[asterisk-dev] DTMF detection and generation code

[Iain Barker]

Iain Barker wrote:
>Tone duration .. is important for other applications. 
>One example: A "Press 0 to buzz in" door entry system

Steve Underwood wrote:
>Why is a door entry system using DTMF? Sounds kinda odd.

Well, most recent multi-tenant apartment or condo buildings in North America certainly have this... It's now common practice for the door-entry lock to be hooked up to a bank of pushbuttons, one for each apartment.

A visitor presses the intercom/doorbell, and is actually initiating a POTS call from the entryphone to the listed phone number of the tenant. The tenant answers their phone, talks to the visitor, and then if they want to open the door they press a single DTMF key. The door latch is 'buzzed open' only for the duration of the keypress.

Lets assume now the tenant has an Asterisk-routed VoIP line, instead of a POTS phone. If Asterisk has arbitrarily decided to clip tones to some fixed duration (ITU Q.24 minimum of 40ms, or RFC2833 minimum of 3x packet period), then the door-entry system becomes broken for that user.

ps. I'm not saying this is a problem right now. I'm just saying be very careful about making assumptions regarding the use-cases.