What is a DSL Filter?



DSL Filter

A DSL filter prevents signals or echoes from an analog device from compromising performance and causing connection issues with DSL service.

Sometimes called a micro-filter, it’s an analog low-pass filter between analog devices and a regular line for your home phone. It blocks interference between devices and the DSL service on the same line, which, at the top or bottom or their frequency range, can disrupt your DSL Internet connection.

If you use home phone service with a splitterless setup, you’ll need DSL filters installed on every device that connects to a DSL phone line, including:

  • Analog modems
  • Answering machines
  • Cable boxes
  • Caller ID units
  • Fax machines
  • Satellite television devices
  • Telephones
  • TiVo boxes
  • Voice band modems

Why do I need a DSL filter?

Think of your phone line like a three-lane highway.

For slow drivers (phone conversations), all you need is the right lane. It operates from 0-4 kilohertz, units of alternating current or electromagnetic wave frequency. This range is perfect for the human voice and touch tones. Music sounds horrible at this level, which is why hold music is often distorted.

A DSL Internet connection needs that next lane, from 26-138 kHz, much higher than the frequency needed for human voice, to deliver upstream Internet signals. Downstream Internet signals come in the 138-1,000-kHz range. So long as these signals stay in their lanes, everything works as it should.

Individual filters, or a universal filter installed at the phone line’s main junction, ensure high-frequency DSL signals don’t go where they are not detected, and low-frequency signals go to the right place.


What do they look like?

A DSL filter is a little device with phone connections at both ends. It goes into the phone’s wall connection, and the home phone line is inserted into the filter. There’s two ways to go about separating the signals on your phone line:



You won’t need a DSL filter in this case. A technician will install a splitter in the main telephone cable. This gives you two lines, one that feeds into standard phone jacks, the other just for a DSL modem.



Because you set up DSL filters for individual devices, no technician is required. Some devices, such as power strips and surge protectors, have integrated filters. Consult the item’s user manual.