This is why you need an ADSL Splitter or Filter

Ever wondered what was inside an ADSL splitter that you are using on a DSL line if you are having both Telephone and ADSL modem? Well, let me tell you what it has and what it does. An ADSL splitter has a most simple circuitry inside it which separates voice and data coming from a single pair of DSL line. Let’s discuss in more detail.


Telephone wires were originally designed to carry voice signal between your Telephone device and Telephone Exchange. This voice signal uses a band of frequencies from 300 Hz to 3.4 KHz. This system is known as PSTN( Public Switched Telephone Network). Then the concept of Internet came and ISP uses a technology called ADSL to transmit both data and voice signal on a single copper wire. The ADSL system uses frequencies much higher than the voice to carry faster data traffic. The ADSL system uses frequencies typically from 25 KHz to 1.1 MHz. Because the ADSL and PSTN systems operate on the different frequency range, they can be carried through same wire at the same time.

Why use ADSL splitter or filter?

If we use an ADSL modem and a Telephone at the same time without ADSL splitter( which we can not), the high-frequency noise used by ADSL might interfere on your telephone and you will end up hearing a lot of noise rather than voice and vice versa. In order to keep these systems apart and stop interfering with each other, it is necessary to separate them from the main telephone line. This is where splitter comes in.

How ADSL splitter splits or filters the signal?

ADSL splitter or filter has an internal circuitry which is called a low pass filter. In simple terms, a low pass filter allows lower frequency below a certain cutoff frequency to pass through and filter out the higher frequency. In this case, the ADSL splitter or filter allows the voice signal which has lower frequency range (300 Hz to 3.4 kHz) allows to pass through to the Telephone output of the splitter. The unfiltered output( 25KHz to 1.1 MHz) will go directly to the ADSL modem output. This is simple, isn’t it?

Now you might be thinking that how could one without a splitter or a filter use an ADSL modem or Telephone only if it is put directly in the main socket. The answer to that is simple. If you use only one device at a time i.e either a Telephone or an ADSL modem there is no one to interfere the signal. For example, if you put your ADSL modem directly to the main socket it will work fine without any splitter or filter because there is no Telephone to interfere the ADSL signal on that line. Same is the case with Telephone.

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