From: Jaap van Ganswijk To: Jake6000 Subject: Re: question At 01:56 1997-05-17 -0400, Jake6000 wrote: >what is the difference between a modem and a codec in terms of wether the >stations are analog or digital, and wether the transmisson line is analog or >digital. A modem converts from an RS-232-signal (so a digital bitstream as described on my UART page, but with levels alternating between -12 V and +12) to a telephone signal. The telephone signal can be everything, but with 300 baud modems it consisted of two alternating frequencies for sending and two others for receiving. Those four frequencies weren't allowed to be harmonics of each other... A codec codes the signals on the telephone line to (generally PCM coded) binary information. So it digitizes the signals on the analog telephone line, whereas a modem uses the analog telephone line to send digital information. To answer your question in these terms: When these things were introduced all telephone lines were analog, so a modem translated from digital to analog and back again and a codec translated analog signals from the telephone line to digital and back again. It's not correct to use these terms for digital lines I think, but the interface between a computer and ISDN line is also called an ISDN-modem sometimes, although it doesn't modulate anything. A codec isn't necessary on an ISDN line, since the data on the ISDN line are already digital. A codec is however necessary in an ISDN-telephone set or in the connection between an ISDN line and a normal analog telephone, but in this case it converts the digital ISDN-signal to analog audio output and the input from the microphone back to the ISDN line again. I hope this explains enough...