Modems - About modems in particular and modems for the Spectrum.

╔════────
   ║
   │ Modems: briefly about the main thing │
                                      ║
                              ────════╝

(C) Sergey Filimonov

 Hello, dear readers of the newspaper
"Optocoupler"!
  So, I decided to write a short article
for this publication. First of all I would like
say that the very fact of the appearance of the Lviv
electronic newspaper is of great importance
for all Speccy users in our re-
gion, perhaps even beyond its borders.
Thanks to "Optocoupler" we are all a little bit
"let's get closer": learn more about each other
(albeit in absentia), who does what, who
what are the plans for our Spectrum
ma and not only, what news and much-
much more.
  Since many of you, dear readers,
whether, are looking forward to the appearance of CDOS-mo-
demos on the Lvov market, and I, as a producer
body of this product, I’m “slowing down” a lot
this process, then let me explain
come before you and tell you briefly about
modems in general and about CDOS modems in particular
tee. Perhaps my story will open to someone
eyes on the concept of "MODEM" and its real
capabilities and speed.


        ──══ About modems in general ══──


 You've probably already heard, dear readers,
they said thatthere are "crazy" speeds
for modems (expensive, of course), for example,
48 kbit/s (yes, yes, exactly kbit/s, not
kbod, since this is not the same thing, but about
below). The telephone channel can
cut yourself to skip the frequency from 300 Hz to
3.4 kHz. (I don’t know how the Zaporozhye
distributors of their modems can
sell at all if they claim that
telephone channel passes frequency up to 5
kHz :-)).
  Subjectively, such a speed seems to
turns out: that is, take a file of length N
byte or in N*8 bits, send it to those
telephone channel via modem, time
transfer name, divide the file length in bits by
transmission time in seconds and get
approximately the speed that was written
sana in the instructions for the modem (if it’s not wrong,
Li), as a rule, less, but not more.
  So how does this work?
  Firstly, due to the compaction of information,
secondly, due to the modulation of the signal in such a way
so that in one baud you can
“push through” two, four, eight bits and yes-
but more, thirdly, the use of many
channel modems. I'll tell you about everything
order, but first you need to explain why
However, 'bit/s' is different from 'baud'.
  I think everything is clear with the concept of 'bit/s':
the modem transmits a file N bits long in M se-
kund, so its speed is N/M bit/s.
  The term "baud" means the number of changes
logical states of the modem per second
du. That is, for one baud or for one logical
logicalmodem status can not be transmitted
only one bit, but also more (but not
less, i.e. cannot be bit/s --- cut ---

Protocol │speed, bit/s│characters/s, max
───────────┼───────── ─────┼───────────────
Bell 103 │ 300 │ 38
V.22 │ 1200 │ 150
V.22bis │ 2400 │ 300
V.29 │ 9600 │ 1200
V.32 │ 9600 │ 1200
V.32bis │ 14400 │ 1800
V.42 │ 2400 │ 300
V.42bis │ 9600 │ 1200
HST │ 14400 │ 1800
PEP │ 19200 │ 2400
Express96 │ 9600 │ 1200
(Hayes H96)│ │

 Note: Some protocols have different
al speed is lower. Givenhere
the number for bit/s takes into account that some
Some protocols perform information compression
tions during shipment, due to which we achieved
there is such a speed (for example, V.42bis -
actual speed 2400).

 --- cut ---

 Ideally, of course, everything turns out beautiful
in (cool modems, high speeds), but
the realities are different, especially on our lines.
Constant crackles, spikes, squeaks,
us, getting into someone else's conversation, etc. -
what primarily interferes with the quality
no connection. For example, I know people who
ry on their Hayes modems connected to
IBM, barely communicate at 1200 speed
bit/s, and between some PBXs there is no
connection via modem.


     ──══ About modems in particular ══──


 Taking into account all the above applications
directly to the Spectrum CDOS modem, perhaps
but to conclude that not everything is so
bad in terms of speeds. First of all,
since the CDOS modem is not hardware, but
software, that is, the information itself
modem cannot be condensed in any way, then
it can be pre-compacted when
power of the ZXZIP packer, and after receiving another
Unpack using your computer (ZXUNZIP). Not-
which text files are packed into 2
- 3 times, and individual screen files or
samples from musical works packaged
are falling outup to 80 - 90%, that is, almost 10
once! If we communicated satisfactorily
at a speed of about 2000 bps, then taking into account
tenfold compaction, one might say,
that the speed of the CDOS modem is 20000 bps
(just kidding, of course). Of course, if you
dear reader, try to condense
already packed information, then it is unlikely
It is certain that this will lead to the desired result.
result.


      ──══ And about modem realities ══──


I'll tell you a little why this happens.
What a delay in the release of modems. In the first
issue of "Optocoupler" talked about how
the first Moscow modems appeared in Lvov
and that initially on those modems on which
who are contacted normally in Moscow, we
Couldn't get in touch at all.
  Since then, a lot has happened in the modem circuit.
a lot of changes. I won't go into
here in hardware subtleties, but believe me,
There were really a lot of improvements.
A trial board was even released, unfortunately
lenition, unsuccessful, because after that
Many more improvements were introduced. On
At this stage the circuit design is practically
completed. But every idea, every
the work needs to be implemented, tried in re-
al scheme and after that - discard
her :), try something else, and so on
further until some level is reached
be an objective decision. Now it's left
literally unresolveda couple of questions. I don't
I will undertake any obligations
regarding the release dates of modems, but
lucky coincidences, I tried
I want to present our Sinclairists with a new
anniversary gift.
  Your reviews, comments, wishes and pro-
which you can convey to me personally on the radio
market (I always go there on Wednesdays).

 ──══════════──