ZX-News #36 02 июля 1997

Модем

Typo - Vicomm-compatible modem from FLASH'a ? Brief explanation of the possibility of a velocity greater than 3200 baud at Vicomm-modem.

            Ochepyatochka out



                         Alexander Mayorov

                             Dmitry Lomov


  In the last issue was written ZxNews
that a group of FLASH (Novosibirsk)
made Vicomm-sovmeestimy speed modem send / receive 4800 baud.

  This information was taken from a letter
Alexey Voskresensky (Las'a), where it was said next "... 
invented a new modem rate ... "



  After this, the editors received a letter from
most Alexey Voskresensky, in which he
We decided to correct it.

  He reported that FLASH CORP. was not invented
modem for 4800, and the method of exchange, allowing
obtain such a high rate on any
Vicomm-compatible modem.

 Also, in a letter dated Las'a were the words:


             ---- C U T ---
So I made out of paper the following conclusions:

 1. Advertising XTRmodema
 2. Made it clear to the user, that too FLASNa
will have to buy a modem.
 3. Buy better in St. Petersburg at the X-TRADE,
FLASH than in Siberia! :)


  So what? " Once again I repeat: FLASH not
invented the modem for 4800! I would like to
seen in the "News" is more correct Old,
than that nonsense. And, incidentally, something like that I 
will send an echo in some thread to nick long wait to buy XTR 
2400 because soon will terminalka for ViComm on

4800!


             ---- C U T ---

  At present the algorithm for 4800 baud
while none of the editorial board has not seen, and, therefore, 
nothing specific about it, we can not say can.


  But from the explanation Las'a, develops
impression that it will be modified
transmission algorithm Vicomm'a (frequency modulation).


  That's just not clear yet, as will be further developed 
algorithm Vicomm'a as to raise the rate of transmission can not 
be - for 2400 Baud is already near the border, and "branded" 
vikomki (Sold Usov) have very weak transmission, which is why 
they have difficulty hearing. 

  However, there are no rules without exception: it is known 
that between 176 -.... and 516 -... Transfer to 2400 when the 
turbo mode (Machines and Profi Turbo2 +) is just perfect (the 
rate is obtained about 3200 baud!)



  Well, when you use the transfer dibitami, due to different 
speeds of computers, may be a lot of errors.



  Although there are other ways to improve the transmission 
algorithm. 


  So, I would like to receive more information.



  Well, it's better - Xtr or Vicomm, decide for yourself. We 
just compare them: 


  - Connecting to a computer and sizes


  Vicomm mount is very simple - just
four wire, well, size usovskih
modems, you know ...

  To connect the Xtr to Scorpion / Kay
you just plug it into the socket. Sizes, respectively, are 
small. If you connect to other types of computers, will reach 
about 15 wires and cut one track on the board.




  - Wire length


  Vicomm conceived as an external modem,
although very long wires should not do.

  If the wires from your computer to Xtr longer than 20 ... 30 
cm, the modem can "fail". 



  - Quality of reception


  As already mentioned, the usovskih Vicomm'ov very quiet 
transmitter, although if modem, modify or make yourself, then

This problem is easily solvable.

  But since Vicomm podlkyuchaetsya to
recorder input, the reception quality
mainly depends on the amplifier circuit! And these schemes, 
there is great variety, and often they are not configured. 

  For example, the same modem
included in the same phone jack, but connected to different 
computers, works quite differently! 


  I Xtr built-in amplifier, which
accordingly adjusted.



  - Reception / Transmission


  Well at the moment algorithm reception / transmission like 
Vicomm'a, and at Xtr'a identical. However, Xtr works better.



  This is explained as follows: transmission algorithm is as 
follows - set the output "1" and keep her X ms, then set the 
"0" and keep Y ms.


  At the time Vicomm'e X and Y are defined in software, and if 
one computer is running faster, the value of X and Y had 
changed, and the second computer could "not understand" that it 
took: one or zero. 


  Well, in Xtr time is given by
built-in timer, which stops
processor at the right time.



  - Dialing / reaction to the call / caller ID


  In Vicomm'e just do not. To implement the first two items 
required gadgets that you can either collect or buy. But it 
still needs to connect and ... 

  That the same concerns AONa, then Vicomm'e
that no one has yet worked.



  - Waveform


  Vicomm can form only
rectangular signal. A Xtr has DAC
16 shades, so that can generate a sinusoidal signal.



  - Price


  Cost Xtr-modem 67.000 rubles, its
sell for $ 70 ... 90 thousand rubles.

  Vicomm can be bought for about 40 ... 60
thousand rubles, or to collect himself, in this case
it will cost even less.


 Brief explanation of the possibility of velocity

    large at 3200 baud Vicomm-modem.


  Generally speaking, implementation of algorithms
FM or FM in the forehead there is a lot of problems to be 
solved only good line of communication (lack of high-Q filters 
at the station). When implementing clever algorithms, such as 
codes Patel or Miller, the problem is the upper limit frequency 
removed, but there are other, more fierce.


  Namely: the phase distortion of signals
predkorrektsii need for transmission
the complexity of the PLL, anti-
constant component, etc.


  Solve these problems purely software
very difficult, even for Xtr with its hardware support.


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