[sdiy] Information Content of Signals

[Neil Johnson]

Easy one...

Tim Ressel wrote:
> I want to know how they can get 53kbps on a phone line with 3khz
> bandwidth. Oh I know they use this funky phase-constellation thingie.

Remember that "baud" != "bits per second".  The proper definition goes
something like "symbols per second".

"bits per second" is a special case of baud, where each symbol represents
only two states (1 or 0).  If we give each symbol 4 states, then we can
encode two bits per symbol.  As DSP gets better, we can decode many more
states per symbol.

Take your phone line, and lets say we have an upper limit of 3,000 symbols
per second.  Now leys say we have 16 states per symbol, which we can
encode 4 bits per symbol.  Now the bit rate is 16 x 3,000 = 48kb/s.

Interesting side note: "baud" is named after the French telecomms man
Baudot, who developed an ecoding scheme using 5 bits per letter, and a
special little typewriter for sending messages.  There are plenty of
references on the web about Emile Baudot.  Indeed, baudot code is still
used by radio hams, and was used for punched-tape.

Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
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