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Comments on Bit-rate vs Baud-rate

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Bit-rate vs Baud-rate

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I am really confused how the terms bit-rate are baud-rate are used in electronics. I never bothered thinking about baud-rate being different than bit-rate before. But recently, one of my teachers told baud-rate being the maximum no. of changes in the bit that can occur during the data transfer.

I really want to visualize how they are different from each other.

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Baud rate is about the speed of bit slots in the channel or symbols. Bit rate is the rate at which actual data is transmitted.

Put another way, baud rate is what you see when you look at a scope trace of a communication signal. Bit rate is how fast data gets from one end to the other.

The bit rate can be both higher and lower than the baud rate due to encoding and packaging schemes. Modern high speed protocols do various fancy things to encode more than one bit of data into each symbol, so the bit rate is often higher than the baud rate.

However, for most "ordinary" protocols (UART, CAN, IIC, SPI for example), some of the bit slots are used to delimit and package the information, so the bit rate is lower than the baud rate.

Consider the common UART protocol of 115.2 kBaud, 8 data bits, no parity, and 1 stop bit. Each character requires 10 bit slots to send: 1 start bit, 8 actual data bits, and 1 stop bit. Since the baud rate is 115.2 k/s, each bit slot is 8.68 µs long. A whole character therefore takes 86.8 µs to transmit. A total of 8 payload bits are transported during that time, so the bit rate is (8 bits)/(86.8 µs) = 92.2 kBits/s.


@TonyStewart‭ wrote:

Can you give an example where Baud Rate is higher than Bit Rate?

I did. See previous paragraph.

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TonyStewart‭ wrote almost 4 years ago

Dear Olin, I think you are using bit rate efficiency with overhead to define effective data rate, both which have nothing to do with the bit/s baud compression technology

Symbols can stuff more bits per symbol such as the Baud symbol in a modem after Émile Baudot who invented the Baudot code (Bodo) for telegraphy in the 1870s e.g. dot dit dot = S which is a character and the dit/dots are binary symbols determined by pulse duration.

Skipping 1 deleted comment.

TonyStewart‭ wrote almost 4 years ago

There are many types of Baud Symbols that compress bits in time, frequency, phase or time in many combinations by sending many per unit of a bit. such as 9600 modems using 1200+2400 Hz tones with multiple levels to get 8 bits per Baud symbol in a 2400~3000 Hz bandwidth. ( I think, I forget)

TonyStewart‭ wrote almost 4 years ago

... So don't byte me for saying your Byte rate example is not related to bit rate vs baud rate or symbol rate. but does affect character rate. characters are not symbols in this sense yet are symbols in language.

TonyStewart‭ wrote almost 4 years ago

Can you give an example where Baud Rate is higher than Bit Rate?

Canina‭ wrote almost 4 years ago · edited almost 4 years ago

@TonyStewart Maybe Morse code can be considered one such example, with variable-length symbols? It would probably be tricky to analyze as such, but I suppose technically it fits the definition...

TonyStewart‭ wrote over 3 years ago

I remember when Qmodem would compute the bit rate differently, and shown higher than output expected. On dialup using 56k modems in high speed mode with a faster serial port, they would count the start and stop bits in the transfer rate, which then of course got translated into , I forget 64 levels of low (1200) baud rate symbols to fit in the telephony bandwidth with trellis coding etc. FCC blocked the 64kbps due to spectral emission issues.