequency component within this range (e.g. 480 + 620Hz), are detected as

Call Progress High Band tones.

Special Information Tone: The nominal frequencies 950Hz or 1400Hz or 1800Hz.

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 FX643 will usually interpret these

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. The

lower level at which absence of an input signal will be registered is not specified.