Baud versus Bits Per Second

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Baud versus Bits Per Second

Just as the title suggests, this posting briefly explains why it's inaccurate to make mention of a "14,400 baud" modem.

Baud and BPS are NOT synonymous, especially beyond 2400. While many use the terms as though they are identical, the measurements are quite different.

Could you perhaps be a bit more specific?

Strictly defined, BAUD defines the number of "signal events" that occur in transmitted signal. However, more than one bit may be encoded in a signal event using any of a variety of modulation techniques.

Consider the transmission of a simple unit amplitude sine wave. In one period, there are two signal events - a positve value (we'll call that a 1 bit) and a negative value (we'll call that a 0 bit.) Now, suppose that in the second period, we shift the phase of the positive half-wave 90 degrees from where it was expected to be (relative to the waveform of the first period). The receiver can detect the shift and we use this capability to define a "2" bit (not really a "bit" now, but bear with me). We can continue this relative phase shifting and create (or more correctly, encode) a number of additional bits while still having only two signal events (the positive and negative values). Furthermore, if we allow our amplitude to vary from one period to the next, we can also encode additional bits. By exploiting the ability of the transmitter and receiver to generate and detect these signals, we can maintain a BIT RATE which is considerably higher than our BAUD.

Current modem technology is based on exactly these techniques, which is why a 2400 Baud modem using Quadrature Amplitude Modulation can be sold as a 14.4kbps device. Compression techniques can also increase apparent bit rate, but that's a whole other tutorial. :)

Hope you can follow this and that it's helpful.


Steve Zimmel
F-22 Flight Simulation        Any agreement between the
Lockheed Fort Worth Company    opinions above and those of
Fort Worth, TX                  LFWC are purely coincidental
(817) 777-1417

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