> The fax requires a 56KHz channel This is quite wrong. I believe you are confusing Hz with bitrate, as the most advanced modems communicate at 56kbit/s using

> The fax requires a 56KHz channel This is quite wrong. I believe you are confusing Hz with bitrate, as the most advanced modems communicate at 56kbit/s using PCM. To clear it up, a regular phone line has a 4000Hz bandwidth. With digital lines, you require sampling at at least 8000Hz (Nyquist theorem). It's impossible to send a frequency above 4000Hz using this line. Each sample is considered to have 7 usable bits of data, giving it a capacity of 8000 * 7 = 56 kbit/s. Most faxes are compatible with the T.30 standard, which doesn't acknowledge the usage of digital lines, and sends data using the older V.34 modem standard. This is either at 28kbit/s or 34kbit/s. This uses a method called Quadrature Amplitude Modulation (QAM), and a maximum baud rate of 3429 symbols per second. Since the centre frequency is set at 1959Hz, you won't have frequencies greater than the centre + bandwidth/2 = 1959 + 3429/2 = 3674Hz. You get 10 bits with each symbol, resulting in 3429 * 10 = 34 kbit/s. Edit: Here's someone attempting to use the slowest supported T.30 standard, 300 bit/s, over his cell phone, and you can see it's not perfect: https://www.youtube.com/watch?v=uQqWHLZjOjA https://www.youtube.com/watch?v=uQqWHLZjOjA