DTMF TESTING (VoIP)

7.9
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 is necessary to ensure consistent calibration in interfacing tape tests. Note that initially tapes were used for these tests. Recently tapes were converted to compact disc (CD) [FR-763-01(2006)] format to improve on playout precision. The digitized versions are also useful with simulations without waiting for the actual hardware and telephone interfaces.
Several test-sets such as those from Sage Instruments [URL (Sage935)] also support basic DTMF tests. Signal and timing parameters are controlled accurately with these instruments. It is useful to test with instruments before
subjecting to tape tests. It is also helpful to incorporate equipment as a telephone line tester to monitor various signal levels used in DTMF tests. One example of such monitoring instrument can be found at [URL (Advent-5120)]. Different countries may use target lines with different impedance characteristics. These instruments usually support one or two country impedances. Testing for additional country-line impedances can be an extra effort. These impedances mainly influence the performance of DTMF total power levels by up to 1 to 2 dB. Implementations typically cater to such small margins. In practice, users create their own test cases as either simple test vectors or test vectors played on telephone interfaces to ensure proper functioning, calibration, and robustness for various impediments.
DTMF rejection tests are based on both measurement and user perception of quality. In measurement-based tests, high-power DTMF digits are passed through DTMF rejection and they should not be detected through in-band packets. After ensuring the timing and signal levels, perception tests are important. Several digit combinations and impediments are generated while listening for DTMF rejection ticks.