home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Ham Radio 2000
/
Ham Radio 2000.iso
/
ham2000
/
fax_sstv
/
ezsstv
/
tuning.txt
< prev
next >
Wrap
Text File
|
1996-04-03
|
7KB
|
163 lines
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.