                Hardware Information
                --------------------

There are now two versions of Pasokon TV 3 which use different
types of hardware:


Pasokon TV Classic
------------------

The Pasokon TV Classic interface fits inside an expansion slot
of a computer.  This design has many advantages over the 
simple single chip circuits used by many other SSTV systems

  - Built-in audio filter to dig signals out of the noise.
  - Hardware decoding of SSTV signal for more consistent results.
  - On-board crystal oscillator for precise timing.
  - Does not tie up a serial port.
  - Built-in relay for PTT and transmit audio switching.


E Z SSTV
--------

E Z SSTV uses a simple single op-amp circuit connected to a 
serial port.  If you are already using one of these, you can 
use the same interface with little or no modification:

     *  AEA FAX   
     *  EZ SSTV (version 2)
        GSH-PC
        JVFAX
        MSCAN
        ProSkan
     *  SSC PC SSTV
     *  SSTV Explorer
        VESTER

* = Note: Reverse power must be selected for these.
  This mentioned below and explained in more detail in
  the file POWER.TXT or from the Help button of the 
  interface Power polarity dialog box.


These are the necessary connections for receiving:

        Signal    25 pin     9 pin   
        Name    connector  connector    Function
        ------  ---------  ---------    --------
        DTR       20           4        + power for op amp
        RTS        4           7        - power for op amp
        GND        7           5        ground
        DSR        6           6        amplified audio to computer


Interfaces marked with "*" in the earlier list use RTS for + 
power instead of DTR.  The software has an option for this.

Note that some interfaces use a full wave rectifier so it will
work with either polarity of RTS and DTR.  


A Very Simple Receive Interface
-------------------------------

This is about as simple as you can get.  The basic idea is to 
amplify the received audio and feed it into one of the pins 
of a serial port.  The computer measures the time between the
zero crossings and decodes an image from the timing.

You might even be able to use an 8 ohm to 1000 ohm audio 
transformer to increase the signal voltage sufficiently and  
forget about the op amp.  I haven't tried this yet.

A well stocked junk box probably has all of these.  Otherwise, 
you can get everything at your local Radio Shack store. The 
resistor and capacitor values are not at all critical.  Use 
either a 25 or 9 pin connector depending on the type of serial 
port available.   Just about any op amp will work; we're just
using it as a limiter at audio frequencies so there is no
benefit to using something with super high speed.  

                                          part #        $
                                          ------      -----
      U1      741 or similar op amp       276-007     0.79
      R1      47K resistor                271-1342    0.39
      C1      0.01 uF capacitor           272-131     0.49
      J1      DB25S                       276-1548    1.99
          or  DB9S connector              276-1538    1.29
      -       Shielded cable for audio     ?           ?

There are many variations on this basic circuit.  Others use 
diodes for reverse voltage protection, capacitors for power 
filtering, and/or transformers for breaking up ground loops.  
This is trimmed to the bare essentials but still seems to work 
well.


                      U1 - top view
              (pins pointing away from you)                25    9
                  notch on this end           J1    name   pin  pin
                  and/or dot near pin 1             ----   ---  ---

Audio                   #----U----+         +-----+
from                    | 1     8 |    +----|--O  |  DTR   20    4
Receiver | |            |         |    |    |     |
  -------| |---+--------| 2     7 |----+    |     |
         | |   |        |         |         |     |
      C1       |   +----| 3     6 |---------|--O  |  DSR    6    6
              <    |    |         |         |     |
               >   | +--| 4     5 |         |     |
          R1  <    | |  +---------+         |     |
               >   | |                      |     |
              <    | +----------------------|--O  |  RTS    4    7
               >   |                        |     |
               |   |                        |     |
   ------------+---+------------------------|--O  |  GND    7    5
Audio                                       +-----+
Ground



Transmitting
------------

When transmitting, audio is available from both the PC's 
speaker and the serial port.  


Transmit audio from serial port
-------------------------------

        Signal    25 pin     9 pin   
        Name    connector  connector    Function
        ------  ---------  ---------    --------
        TxD        2           3        transmit audio
        RTS        4           7        + during xmit to drive
                                           transistor for PTT
        GND        7           5        ground

Advantages:

   - No need to connect a wire inside computer.
   - Safe.  Can't harm serial port by shorting wires to ground   
     or each other.
   - Can be used with laptops or other computers where it is
     very difficult to get at the internal speaker.

Disadvantages:

   - Tones are less accurate than from speaker.  Transmitted
     image quality will not be as good.

The signal from the serial port is about 20 volts peak to peak
so this must be attenuated quite a bit before applying it to 
the transmitter.  Use something like this:


        Serial Port                   | |
        TxD signal   -----/\/\/\/-----+ +-------+
                                      | |       |
                          100k                  |   10k pot
                                     0.1 uF     <
                                                 >
                                                <
                                                 > <-----------
                                                <
                                                 >      Audio
                                                |        to
                                                |       Transmitter
                         Ground ----------------+


Some designs also provide some low pass filtering to reduce
the harmonics present in the square wave signal.  I don't know
if this is really necessary because the transmitter has 
limited bandwidth anyhow and will round off the edges.


Transmit audio from internal speaker              
------------------------------------

Advantages:

   - Tones are more accurate than from serial port.
     This results in much better quality transmitted images.

Disadvantages:

   - Must connect a wire inside computer.  
   - You can damage the computer if this is done improperly
     or the speaker wire is accidently shorted to ground.

CAUTION !!!  v    v    v    v    v    v    v   v    v    v

        Be very careful if you decide to connect a wire
        to the internal speaker.  If the designers wanted you
        to connect something to it, they would have provided
        a jack.

        In my computer (don't know about others) one side of
        the speaker is connected directly to 5 volts.  If you
        accidentally short this to ground, the computer might
        be damaged.  I would strongly recommend that a 1k
        resistor and a capacitor be used between the speaker 
        and the wire  coming out of the computer.  This will 
        limit current and prevent damage if the wire is shorted 
        to ground.

CAUTION !!!  ^    ^    ^    ^    ^    ^    ^    ^    ^    ^


You will have to experiment to see which side of the speaker
has stronger audio.   Be careful!!!  One side of the speaker
might be connected directly to 5 volts.  Shorting this to
ground could cause extensive damage.

WARNING!  Connect only a single wire to the speaker.  DO NOT
connect a ground wire to the other side!!!

                                             .
                             Inside computer . Outside
                                             .
                                             .
        PC Internal                | |       .
        Speaker   -----/\/\/\/-----+ +-----------+
                                   | |       .   |
                         1k                      |   10k pot
                                  0.1 uF        <
                                                 >
                                                <
                                                 > <-----------
                                                <
                                                 >      Audio
                                                 |        to
                                                 |       Transmitter
                         Ground -----------------+
                     (not other side of speaker)


I would suggest placing the resistor and capacitor inside the 
computer and insulate everthing well with heat-shrink tubing.
With the resistor and capacitor to limit current, you won't
have to worry about damaging the computer if the external
wire is accidentally shorted to ground.


Push To Talk (PTT)
------------------

There are 3 different ways to activate the transmitter when
sending an SSTV image:

   - Manually.
   - VOX activated by SSTV audio signal.
   - Computer controlled PTT.

If you want the computer to control the transmitter with the
PTT line, use the following circuit.  This should work with
most modern transceivers with low voltage, low current PTT
circuits.  For older rigs you might have to use a relay to
handle higher voltage or current.

                                                               Xcvr
                                                   /---------- PTT
                                               |  /  c
        Serial                  |\ |        b  | /
        Port RTS  ----\/\/\/\---||>|-----------|<       2N2222
                                |/ |           | \      or similar
                       10k                     |  _| e       NPN
                               1N4148               \
                                                     |
                                                   Ground

The diode is important because applying a negative voltage
to the base of the transistor might damage it.


Configuration File
------------------

The serial port number, and possibly other attributes, must be
specified in the PKTV.CFG file.

See CONFIG.TXT for more details of the COM_PORT option.

