# bit rate\baud rate

Discussion in 'Electronic Basics' started by Ratish Kumar Srivastava, Jul 14, 2005.

1. ### Ratish Kumar SrivastavaGuest

Can any body explain difference between Bit and baud rate with an example?

Best Regards,
Ratish

2. ### BobGuest

Can any body explain difference between Bit and baud rate with an example?
When digital information is transmitted it is sometimes more efficient to
encode the bits in a way such as to reduce the total bandwidth necessary for
that transmission.

If you use only two different values of a signal's characteristic (let say
its amplitude) to encode a binary signal then the bandwidth required to send
that signal is fairly high.

If, on the other hand, you use more than two different values of that signal
to encode a binary signal then the same bitrate can be transmitted using a
lower-bandwidth channel.

For example, a two-level voltage signal's spectral content is easily
calculated using the Fourier transform of the time-domain signal. Use a
1-0-1-0-1-0... data pattern with a given rise/fall time to see the highest
needed bandwidth of the channel (and get the signal through with minimal
time-domain distortion).

Now, try using four levels to encode the bitstream. Each pair of bits can be
encoded with a single voltage level. Do the same Fourier transform and see
its reduced spectral width. Note that this type of encoding requires the
bitstream to be scrambled such that if you have each pair of bits repeat
themselves (over and over) you always get some voltage level changes (which
is necessary to recover the level-change boundary information [aka, clock
recovery]).

A baud is the symbol rate (per second). With encoding, you send more than
one bit/second (bps) per baud.

If you send 1M bit per second, but encode 16 bits per symbol, then the baud
rate is 62.5K baud. This could be done (for example) by using 4 voltage
levels per symbol or by using 4 different phases per symbol.

If I recall correctly, fast dial-up modems (say 38.4Kbps and 56Kbps) are all
2400 baud. This high bps per baud encoding is required because dial-up lines
only have about 3KHz of bandwidth (they're all digitally sampled at 8Ksps).

By the way, if this is a homework question then I hope (at least) you force
yourself to really understand this. If my explanation doesn't click with
you, then do some research. Please don't just copy this text and paste it.

Hope this helps.

Bob

3. ### BobGuest

I goofed up, in the description below.

What I said is 4 levels for 16 bits should be 65536 levels for 16 bits. Duh!

Bob

4. ### PeteSGuest

In addition to what Bob said, the most basic definition, when
considering communications systems, of course:

Bit rate : The actual data rate in bits per second that is to be
transmitted

Baud rate : The maximum number of line signaling symbols per second
when transmitting the above data in the transmission medium. (Sometimes
we refer to line transitions, but that gets a little sticky for FSK,
but the principle holds)

We encode data so there are multiple bits per line transition, usually
for bandwidth reasons (there's a correlation between encoding and S/S+N
degradation). The Baud rate is sometimes also known as **the Symbol
Rate**, the imply the number of symbols, not the original data rate.

Just to aid with the homework, some encoding techniques that are common
(not line coding, but multi-level codes) are:

QAM
PAM
PSK
QPSK
DQAM
DQPSK
DPSK

There's a lot more - I suggest some light reading

Cheers

PeteS

5. ### Roger JohanssonGuest

Baud has been misused a lot, as a fancier word for bps. Especially when
talking about modems you see that Baud is often used incorrectly.

"Data speed used to be specified in terms of baud, which is a measure
of the number of times a digital signal changes state in one second.
Baud, sometimes called the "baud rate," is almost always a lower figure
than bps for a given digital signal because some signal modulation
techniques allow more than one data bit to be transmitted per change
state."

The advertising departments of modem manufacturers wanted to tell the
users about the higher number, the bps, but they also wanted to use the
fancier word Baud, so they often used Baud in place of bps.

From http://www.essaysample.com/essay/001633.html
"While taking about modems, the transmission speed is the source of a
lot of confusion. The root of the problem is the fact that the terms
"baud" and "bits per second" are used interchangeably. This is a result
of the fact that it's easier to say "baud" than "bits per second,"
though misinformation has a hand in it, too. A baud is "A change in
signal from positive to negative or vice-versa that is used as a
measure of transmission speed" and bits per second is a measure of the
number of data bits (digital 0's and 1's) transmitted each second in a
communications channel. This is sometimes referred to as "bit rate."
Individual characters (letters, numbers, spaces, etc.), also referred
to as bytes, are composed of 8 bits. Technically, baud is the number of
times per second that the carrier signal shifts value, for example a
1200 bit-per-second modem actually runs at 300 baud, but it moves 4
bits per baud (4 x 300 = 1200 bits per second)."

6. ### Michael A. TerrellGuest

You forgot FQPSK ;-)
--
Link to my "Computers for disabled Veterans" project website deleted
after threats were telephoned to my church.

Michael A. Terrell
Central Florida

7. ### Michael A. TerrellGuest

At one time baud and bit rate were the same, in the days of 60 mA
current loop TTY interfaces and simple analog MODEMs

--
Link to my "Computers for disabled Veterans" project website deleted
after threats were telephoned to my church.

Michael A. Terrell
Central Florida

8. ### Rich GriseGuest

Yeah - you missed BTFSPLK ;-)

Cheers!
Rich