DSL needs SPECIAL filter?

[miamicanes]

I've seen a few posts here and elsewhere that imply that a M1G used on a phone line with DSL needs some special kind of filter or interface, allegedly because it needs to be able to seize the line, and that it's not adequate to simply use normal DSL filters between the alarm controller and demarc.

I'm at a total loss trying to understand this. Why would the alarm need to be able to "seize" the line away from a device that never interferes with the ability to place voice calls anyway? The only reason filters are needed is to keep anyone who picks up a phone directly connected to the line from hearing high-pitched noise (the lower end of the frequencies used by DMT)? It's not like the alarm could be relying on frequencies above 6KHz to transmit info to the monitoring service... by definition, the phone company digitally attenuates anything above 6KHz before it leaves the phone company's substation anyway.

Someone please tell me why it WOULDN'T work to split the line at the demarc, pass one branch straight to the DSL modem, and pass the other branch straight to the alarm controller (through an inline DSL filter), using other jack on the alarm controller to feed the rest of the phones in the house? If the controller wants to make a call, it will be able to grab the phone line away from any voice calls in progress anyway?

Or is this just an issue cooked up by UL or somebody's paranoid lawyers, insisting that the alarm be in total control of the line "just in case" a fire damaged the DSL modem enough to make it grab the phone line like a normal phone & prevent the alarm from dialing out? Because I really can't see how a normally-functioning DSL modem could interfere with a device that's ultimately just making a normal voice call.

 

[nightwalker]

Basically you want the alarm to be able to seize the line so if there is an alarm event it's able to call out to the monitoring company. If the alarm is not in control of the phone line then someone talking, or the phone line being used for any purpose, or even a phone off the hook by accident would prevent the alarm from being called in. If it's critical to you for the alarm to be able to report an emergency then the panel must be in control of the phone line.

 

[rfdesq]

Why would the alarm need to be able to "seize" the line away from a device that never interferes with the ability to place voice calls anyway?

The line seize ability is not seizing the line from the DSL portion but is seizing the line from the bad guy. If the first device was not the alarm panel then picking up any extension in the house would interrupt the call being made to the central station. Here is a link to a picture of a DSL filter/splitter that I like http://www.homephonewiring.com/images/PS-15.jpg. This device gives you line in, a clean dsl line to the modem, and a clean pots line to the alarm panel.

 

[CollinR]

I hadn't replied as I think you have your mind made up but, yes. Also you'll need an RJ31x which you may also think is not really required and the system will function correctly without it.

 

[Steve]

The alarm needs to be filtered. It may intermittently work being unfiltered, but as others have said you want it filtered an on RJ31x. Search here on RJ31 and you will find some other explanations and links.