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PICList Thread
'BAUD ? BPS'
1997\02\01@134015 by Kalle Pihlajasaari

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Hi,

Not on toPIC, sorry.

> >*I used bps and not baud - these two are not the same all the time.
> >Starting with rates above 1200 bps, baud and bps are not the same.
>
> I disagree!
>
> Originally Baud (shortened from baudot) was used to describe the 5 bits
> Baudot codes used by teletypes.  Then along came EBCDIC and ASCII and a baud
>
......
>
> the number of bits per character will increase and, therefore,  the
> characters per second will decrease,  but the number of bits ber second is
> still the same.

What was said here about baud was rather, ......   well something anyway.

Here is a link that I found on the net after finding 3 incorrect
descriptions that is short but correct.

  http://libweb.macarthur.uws.edu.au/comp1.1/cpw11p1.htm

BPS is for the digital guys and just leave baud to the analog modem guys.

BAUD is equal to BPS for unmodulated data.  When you modulate the
BPS does not change but you have a choice of encoding multiple
(yeah or fractional) bits with one change in the signal.

Take the classic simple pulse amplitude modulation with 4 levels
each interval on the channel can be one of 4 levels and so contain
the information for 2 bits.  Hence in this case you would have
a baud of half the bps.

Take a consumer remote (I am quoting a rather unsuaual one that
was used in a set top decoder her in SA that was only singly
modulated, usually they are twice modulated) that has a IR carrier
frequency of 38 kHz and a bps of 880.  The modulation technique
here was CW (continuous wave, turn the carrier on or off) and the
level would change in the channel at 76000 times per second hence
76000 baud but still only 880 bps.

What this means is that the bps signal is what you get when connecting
most digital equipment together with garden variety serial connections
and the baud only comes into play if you are about to modulate the
signal.

This has very little directly to do with PICs or microcontrollers
but serial communications do get used often and it is safer in
almost all cases to just use Bits Per Second unless you are into
modem design.

Sorry to ramble but I was getting a bit queasy reading the
urban legends being promulgated as fact.

> >I refer anyone wanting more info to The Modem Communication Book (don't
> >have exact title or author cause that book is at work) for protocol info
> >and National Semiconductors Communications Databook - good info on EIA
> >and RS standards.

Good advice, the net seems to be pretty weak on this topic with
60% of the first 5 sites I checked getting it wrong.

Cheers
--
Kalle Pihlajasaari   spam_OUTkalleTakeThisOuTspamip.co.za   http://www.ip.co.za/ip
Interface Products   P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750   Fax: 402-7751    http://www.ip.co.za/people/kalle

DonTronics, Silicon Studio and Wirz Electronics uP Product Dealer

1997\02\01@152053 by Eric Smith

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Kalle Pihlajasaari <.....kalleKILLspamspam@spam@DEVICE.DATA.CO.ZA> wrote:
> Take a consumer remote (I am quoting a rather unsuaual one that
> was used in a set top decoder her in SA that was only singly
> modulated, usually they are twice modulated) that has a IR carrier
> frequency of 38 kHz and a bps of 880.  The modulation technique
> here was CW (continuous wave, turn the carrier on or off) and the
> level would change in the channel at 76000 times per second hence
> 76000 baud but still only 880 bps.

I don't think that would usually be considered to be 76000 baud.  The baud
rate is the rate of symbol changes per second, independent of exactly how the
symbols are encoded.  If each bit is transmitted in a unique, independent time
period, the symbol is exactly one bit, and the bit rate is equal to the baud
rate.  In your example the symbol encoding is done by the presence or absence
of a carrier frequency, but the carrier frequency itself does not convey any
symbols.

For instance, the old Bell 103 FSK modems were used at 110 or 300 bps, and
since it was simple FSK they only transmitted one bit at a time, so they were
also considered to be 110 or 300 baud.  However, there were four different
carrier frequencies, two for the 0 and 1 bits transmitted by the originate
side, and two for the 0 and 1 bits transmitted by the answer side.  By your
reasoning, Bell 103 would actually have a different baud rate for the signals
transmitted by the originate and answer modemds, and even a variable baud rate
depending on the data!

The bps vs. baud distinction is perhaps better illustrated by V.32 modems,
which use QAM (quadrature amplitude modulation) to operate at 9600 bps using
an 1800 Hz carrier.  Four bits are transmitted at a time (ingnoring the
possible use of trellis coding), so there are sixteen symbols.  2400 of these
symbols are transmitted per second, so the baud rate is 2400.  If the carrier
frequency was considered significant in determining the baud rate, rather than
only considering the symbol changes, the baud rate would have to be at least
the LCM of 1800 and 2400, which is 7200.

Cheers,
Eric

1997\02\01@154927 by John Payson

picon face
> The bps vs. baud distinction is perhaps better illustrated by V.32 modems,
> which use QAM (quadrature amplitude modulation) to operate at 9600 bps using
> an 1800 Hz carrier.  Four bits are transmitted at a time (ingnoring the
> possible use of trellis coding), so there are sixteen symbols.  2400 of these
> symbols are transmitted per second, so the baud rate is 2400.  If the carrier
> frequency was considered significant in determining the baud rate, rather than
> only considering the symbol changes, the baud rate would have to be at least
> the LCM of 1800 and 2400, which is 7200.

The place I read was the earliest use of "baud" was for signalling flags on
ships.  If the person with the flags could give one signal per second, that
was one baud; the number of bits per second would depend upon the number of
different signals he could give, but the baud rate was the number of signals
per second.

1997\02\02@131536 by John Dammeyer

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At 08:32 PM 01/02/1997 +0200, you wrote:
>Hi,
>
>Not on toPIC, sorry.
>
>Here is a link that I found on the net after finding 3 incorrect
>descriptions that is short but correct.
>
>   http://libweb.macarthur.uws.edu.au/comp1.1/cpw11p1.htm

Hilarious! When you find something that matches your definition then "it's
short but correct", and when it doesn't then it's "incorrect".

>
>BPS is for the digital guys and just leave baud to the analog modem guys.

>
>BAUD is equal to BPS for unmodulated data.  When you modulate the
>BPS does not change but you have a choice of encoding multiple
>(yeah or fractional) bits with one change in the signal.

[snipped ... well ... stuff]

>
>What this means is that the bps signal is what you get when connecting
>most digital equipment together with garden variety serial connections
>and the baud only comes into play if you are about to modulate the
>signal.

Who cares really?
>
>This has very little directly to do with PICs or microcontrollers
>but serial communications do get used often and it is safer in
>almost all cases to just use Bits Per Second unless you are into
>modem design.

I agree!

Baud/bps. Until a particular group of 'experts' evolves in this industry the
definitions of what happens or what things are are fairly logical or are
named after a scientist or innovator.

For instance in the communications industry, bytes or characters are _not_
transmitted.  Instead they use octets. (Internetworking with TCP/IP Volume 1
- Principals, Protocols and Architecture - by Douglas E. Comer  ISBN #
0-13-216987-8). Also, Comer states that:

"baud - Literally, the number of times per second that the signal can change
on a transmission line. Commonly, the tranmission line uses only two signal
states (eg. two voltages), making the baud rate equal to the number of bits
per second that can be transfered.  The underlying transmission technique
may use some of the bandwidth, so it may not be the case that users
experience data transfer at the lines specified bit rate.  For example,
because asychronous lines require 10 bit times to send an 8-bit character, a
9600 baud asynchronous transmission line can only send 960 characters per
second."


Here he gives a definition that doesn't even use the word modulate and the
actual number of data bps is 7680.  Program your PIC to bit bang a uart at
7680bps and you won't properly receive the data. Program the PIC to bit bang
at 9600 baud,  strip away the transport information and you get 960
characters per second - correctly.

>Sorry to ramble but I was getting a bit queasy reading the
>urban legends being promulgated as fact.

Quite right.  In the future you may write that we're all so silly to talk
about characters and bytes when they are really _octets_ and that bytes and
characters are urban legends.


>> >I refer anyone wanting more info to The Modem Communication Book (don't
>> >have exact title or author cause that book is at work) for protocol info
>> >and National Semiconductors Communications Databook - good info on EIA
>> >and RS standards.
>
>Good advice, the net seems to be pretty weak on this topic with
>60% of the first 5 sites I checked getting it wrong.
>

Right on.  After all I found a source that states we send octets, not
characters so all the other sources that say we send bytes or characters are
also wrong.  8-)

In either case to get back on toPIC.  If asked to interface to a modem or
other device that is sending data to me at 38K baud I'll still ask if one or
two stops bits are being used, then if parity is included in the 8 bit byte
(or octet) or if it takes an extra bit position and determine if my
application can handle the data load.  Past that for a PIC, who cares if
it's fiber optic light beam on/off at 38K baud or modulated something or
other.  I still get a large number of bytes in per second that my
application has to handle and I may have to strip a certain amount of
transport information to get at my data.


IMHO

John


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