7.10 SUMMARY AND DISCUSSIONS
A DTMF signal is a burst of two pure sinusoidal tones. DTMF generation is always very simple in implementation. DTMF detection and rejection operations work in close coordination. DTMF generation and detection are essential in FXS and FXO interfaced VoIP systems. DTMF rejection is optional in some implementations. In the process of analyzing DTMF detection and rejection implementation, engineers may come across several contradicting requirements that force incorporation of extra logic to tolerate deviations in algorithmic steps. The DTMF processing effort is low compared with voice compression and echo cancellations. Standard single multiplier accumulator (MAC)-based digital signal processors (DSPs) require 1 to 4 MIPs (million instructions per second) processing for DTMF detection and rejection functions. DTMF modules occupy about 1000 lines of code in the DSP assembly implementations. It is difficult to justify the best scheme for DTMF. Every scheme discussed in this chapter had certain merits and demerits. Implementers always tweak certain logic and algorithms to tune the algorithms. It becomes an implementation-specific proprietary effort to achieve good overall performance from the selected DTMF detection and rejection method.
VoIP Voice and Fax Signal Processing, by Sivannarayana NagireddiCopyright © 2008 by John Wiley & Sons, Inc.
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