Digital Subscriber Line (DSL) filters, also known as micro-filters, are small in-line devices sometimes required for DSL installations. They filter line interference to standard telephone equipment when phones share the same lines as DSL service. Not all setups require the use of a DSL filter, though they are often needed for each phone connection.
Basic Design
A DSL filter is a small, rectangular device that has phone connections on both ends. The person setting up a DSL system simply unplugs a telephone line from a wall jack, inserts the filter into the wall connection, and then plugs the telephone line into the filter. Fax machine lines and answering machines tend to also require DSL filters. Filters separate the voice and data signals sent through phone lines, ensuring that neither signal interferes with the other.
Split DSL Setups
There are two methods for installing DSL service: split, or splitterless. A DSL filter is not required if the split method is used. In this case a technician visits the premises and installs a splitter in the main telephone cable. Two lines emerge from the splitter; one line feeds into standard phone jacks, while the other line is used exclusively for a DSL modem. A DSL filter is not required in this case because the splitter has separated the voice and data signals.
Splitterless Setups that Require Filters
The second method, which has gained popularity because it does not require a technician, can be user-installed. In this case, the end-user can setup a DSL modem along with several micro-filters. DSL service is sent through the main service line so the end-user only has to connect the modem to any existing phone wall jack. However, the signal reaches all connected phone jacks, creating possible bleed-over noise on telephone, answering machine, and fax machine lines. A DSL filter connected to each of these lines blocks interference that might be created by the service.
Advantages of DSL Setups
DSL has three great advantages over dial-up service: it is always on, it is up to 50 times faster, and it does not tie up a phone line, even though it uses the same connection. Standard telephone equipment uses an analog signal, while DSL communicates on a higher digital frequency. This lets a user navigate the Internet using DSL service, while talking on the phone at the same time.
Other Filters and Troubleshooting
A DSL filter is typically a small device, though some other equipment can have filters built into them. Some surge protectors or power strips, for example, include a filter to help with DSL connections and to protect equipment in case of a power surge through a phone line. Issues that arise with signal clarity may be related to faulty or improper filters, though flaws in telephone cables or damaged hardware can also result in poor performance.