                Are You On Frequency?
                ---------------------

I recently received a letter from someone (not using
Pasokon TV) complaining that many Pasokon TV users often
transmit off frequency.  The earlier documentation did not
explain this topic clearly enough.  Here is a quick 
attempt to explain the situation and how to prevent it.

Part of the letter says:

        "All stated they use Pasokon TV and they have no 
        trouble receiving.  They assume, therefore, that 
        if they don't trigger me, it is my fault."  

Pasokon TV has Automatic Fine Tuning (AFT) and is very tolerant 
of mistuned signals.  Most of the Pasokon TV users don't realize 
that some people, including themselves, are way off frequency.  
Users of some other systems miss the off-frequency transmissions 
so they are forced to be more aware of mistuning.  Or they
just miss a lot of pictures and don't understand why.


Pasokon TV Tuning Indicator
---------------------------

The Tuning Indicator looks something like this  (sorry,
no images in this text-only file):

                        +-------+
                        |   =   |
                        |   #   |    \
                        |   #   |     \
                        |   #   |      >   Gray Scale
                        |   #   |     /
                        |   #   |    /
                        |   =   |
                        |   <   |          Red Triangle
                        |   =   |
                        +-------+

The vertical position represents the audio frequency:

  - The gray scale area represents the frequency range of
    1500 Hz (black) to 2300 Hz (white).
  - The red region represents 1150 to 1250 Hz.
    The center of the red region is the ideal sync
    frequencey of 1200 Hz.


The Right Side
--------------

The right side of the tuning indicator contains a histogram of 
the received audio frequencies.

For example, suppose you were receiving a black and white
checker board pattern.  The received signal would cycle
between 1500 Hz (black), 2300 Hz (white) and 1200 Hz (sync).
The right side of the tunining indicator would look something
like this:  (use your imagination)

                        +-------+
                        |   =   |
                        |   #---|    White - 2300 Hz
                        |   #   |     
                        |   #   |    Gray - in between
                        |   #   |     
                        |   #---|    Black - 1500 Hz
                        |   =   |
                        |   <---|    Sync - 1200 Hz
                        |   =   |
                        +-------+

The lines on the right side show the received tones.
These will constantly change.  In this ideal case we have
clean lines at white, black, and sync.  In reality,
noise and distortion would cause them to spread out a little.
Most images are not full white and full black so there will
be many more lines in the gray scale range.

(Note: This histogram is run as a lower priority task than
image decoding.  It may freeze during reception of some modes
on slower machines.  This is nothing to worry about.)


The Left Side
-------------

The left side reveals two things:

  - What is happening with the Automatic Fine Tuning (AFT).
  - Quality of lock on the signal.

                        +-------+
                        |   =   |
                        |   #   |    
                        |   #   |     
                        |   #   |    
                        |   #   |     
                        |   #   |
                        |   =   |
                        |---<   |
                        |   =   |
                        +-------+

The vertical position shows the recent average frequency of
the sync pulses.  Ideally this line should be at the tip of
the red triangle.  This will be higher, or lower, if the
sending and receiving stations are not on exactly the same
frequency:

                        +-------+
                        |   =   |
                        |   #   |    
                        |   #   |     
                        |   #   |    
                        |   #   |     
                        |   #   |    
    Tones too high ->   |---=   |
                        |   <   |    
    Tones too low ->    |---=   |
                        +-------+

If you watch the line on the left side when receiving signals
from several different stations, you will probably notice 
it is in a different position for each station.  This is because
they aren't all tuned to exactly the same frequency.

Why would they be on different frequencies?  Transceivers aren't
all calibrated exactly right, and SSB signals are tuned in by the
sound of voice, which isn't too accurate.  This is how to get
everyone on the same frequency:

  1. Designate one station, a net control station for
     example, as the standard.
  2. The "standard" station transmits a picture.
  3. Everyone else adjusts their transceiver dial until
     the line on the left is lined up with the point on
     the red triangle.
  4. Voila!  Everyone is on the same frequency.

Unfortunately, this doesn't work for two reasons:

  1. Most other SSTV systems have no tuning indicator - or
     other facility - that allows the receiving station
     to get on the same frequency as another station.
  2. Many people just don't understand the concept.  This is
     my fault for not making the description in this file
     clear enough.


Automatic Fine Tuning
---------------------

Suppose that mistuning caused all the received tones to be
100 Hz too high.

                        Ideal   Actual received
                        -----   ---------------
                Sync    1200    1300
                Black   1500    1600
                White   2300    2400

Some other, less sophisticated SSTV systems have no capability
of compensating for this situation.  The image would become
too light.  There would be no full black and all the very
light shades of gray would all become full white.

Or, the opposite example:  Suppose the received tones were
100 Hz too low.  Many other systems would display the image
too darkly.  Details in the darker regions would be lost and
full intensity never achieved.  (That is, assuming it displays
anything at all.)


Pasokon TV to the Rescue
------------------------

The Pasokon TV system computes the difference between the 
expected sync frequency of 1200 Hz and the average actual sync 
and shifts all the tones by this amount before decoding the 
signal.  You can twist your transceiver frequency dial back and 
forth slowly while receiving a picture.  The line on the left 
will track the changes and the colors will be correct.

Back to the letter someone sent to me...
Question:

        "... does Pasokon TV have an AFT that makes 
        it easy to tune to a video signal?"

Answers:

   *  It has an on-screen tuning indicator that makes it very 
      easy to tune signals properly.

   *  However, the Automatic Fine Tuning usually makes it 
      unnecessary to do any manual fine tuning  for proper 
      reception.

This makes the Pasokon TV system very "user friendly", easy to 
use, and reliable.  However, many users end up transmitting 
significantly off-frequency because they are not aware of the
problem.

It's good operating practice to try to get everyone as close 
to the same frequency as possible.


Alignment Tones
---------------

From letter:

        "... I send an alignment tone so everyone 
        gets on the same frequency. ... I find many 
        of them never get on frequency."

The details depend on what you mean by "alignment tone" and 
the type of tuning indicator on the receiving end.  The "A"
button, next to the "Xmit" button causes Pasokon TV to transmit
an alignment tone for about 10 seconds.  It follows the AVT
Master precedent of mostly 1500 Hz with short bursts of 1200 Hz.

As I recall, the 1200 Hz pulse timing didn't correspond with any
of the standard transmission modes.  The receiving station would
tune his/her SSB transceiver so the lines on the right of the
tuning indicator line up with the black end of the gray scale
and the middle of the red region.

This special "alignment tone" seems pretty useless to me.  All
modes other than AVT use a 1200 Hz "sync" tone between scan
lines.  The AVT modes have a digital header with known tones.
What else do you need?

If you want to transmit mostly 1500 Hz with some 1200 Hz, send
a completely black image in 8 second B&W mode.  I'm not aware
of any other system that has implemented the AVT alignment tone
and have never heard anyone send one.  Don't be surprized if the
little "A" button disappears in a future version.

During a round table discussion, one station should be 
designated as the "standard" frequency and stay put.  Everyone 
else should fine tune their transceivers to this one station.  
Otherwise, everyone will be following everyone else all over 
the place.

Question:

        "... for purposes of assuring single frequency 
        (net) activity, could you cut off the AFT while 
        we do the 'alignment tone' routine?"

There is no need to disable AFT.  First, the Automatic Fine 
Tuning does not affect the right side of the tuning indicator,
only the decoding of the image.  Second, the AFT does not affect
transmitted tones.  Pasokon TV always generates precise crystal
controlled tones of 1200 Hz for sync, 1500 Hz for black, and so
on.  It does not try to immitate inaccuracies of other recently
received signals.

The line on the left side of the tuning indicator shows something 
else besides the average received sync frequency.  The length 
reveals the confidence level that a valid signal is being received.  
This is used for an option called ...


Sync Squelch
------------

When this option is off, Pasokon TV will make an attempt to 
display an image for even very poor signals.  This is fine 
when you're sitting there watching but not so good when you 
turn on "auto save" and go away for a long time.  Your disk 
would soon be full of images of random noise.

The Sync Squelch option causes images to be displayed only 
when there is a fairly high confidence level that a valid signal 
is present.  We're dealing with noisy HF SSB signals so it's 
all based on statistics of the sync pulse spacing.  The length 
of the line on the left side of the tuning indicator shows the 
confidence level that a valid signal is being received.  When 
the Sync Squelch option is turned on, an image is displayed 
only when this line exceeds a certain length.

Based on comments heard on the air, some people really don't 
understand the option called ...


Free Run
--------

All SSTV modes, other than AVT, have sync pulses separating 
the scan lines.  One reception software strategy is to sit 
there in a loop of

 * wait for sync pulse.
 * gather and display a scan line.

This is OK in theory but results aren't so good in practice.  
If a sync pulse is missing, a line won't be displayed at all.  
Inconsistent detection of the sync edge results in lines not 
being aligned properly.  This is called jitter and straight 
vertical features become jagged.  When the Free Run option 
is off, Pasokon TV follows the average sync locations to 
minimize the image degradation from missing and corrupted 
sync signals.  However there is still some pixel jitter.

With the Free Run option on, a precise crystal oscillator 
is used instead for timing and the exact sync position is 
ignored once locked onto the signal.  Pixel jitter is reduced 
and image quality improved.  There is one disadvantage.  If 
the transmitting station is not calibrated properly, the 
image will be slanted.  New users of the low-end systems often 
transmit a few slanted images but other helpful hams get them 
straightened out.

Unless you need to compensate for signals with improper timing, 
leave the Free Run option on for best results.
