0

I know that there is lots of questions and answers that is related to this topic, but i can not clearly and distinctively find answers to my questions, yet. In addition i found some answers which seems to be contradictory to each other.

I know that baud is number of symbols per second in a data transmitting signal, As here is mentioned: https://electronics.stackexchange.com/a/282382/254779, Then its unit could not be "bits per second", right?

But these two answers bellow say that the baud rate unit is "bits per second": https://electronics.stackexchange.com/a/273817/254779, https://electronics.stackexchange.com/a/117245/254779.

Further more, I have ambiguity about the way measuring baud rate. In some web sites is said to be measured by number of times line changes per second and some where else as signal units per second. But i guess that they are not completely the same, right? Because this really means that every signal unit should be correspondent to only one signal change.

So for example how baud rate is measured for Manchester encoding? I guess that in this encoding number of signal changes per second is not the same as number of signal units per second, right?

And the last one is that i found some answers which said meta data bits like start and stop bits were not taken into account for measuring bit rate, however, I saw somewhere else that all bits were used for measuring bit rate. So which of these is correct at the end? For example is baud rate / bit rate = 1 in the UART protocol?

I have these questions in my head for long, and i decided to ask them once for all.

Thanks for reading.

A.R.S.D.
  • 53
  • 4
  • baud rate is the number of bits that can be transmitted per second ... that includes start, stop and parity bits ... but you could be transmitting one character per hour ... bitrate may be referring to data bits – jsotola Jun 08 '20 at 19:21
  • The second two questions and answers are for bare serial transmission and there is always equality with baud rate and symbol rate. For more complex transmission systems, sometimes there are more than 1 bits of data per symbol. – Andy aka Jun 08 '20 at 19:22

3 Answers3

1

"Baud rate" means "raw symbols per second". For binary symbols (like RS-232), it's synonymous with "raw bits per second".

Manchester encoding is a poor fit to this model, but if you wanted to squeeze it in, then a symbol period is equal to a bit period, and there's all the weird Manchester encoding rules that need to be followed.

For RS-232, baud rate = raw bit rate, and that includes the start & stop bits. For instance, 1200 baud means a bit time of \$833\mathrm{\mu s}\$, and that applies for all bits. So if you're sending 1 start, 1 stop, and 8 data bits, the fastest you could transmit payload bits would be \$1200\frac{8}{10} = 960\$ bits/second.

Typically, in the literature the term "baud" has become clouded because it's often conflated with "bits/second", and with more complicated modulation schemes like OFDM the exact meaning of a "symbol" can be up for debate -- so if you're authoring a paper, it's a good idea to make it clear what you're talking about independent of the term.

TimWescott
  • 44,867
  • 1
  • 41
  • 104
  • You said "then a symbol period is equal to a bit period", So do you mean that "line changes/second" is not equal to "symbols/second" in Manchester encoding? because there is a line change at the middle of symbol period in Manchester encoding. – A.R.S.D. Jun 09 '20 at 07:24
  • I purposely left Manchester encoding out of my discussion, because it's ambiguous!! Usually you encounter the notion of "symbols" in a context where you can easily sort them out using linear operations (like matched filters). Manchester encoding doesn't fit that paradigm well. If pressed, I'd *define* a symbol in Manchester as being either high, low, low->high, or high->low, subject to further decoding. This would *make* the symbol length equal to a bit period. – TimWescott Jun 09 '20 at 19:51
1

Baud is symbols per second. This can, but does not have to be bits per second.

Example: You have a symbol which can have 4 states (e.g. red/green/blue/white). If you transmit this symbol 2 times each second, you got 2 baud, but 4 bits per second.

A real life example: The ancient 56k modems, the bitrate of them is 56 kilobit/s while the baudrate is 8000 baud/s.

For UART where you have a binary symbol (0 or 1), bitrate is the same as baudrate.

Stefan Wyss
  • 6,471
  • 1
  • 11
  • 22
  • 1
    A real life example: The ancient 56k modems, the bitrate of them is 56 kilobit/s while the baudrate is 8000 baud/s – user17645 Jun 08 '20 at 20:13
  • How about bit rate, Are the payload bits considered for measuring or all bits? I mean is bit rate / baud rate = 1 in the UART protocol? – A.R.S.D. Jun 09 '20 at 07:33
0

Baud rate can equal bit rate, it just assumes the simple case of one bit per symbol such as for UART comms. Baud rate is the rate of symbols per second and for complex modulation techniques you can send multiple bits per symbol. A modem can send 4 bits per symbol so 600 baud modem can send 2400 bits per second. You can press one key on a piano, and assuming the piano has 64 keys then each symbol is worth six bits. You can type single ASCII letters (symbols) using a keyboard, but each symbol is 7 bits.

Justme
  • 127,425
  • 3
  • 97
  • 261