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

I recently received a letter from someone complaining that 
many Pasokon TV users often transmit off frequency. The manual 
does 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 conscientious.


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

The vertical position represents the audio frequency.  The gray 
scale area represents the image tones of 1500 to 2300 Hz.  The 
red region is 100 Hz wide centered around the 1200 Hz sync 
frequency.  The sync of a correctly tuned signal will be in the 
middle of the red region. 

The right side of the tuning indicator contains a histogram of 
the received audio frequencies.  The left side reveals what is 
happening with the Automatic Fine Tuning (AFT).  The line on 
the left side shows the recent average frequency of the sync 
pulses.  This is used to compensate for mistuned signals.  A 
properly tuned signal will have a line in the middle of the red 
region on the left side. 

For example, suppose the received tones were 100 Hz too low.  
Most 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.)

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.

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 don't see the problem.

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

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.  I can speak 
with authority only for one particular system.  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 sync spacing didn't correspond with any of the 
transmission modes.  The receiving station should tune his/her 
SSB transceiver so the lines on the right line up with the black 
end of the gray scale and the middle of the red region.

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 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 imitate 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.
