Technical Helpweb

Dialogic® DM3 and JCT Media Boards

Functions for Detecting User Defined Tones

1. In functions where cadence is not defined (dx_bldst(), dx_blddt()), the "on time" is also not defined. This means a tone can be detected if the defined frequency is present on the line for as little as 40ms! This is especially problematic with dx_bldst(), where even human voice can trigger a single frequency for that period of time.
   
   When user-defined tones are used in an application, they are usually present on the line for significantly longer than DTMFs, since DTMFs are usually about 100ms long and user-defined tones are often 200-500ms or longer.
   
   
2. In functions where only one frequency is defined (dx_bldst(), dx_bldstcad()), there have been issues because of the way the tones are detected. All tones are detected as dual-frequency tones. When the dx_bldst*() convenience functions are used, the second frequency is automatically filled in as 0. Usually for single frequency tones, the second frequency equals 0, so there would be no problem.
   
   Sometimes, however, a second frequency is detected that is very close to the first frequency. This has been reported in the field, and can be resolved by making two definitions to dx_blddtcad().

• Use dx_blddtcad() for all tone definitions. The small amount of extra data entry far outweighs the possibility of poor tone detection in the field.
• Using dx_blddt() and dx_bldstcad() are relatively safe, but may have potential drawbacks that make it safer to use dx_blddtcad().
• Generally, one would avoid using dx_bldst().