Message 26121 in TECHNI.WW
From: G8MNY
Date: Tue, 21 Jul 26 07:38:00 Z
Newsgroups: TECHNI.WW
Subject: Modem Bauds & Bits
Message-ID: <65438_GB7CIP>
Path: N2NOV|VE2PKT|VA3BAL|VE3CGR|OK2PEN|GB7CIP
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jan 21)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
BASEBAND
This is the starting point for data transmission. If the comms channel can
handle DC and have a flat phase & amplitude response in Hz to beyond half the
data rate in bits/S then the link should work. Over very long lines repeaters
may be needed to straighten up the rounded edges and these can be straight
Schmitt triggered amplifier or better still re-clocked (retimed) gated
amplifiers.
GOOD DATA ROUNDED DATA TIMING ERRORS
The trouble with the need to find edges in the data to reclock & sync, means
either a clock must be included with the data, or the data must have enough
alternations (0 & 1s) that the clock can always be extracted.
When no DC path is available then all is not lost if the data is short and has
no substantial DC component, then a DC clamp & restore circuit will work, as
will random encrypted data system (spread spectrum) that has no net DC.
Anything else needs a Modulator & Demodulator "Modem" to translate the baseband
to higher frequencies suitable to telephone line or radio path.
VSB (AM)
It is possible to send the data as Vestigial Side Band (part of AM signal),
utilising nearly all the available spectrum to carry an information sideband.
With VSB a carrier is sent near to the edge of the passband, this ensures the
DC component is sent OK.
Channel Width Only a tiny fraction of the LSB is sent enough
300 400 2700Hz bits per second. In single level AM, AFSK or FSK
<-baseband data-> there is a 1:1 relationship, 1 bit/s = 1 baud.
In this example 2300 Hz of baseband band data gives 4600 baud or bits/s on a
narrow SSB channel.
However there are loads of problems with AM & VSB modes for data, so FM with
its 2 sidebands became the dominant mode...
FSK 1200B/S
This is fairly typical of what happens as
0 1 the FM idle carrier at 1800Hz causes 2 AM CW
0 300 600 1k2 1k8 2k4 3k (1800Hz carrier is used as it is the fastest)
(frequency on most telephone lines.)
In this example the data rate would be locked to the carrier frequency, if not
locked system then similar but not exactly the same modem frequencies are used.
For this FSK to work, a bandwidth of 600Hz - 3kHz is needed, it does not have
to be too level flat, as FM can be hard limited, & that removes level changes
as well as almost all noise effects.
So a bad line/comms link with more than 15dB frequency spread and worse than
15dB S/N is still error free despite being very poor for phone use.
However differing delay times for the 2 tones is critical, and a Group Delay
difference of less that 1mS between 1200Hz & 2400Hz will garble the data, as
the modem will Rx no tone, then 2 tones etc. This has no affect on comms phone
audio!
Typical Group Delay of a long loaded line, or via a comms Rx IF filter!
Delay
300 500 800 1k2 1k6 2k 2k4 2k7 3k 3k2 3k4 Hz
Sent data Rx Data with 833uS of group delay!
______~~~~~~____________~~~~~~~~~~~~ ______????????????______??????~~~~~~
0 1 0 0 1 1 0 no 0+1 1 no 1
1200 2400 1200 1200 2400 2400 1200 Sig 1k2+2k4 Sig 2400
Even so this is one of the simplest and robust standards around (this why it is
still used), however it is important to have all the sidebands that make up the
signal turning up all at the same time and at simular levels and also not
completely drowned out in noise or over distorted.
GOING FASTER
If we look at what a telecomms line can pass e.g. 300Hz - 3.3kHz with some
frequency attenuation & group delay. Then that would support a max baud rate
0 300 1k8 3k3 Hz
But by using AM multiple levels as well as PM together, it is possible to make
a "single baud of carrier" represent 1 of many states, e.g. 16 AM levels & 16
phase angles, gives 256 (256QAM) states or symbols, which is equal to 8 bits
of data per baud! That would make a data rate of 8x3k = 24kB/S.
Phase & level diagram (dartboard vector) for just 4x 4 (16 symbols, 16QAM)
LEVELS-4 -3 -2 -1 0 +1 +2 +3 +4
LEVELS-4 -3 -2 -1 0 +1 +2 +3 +4 at once!
However you don't get something for nothing, using 16 levels means the
distortion & all noise must put the level into the next 6.25% level window, or
3.1% (-31dB peak). And the same goes for multiple phase mod, due to jitter and
multipath etc. that must not to exceed 1/32 of a cycle peak @ 1800Hz! A very
tight specification for radio path!
EQUALISING (training)
As the line will not be flat or have no group delay it is important that the
modem has an equaliser that has the opposite characteristics to the line. This
used to be an analogue nightmare, but with digital A-D convertion of the line
audio signal, it is relatively simple to flatten in software!
e.g. a Line with bump at 1kHz
Level Add in a % of signal data from 500mS ago and the
0 250 500 1k 2k 4kHz
Training can also be done on live data, by making small adjustments and seeing
ifthe data quality (how near Rx symbols are to their pigeon hole's ideal
centers)
Ideal Symbol Amplitude Phase Noise & a Symbol
in Centre of Trouble Trouble Distortion in Error
Pigeon Hole up/down Sideways
from centre ref centre
So you see that although modern modems are super at equalising and giving 56
kB/S max on a good short line, they can only do that over clean copper circuits
and high quality PCM audio systems that give <1% THD >50dB S\N and no phase
jitter!
In practice most lines can't do this, so the modem is designed to drop back the
baud (symbol rate) and or the number of levels/phase angles (symbols). Also the
modem has error detection/correction, so missed errors that are passed through
are low. The use of an encryption algorithm in some modems also minimises
continuous retries of data patterns that might have high error rates.
BROADBAND
These systems are different and use much wider frequency range and multiple
carriers each carrying some of the data in QPSK. But there are similarities,
and nasty line problems will greatly reduce the usable data rate. A common
cause of broadband data rate changing/hiccups is the need for the modem to
re-establish what frequencies can be use due to changing QRM, e.g. Nightime AM
station, or an old analogue cordless phone occasionally in use sending signals
on the same cable as your broadband.
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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