Bit Rate vs

Baud Rate (Bd): In communication theory, Baud Rate (Bd) refers to quantitative measure of information being transmitted/received by a carrier. A carrier is said to be conveying information when the modulating signal (the information) brings changes in its amplitude, frequency or phase. Every change in the carrier is an information bit (the baud).

In asynchronous serial communication, there is no carrier. The bits being carried out over the TX-line are all information bits. So, the Baud Rate is equal to the Bit Rate.

Figure-1: Waveform for asynchronous data frame for character A (TTL Logic)

(a) Assume Bd = 4800; Even-parity (number of 1s in the data field is even).

(b) Frame Length : 11 (1-Start Bit, 8-Data bit, 1-parity bit, and 1-Stop bit)

(c) Bit Rate = Baud Rate = 4800. Bit Period = 1/4800 = 208 µs.

There are cases where Baud Rate is different from Bit rate. For example: The Bell 212A type modem uses 1200 Hz sine wave carrier for transmission. It uses the so called 'dibit encoding', where '2 consecutive bits' in the data stream is treated together. In this scheme, the value of these two bits determines the amount of phase shift to be occurred in the carrier. The relationship is:

dibit value phase shift

00 0

^{0}01 90

^{0}11 180

^{0}10 270

^{0}Now, assume that the bit pattern 00 11 01 01 11 01 would be transmitted using the dibit encoding scheme. The relationship between the values of the dibits and the phase shifts of the carrier is depicted below in Fig-7.1. The value of the 1st dibit is 00, so there is no phase shift of the carrier. The value of the 2nd dibit is 11, so the phase shift of the carrier is 1800 and it has occurred at the elapse of 1800 from point B. In a similar way, we have indicated the phase shift points for the remaining dibits of the data stream.

Figure-2: Waveform for simple phase-shift modulation

Now, we are in a position to explain the difference between the

Bit Rate and the

Baud Rate. At 1200 Hz carrier frequency, one cycle period is required for the shifting of 1-bit data into the phase-shift keying modulator. In the example of Fig-2, we have 12-bit data; we need 12 clock cycles to shift them into the modulator. So, the Bit Rate: 1200 bits/sec.

Now, let us look at the carrier; we observe that there are only 6 changes in the carrier. The 12-bit data have brought only 6 changes in phases of the carrier. The information content is 6 Baud. The Baud Rate is half of the Bit Rate and it is: 600 Bd.