What is a codec and how do they work?
Whilst we’ve said above that you can use ISDN lines for ordinary telephone calls, and that the quality is better, the audio will still sound like a telephone call. Typically ISDN telephony provides a bandwidth of 3.1kHz. You can calculate the bandwidth you would need to carry live stereo CD quality audio as follows: CD quality is two channels of 16-bit audio send at a sample rate of 44.1kHz. This comes to 1.4 mbits per second. Much more than the 128 kbits you have with the two B-channels available on BRI ISDN. The codec’s function is to compress and un-compress (or COde and DECode) the 1.4 mbits of data into the 128 kbits of space available over the ISDN line.
