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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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In short:

  • The baud rate is the symbol transmission rate.

  • The bit rate is the data transmission rate.

(A "symbol", in this context, is simply a group of some relatively arbitrary, non-zero number of bits, transmitted as one atomic unit.)

While in simple cases the two can be identical, as soon as one symbol encodes more than one bit, they diverge. And that is incredibly common in modern data transmission systems.

Therefore, in practice, the bit rate of a channel will almost always be higher than its baud rate.

The baud rate matters more when you are interested in the requirements imposed on the data transmission physical link layer (such as a radio link's spectrum bandwidth, or the transition times in a modulator). The bit rate, on the other hand, matters more when you are interested in how much data can actually be transmitted within a given time period. Both are useful metrics, but as they measure different quantities, they answer different questions.

As an example, consider quadrature amplitude modulation; 4096-QAM encodes 12 bits (212 = 4096) per symbol. Therefore, the baud rate for a 4096-QAM channel is 2-12 = 1/4096 of the data rate of the same channel.

In practice, some of those bits being transmitted will, in turn, likely be used for various kinds of metadata; things like frame headers, checksums, start and stop bits, on-bus addressing, forward error correction, and so on. In places, data is deliberately stretched out, not uncommonly to improve error handling or simplify synchronization. For example, SATA uses 8b/10b encoding on the physical link layer, and CDDA extends data multiple times for various reasons between the raw audio sample data and what's actually on the physical storage medium. Designs like these will reduce the rate at which useful payload data can be transmitted below that of the raw channel bit rate, and that is relevant for the amount of time it takes to usefully transmit some given amount of data, but they do not change the bit rate of the channel itself.

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Elliot Alderson‭ wrote almost 4 years ago

In the olden days I liked to use the example of a touch-tone telephone. Each tone was a baud, and each tone transmitted 4 bits. Conversely, plain old UARTs have a bit rate that is lower than the baud rate.

Canina‭ wrote almost 4 years ago

@ElliotAlderson At which point you get puzzled looks until you tell people that the DTMF value set is actually 0-9 plus *, #, A, B, C and D, which adds up nicely to 16 distinct values.