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Relationship between baud and characters-per-second? Other questionstoo.

Discussion in 'Electronic Design' started by Green Xenon [Radium], Sep 13, 2007.

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  1. Hi:

    What is the relationship between baud, bits-per-character, and

    Carrying more bits-per-symbol saves bandwidth as opposed to using more

    Does carrying more bits-per-character use up less bandwidth than
    carrying more characters-per-bit?

    “Conveying more than one bit per symbol has advantages. It reduces the
    time required to send a given quantity of data, and allows modern
    modems, FDDI and 100/1000 Mbit/s Ethernet LANs, and others, to achieve
    high data rates. An optimal symbol set design takes into account channel
    bandwidth, desired information rate, noise characteristics of the
    channel and the receiver, and receiver and decoder complexity. A typical
    2400-bit/s modem transmits at 600 baud (600 symbol/s), where each
    quadrature amplitude modulation symbol carries four bits of information.
    1000 Mbit/s Ethernet LAN cables use many wire pairs and many bits per
    symbol to encode their data payloads. 1000BASE-T uses four wire pairs
    and two data bits per symbol to get a symbol rate of 125MBd.”

    “Representing one bit by many symbols overcomes signal noise like radio
    jamming, and is common in military radio and CDMA radio, including cell
    phones, despite using more bandwidth to carry the same bit rate. In
    these systems, each signal is called a "chip" and the baud the "chip rate".”

    Unfortunately I don’t think this answers my question. Unless of course,
    if symbols and characters are the same thing.

    Are “bits-per-symbol” and “bits-per-character” the same thing? I did a
    Google search for “bits-per-character” and “characters-per-second” but
    didn’t see much in terms of telecommunications, baud and modems."characters+per+second"&btnG=Search

    OTOH, “bits-per-symbol” gave tons of links about telecommunications,
    baud and modems.


  2. Bob

    Bob Guest

    Are you trying to get some homework done or do you really want to understand
    these concepts? It seems from your post that it's the former.

    Either way, here are the basics. From this you should be able to deduce the
    answers you seek.

    A symbol can represent one or more bits. For example, if you decide to
    transmit data using only two signal level regions (e.g. 3.3V LVTTL -- where
    a low = -0.5V to 0.8V and a high = 2.0V to 3.8V) then each symbol will carry
    one bit of information. However, what if you allowed sixteen different
    signal level regions instead of only two? Then, each symbol will carry four
    bits of information. It mostly depends on the transmission path as to how
    many bits you can represent with each symbol. If you're sending data through
    a digital IC then you can only get one bit per symbol, but if you're sending
    data through some type of analog medium then you can get more than one bit
    (i.e. two signal levels) through the channel.

    A baud is equal to the number of symbols sent per second.

    A "character" can be made up of as many bits as is required. Typically, a
    character (like an ASCII character) is eight bits.

    Think about this for a while until it makes sense to you.

  3. Bob

    Bob Guest

    It's just simple math. You haven't thought long enough, otherwise it would
    be obvious to you.

    It is very important that you do this for yourself. Someone once said:

    "Give a man a fire and he'll be warm for a day. Set a man afire and he'll be
    warm for the rest of his life."


  4. Bob wrote:

    No. It's the latter.
    What about characters-per-second?
    Most of it does make sense, except for the characters-per-second.

    If there is a difference between "characters-per-second" and
    "symbols-per-second", what is this difference?
  5. _

    _ Guest

    simple maths

    char/sec = (sym/sec) / (sym/char)

    Are you *sure* this isn't homework?
  6. Tam/WB2TT

    Tam/WB2TT Guest

    Baud refers to how often the actual line signal changes state; the same as
    symbols per second. Bits per character is really an animal of a different
    color, and depends on the original encoding. For ASCII, a common value is 8
    bits per character. For instance, the 8 bit ASCII code for the numeral 1 is
    00110001. For 8 bit encoding the number of characters per second is 1/8 th
    the bits per second, but usually not 1/8 the symbols/second. I am ignoring
    any extra bits and characters which are often added.
    Suppose you transmit a 4 level signal. Call these A,B,C, and D. Suppose the
    bits are packed into the symbols so that A=00, B=01, C=10, and D=11. You can
    see that each symbol carries 2 bits, so that the baud rate would be 1/2 the
    bit rate. For a 16 level signal, each symbol carries 4 bits.

    Baud is symbols/second. Any time you increase the baud rate you increase the
    You will have to work this out, but having LESS bits per character reduces
    the bits per second and bandwidth. The original Teletype code used 5 bits
    per character. This only allows for 32 characters, so, one of the characters
    was a SHIFT character.

  7. Why not 1/8 the symbols/seconds? Wouldn’t using fewer symbols-per-second
    make it easer to transmit/receive in high-speed? The less
    symbols-per-second, the less bandwidth is used, regardless of the amount
    of bits-per-symbol.
    If you have more bits-per-character but less characters-per-second,
    would this be more efficient in terms of bandwidth usage?

    I ask because I am aware that more bits-per-symbol but fewer
    symbols-per-second makes better use of bandwidth. So I am guessing that
    this is analogous.
  8. Tam/WB2TT

    Tam/WB2TT Guest

    If you want to keep the overall data rate the same, then if you go to 1/8 as
    many symbols/second as before you now need to have 8X as many bits per
    symbol as before. Actually, this kind of thing is done, but there is a limit
    as to how far you can go because every time you increase the bits/symbol,
    the effective signal to noise ratio gets worse unless you increase the
    power. For ATSC HDTV they go 3 bits/symbol for over the air transmission;
    that means each symbol can have one of 8 values (2**3=8). If you wanted to
    have 8 bits/symbol then each symbol must be capable of having one of 256
    values (2**8 = 256). If you keep the power the same, then the resultant
    signal will be 256 times more susceptible to noise than straight binary.

  9. Ignoring the SNR, is there a physical limit to how many bits-per-symbol
    a device can transmit/receive/process/record/playback if the symbol rate
    is only 1 baud [1 symbol per second]?

    Is it possible to have only 1-symbol-per-second but

    Also, my previous question -- [*most important and interesting to me*]:

    If you have more bits-per-character but less characters-per-second,
    would this be more efficient in terms of bandwidth usage?

    I ask because I am aware that more bits-per-symbol but fewer
    symbols-per-second makes better use of bandwidth. So I am guessing that
    this is analogous.
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