VoIP Voice and Fax Signal Processing

7.9 DTMF TESTING

Even though DTMF is of a simple two-tone combination, it demands a significant amount of testing because of its interaction with voice and various line impairments and conditions. DTMF testing is typically conducted with the following combination of tests.

For testing Matlab, C, and assembly software implementations, engineers often create various test vector files for digit combinations and digit deviations/impediments. It is easy to create required tests for simulation purposes.

Once implementations are ready and working on the processors in real time, digits are fed to actual hardware through several telephones. A user may test with phones meeting the requirements of several countries, redial mode, digits of long duration, and short duration. It is common practice to test DTMF with Telcordia and Mitel test tapes as given in [FR-763-01 (2006), Mitel-tapes (1980)]. These tape tests are used for validating detection and for testing the robustness with several impediments.

For testing robustness, speech files are used that run for several hours. For these tapes, a false detection count benchmark is provided that is similar to the listed specifications in Table 7.2. Implementers always try to achieve a lower false digit count with a clear margin. However, it is difficult to ensure test tape signal levels and other playback parameters. The interfaces for connecting the tape players with telephone interfaces may create a mismatch in calibration of signal levels. It ...