Re: Comcast Telephone Service????

["Robert L Bass" <robertbass1@xxxxxxxxxxx>]

> Even when using a "full rate" codec such
> as the G.711, it helps if "echo cancellation"
> and "silence suppression" are turned off,
> but I have no idea who does that, or how...

Silence suppression is a means of making a VoIP call sound more like a telephone call by creating background noise (BGN) similar to
what is heard on a POTS call when no one is speaking.  BGN is sometimes referred to as "comfort noise" because it makes the user
comfortable that he is still connected.  Needless to say, BGN is undesirable in a digital call between an aloarm system and the
central monitrong station receiver.

Echo cancellation has been used on POTS phone systems for years.  There is an inherent delay in processing long distance calls.  The
microphone picks up some of the receiver's signal and sends it back, a little bit later, creating an annoying echo.  Echo
cancellation systems are employed by telco's to kill the echo.  The process typically employs two technologies.  An adaptive filter
picks up the echo, recognizes it as a delayed copy of previously transmitted waveforms (or, in more modern systems, bit patterns)
and cancels most of the unwanted signal.

Adaptive filters are not perfect.  They leave an aufdible component of the original echo (hmm; is "original echo" a contradiction in
terms:))?  A non-linear amplifier is employed to clean up what remains.  The precise manner in which this is done is beyond my
limitd knowledge of the subject but it involves suppression of the local signal and the remaining echo component.

One problem with adaptive filters and echo suppression apparently is they tend to see repetitive waveform (data signals are often
repetitive) as noise and kill the signal.  Also, some systems filter out high frequency sounds since they're not an expected
component of normal speach.

Almost every ATA device's CODEC can handle DTMF (touch-tine) signalling.  Becaue some alarm transmission formats employ DTMF, these
may be better suited for use with VoIP service than others, especially pulse formats.  NBo guarranty here that some particular unit
will work.  You just have to experiment.

> So, the key to determining whether an alarm system will work on a VOIP
> system may be to determine the type of codec that is used by the VOIP
> interface.  Assuming the cable companies won't tell or don't know, googling
> the part number might produce some useful information.

Most major manufacturers list the CODEC's and lots of other stuff about their VoIP devices online.  Worst case, you have to make a
phone call.

--

Regards,
Robert L Bass

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