Every time we place a call we instinctively perform call-progress analysis (CPA). We listen for dial tone before dialing, detect ring back, busy, answer, and so on, without giving it much thought. But duplicating the nearly unfailing precision of human-based CPA in a multiline communications system requires considerable care in its implementation.
Telephony applications with outbound calling that use the public switched telephone network require the ability to accurately and rapidly interpret the call-progress tones, such as ring back and busy, that are delivered by the network to the calling entity. This call-progress analysis is complicated by the lack of the worldwide use of standard tone frequencies and patterns. The design of CPA signal-processing DSP software must, therefore, address the tradeoff between speed of analysis and generality. Commetrex’ 60080 CPA is designed to minimize the vendor’s support requirements by providing an analysis resource that does not require tuning for over 99% of applications, even for global deployments, while providing accurate responses within 1.5 seconds (for busy) to 5.0 seconds (for ring back-off-hook-no-voice) when using the default parameters.
60080 CPA uses a low- and high-pass frequency discriminator together with energy measurements over time to qualify in-band signaling tones. Three signal characteristics are used for tone detection: the tone frequency, timing (Tone_On, Tone_Off) and period and consistency (the number of times a tone cycle repeats before being qualified).
The design of Commetrex’ CPA is based on these characteristics of call-progress (CP) tones: Nearly all CP tones are comprised of one or two frequencies between 200 and 500 Hz. Although the timing of call-progress tones varies between manufacturers and PTT administrations, there is a nearly universal signal-timing hierarchy that can be relied upon for discrimination between the various signals.
In addition to discerning all the usual call-progress tones and modem-answer tone, this product has a voice-detect capability that enables the using application to determine whether the call has been answered by a residential user, an answering machine, or a business.