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bit rate\baud rate

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

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

    Best Regards,
  2. Bob

    Bob Guest

    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

    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.

  3. Bob

    Bob Guest

    I goofed up, in the description below.

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


  4. PeteS

    PeteS Guest

    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

    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:


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


  5. 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

    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.

    "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. 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. 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 Grise

    Rich Grise Guest

    Yeah - you missed BTFSPLK ;-)

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