Mark and Space refer to the duration a DTMF tone is produced, as well as the duration of the silence between individual digits.
The time which a DTMF digit tone is actually producing sound, is called the “Mark” time. The silence between each one of the digits is called the “Space”. Most DTMF decoders and controllers will list a minimum Mark/Space speed, expressed in milliseconds. Mark/Space is pronounced “Mark and Space” not “Mark divided by Space”. It is not a ratio between the two speeds, but rather the duration of each step in the DTMF code.
The standard for most radio decoders, as well as most telephone equipment is 40/40. The decoders expect the DTMF tones to exist for at least 40 milliseconds, with 40 milliseconds of silence in between each DTMF digit.
While it is a generally accepted practice to have the Space duration be the same as the Mark, this is really not necessary. The purpose of the Space is to give the decoder notice that a DTMF digit just ended. A Space need only be long enough for it to accomplish that purpose. On a fast decoder, a Space can be 5 msec or less and still perform it’s function. While a 40/40 timing is just fine, if max speed is necessary you can consider adjusting the Space timing of your encoder to a faster rate.
Some manufactures such as Whelen Electronics increase the speed to 40/20 for their siren decoders. This increase sends the codes at a faster rate, and uses less “air time” of the transmitter. Some decoders have a problem working with this “Fast Whelen” rate, however all Genave products can handle the Whelen speeds, and even faster, without any problem.