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**Question of the Week / Re: QotW - 12.09.02 - How can I use one telephone line for multiple uses?**

« **on:**10/09/2012 11:15:11 »

Amplitude and Phase modulation of a Carrier Frequency works well in hardware, when there is a small amount of information in a fairly narrow frequency band (compared to the carrier frequency).

The ADSL transmission used on most telephone lines spans quite a wide band - from 25kHz to 125kHz, and so modulating a single carrier would not work very well. This wide range is broken up into sub-bands.

Modern Digital Signal Processors use a mathematical technique called the Fast Fourier Transform (FFT) to generate signals in all of the sub-bands simultaneously. The inverse FFT is used to decode them off the telephone line, while accounting for the variable frequency and phase response over this wide frequency band, and cancelling the interference from the adjacent sub-bands.

Like an increasing number of things today, this is a software-based technique, rather than a hardware-based technique. Most of the hardware effort goes into making sure the software can run as fast as possible, while consuming as little power as possible.

The ADSL transmission used on most telephone lines spans quite a wide band - from 25kHz to 125kHz, and so modulating a single carrier would not work very well. This wide range is broken up into sub-bands.

Modern Digital Signal Processors use a mathematical technique called the Fast Fourier Transform (FFT) to generate signals in all of the sub-bands simultaneously. The inverse FFT is used to decode them off the telephone line, while accounting for the variable frequency and phase response over this wide frequency band, and cancelling the interference from the adjacent sub-bands.

Like an increasing number of things today, this is a software-based technique, rather than a hardware-based technique. Most of the hardware effort goes into making sure the software can run as fast as possible, while consuming as little power as possible.