Named after Emile Baudot, the inventor of the Baudot code for telegraphy, a baud is a unit that is synonymous to symbols per second. Therefore, if there was something with 15 Bd, that would mean that something had 15 symbols per second. Not to be confused with bits per second, the baud — which, when using the symbol, must follow the SI nomenclature with a capital first letter and a lowercase second letter — can be any of the many different kinds of symbols while a bit is only 0 or 1. In older modems, they were all the same, but as new modems have been released, it has become important to split the two up as not to confuse them.
Another way to look at baud rate is through numbers. For example, if something is known to have a 1500 baud rate, that means that in one second, it goes through fifteen hundred different symbols. This sounds large, but this is on the low side of what baud is capable of. Another way to think about it is that when there is something with a 1500 Bd, it means it can change state 1500 times in a second. That means change from 1 to 0, 0 to 1 and all the way up to X. So, 0 and 2. There are a lot more options when dealing with the baud.
How to Calculate Symbol Duration Time
The symbol duration time is the amount of time it takes to complete one symbol change. Because baud stands for the number of signals per second, the symbol duration time tries to calculate the time between each signal in a second. In other words, you're finding out the time for each individual symbol in connection with the entire number of symbols in that particular second.
The equation looks like this:
Ts = 1 / fs
In other words, the symbol duration time is equal to 1 divided by the number of symbols that you have per second. Therefore, with an arbitrary number of 1,000, the duration time of the single symbol would be:
Ts = 1 / 1000 = 1 symbol / millisecond
The same mathematics can be calculated if you only know the symbol duration time. You can figure out how many baud there is. For example, if the symbol duration time is half a second, you'd be able to calculate that the baud is 2 symbols per second or 2 Bd.