Usually, the first thing you hear when a newcomer to SSVideo sends his/her initial pix, is that the picture is "slanted" or its left and right edges are jagged or saw-tooth shaped. I have listened to hundreds of operators trying to help someone correct their "slant" problem. Very few operators, however, try to make the process of communications easy for the newcomer. Either the newcomer wont read instructions or -- the more likely case -- articles about the technology itself are not available to them. There are three things the new SSTV Operator will see and not understand.

1. Slanted pictures (and saw-tooth vertical edges) being received and transmitted.

2. Mis-registered pictures where the three color panels -- Red, Green and Blue -- are not perfectly aligned (i.e. congruent) with each other.

3. Split pictures: the Right side of the original  picture appears on the Left side of the screen and the remainder of the pic has a bluish haze along with funny (mis-registered) colors.

Let me try to explain these peculiarities for the Newcomer who wants to know something about the Mode. The first problem they face is a new set of words/terminology required to explain what they are seeing. This article will familiarize you with some of the terminology a la BEJO!

   
       View #1                                                          View #2              

SLANT (See above Examples)  is caused by a mistiming problem. Your PC has to be calibrated so it's line scan sweep corresponds to the standard timing inherent in the particular picture format (i.e. Scottie 1, Martin 1, etc).

(Since I will refer to a VIS CODE in this discussion, I want you to listen to and observe SSTV transmissions for at least an hour or more so you hear a variety of sounds or tones used to initiate transmission or receiving of a picture. I listened to SSTV for three months before I ever saw a good picture in 1992! In the beginning, you will hear a series of what is called "the VIS CODE" that stands for "Vertical Interval Signaling" Code. This was a ROBOT RESEARCH innovation that sets up or "tells"  your Receiving system  what picture format is being sent (i.e. whether it is Scottie #1, Martin #1 etc). It also indicates: 1) whether the pix is going to be Black & White or a Color picture; 2) sets the vertical synchronization of the receive system. Some systems send the VIS CODE two or three times in case signal strength is weak or erratic. This code is the first thing you will hear!) As you watch the scan lines appear, you will see and hear the "initialization signals that virtually take control of your receiving system!)    

1. First, when we describe the "slant" on a picture, we should use a uniform or same- way to talk about it. Describe the condition from the standpoint of what has happened to the BOTTOM of the picture. For example, if the bottom has shifted to the right, say  " The bottom has shifted to the right." (The top is always in-place because that's where all pictures begin with the initial linesync.)

2. A slanted picture means the timing of the picture being Received is not the same as that of the Sending station -- i.e. they are not in-sync. Either the pix is being transmitted (or received) too fast or too slow with reference to the designed speed of that particular format. If the picture has a saw-tooth edge on the sides, that means that you have a severe out-of-time problem. Many programs try to automatically correct the problem, thereby, causing a saw-tooth effect. When you see this type of picture developing, look at the Histogram or Tuning Aid and you should see the bar change to GREEN which means your are inside the area where you are getting a good "linesync". If it is not Green, tune the signal in so the Sync Pulse is on the 1200 kc calibration marker (that's the vertical line to the left of where the signal activity appears. 

     a. A pix that is shifted to the Right means the timing is too slow. Put another
         way, the time between each linescan is too long or it is "running late".
         Therefore, the effect of a slower interval between each line is the left edge
         of the picture is progressively shifting toward the Right.
 
    b. A pix that is shifted to the Left means the timing is too fast. Put another
        way, the time between each linescan (or Sync Pulse tone) is too short.
        Since the length of the scan line is fixed at, say, 316 pixels, when timing is
        off, the position of the "sync" pulse mark will become increasingly
        displaced, resulting in a slanted picture. (Yes! It is a bit confusing!)

3.  What "timing" are we talking about?  Good question!
     Let's take the Martin 1 mode as an example. M1 mode requires 117 seconds to
     send a complete picture. If it is 5 seconds fast (i.e., is completed in 112 seconds),
     the bottom of the pic will be shifted toward the Left considerably.
 
    The timing of the receive system must be calibrated because the PC clock must be
    running at the same speed of the Sending station. This presumes the
    Sending station is sending accurately timed pictures. The clocks must be set
    within the SSTV program so that the program and the PC achieve perfect
    timing. SSTV requires precise timing! Once this calibration is done, it should
    not require further attention unless you make changes in your setup.

All the above would be fine except, you sometimes hear one operator say he received the picture slanted while another says he received it straight! What's going on? (Very few new Scanners today understand this! No problem!)

Nothing is simple anymore, Pilgrims! The reason one is straight and the other slanted is because the two fellows are running their receive systems differently.

There are two ways to run an SSTV program for Receive?  Yep! They are directly related to Timing and what is controlling the timing (i.e. the Sync Pulse. To receive a picture properly, you must pick up the initial Sync Pulse (that starts the pix and starts the first linescan after the VIS signal) so that each line begins at the right time and place!
     

Most of the time, the program automatically handles which method is being used. Most start when triggered by the VIS code, then begins a few lines with linesync method, then it switches to "synchronous" or "free running". This is a Hybrid system. When the Standby button is blinking in a ROBOT 1200C, it picks up the VIS code, then runs (usually) in Linesync. If the Standby pad is NOT blinking, the operator must press the Receive pad to start the linescan process. Even if there is no picture coming in, a line is scanned across the screen, usually in grey if there is no signal or Noise. It is in "free running or synchronous" mode, then.) 

If you are running in LineSync mode and the sending station is sending a slanted picture, you could receive it straight! If another station is receiving the same picture, but is using "free running" mode, the picture will be slanted. That's why, when you get a "slanted picture" report, always get several more reports before tearing everything apart !

TUNING versus TIMING

At this point, the Newcomer should know that: Tuning is Critical in SSTV! Failure to be on-frequency can produce a number of other weird pictures! Today's SSTV programs provide a SPECTRUM or Histogram that displays the tones being transmitted. The best SPECTRUM of the many I have used is in the MSCAN programs. It is an important tool! It is resizable and separate from the other frames. You must learn how to "read" it along with listening to the tones! MMSSTV also give s you these options!

The most critical item on the MSCAN SPECTRUM is where you place (or tune-in) the SYNC PULSE. To the left side of the Histogram is a line (along the baseline) about 100 Hz wide. The Sync Pulse (from the sending station) should be tuned to the center of this line. Automatic Frequency Control is provided in the software to deal with the fact that many stations will be slightly high or low. Since tuning for SSTV is critical within plus/minus 10 Hz  YOU must always center the Sync Pulse. A well-operated NET will usually remain on-frequency plus or minus 10 Hz. A well-operated Net will begin with a calibration session so everyone is on the NET frequency!

Most histograms display a small red marker in the Sync area that seems to move about sluggishly. This marker is reporting the averaged value of the Sync Pulse frequency. Phase shifts due to propagation and other phenomena act upon the signal and it shows up in the Spectrum. Use this marker if the dancing Sync Pulse is too difficult to follow.

 
    View #3

MIS-REGISTRATION OF THE RGB PANELS  (see View #3 above)

The second anomaly the Newcomer faces is the picture with a blue haze dominating the image. The usual reason for this is that you were not tuned to the frequency of the sending station.
Learn to use the Spectrum/Histogram! (ROBOTs don't have a histogram or any indicator except a SCOPE function, however, it cannot be viewed while a picture is coming in. Instead, operators use the blinking RECEIVE pad as a tuning aid.)

What is "mis-registration"?  Another good question!  Some refer to it as a pic with crazy colors!

An SSTV color picture is created by overlaying three panels of Red, Green and Blue. These three panels (composed of scan lines of those colors) must be placed on top of each other so the images match or align perfectly. The reason for mis-registration -- most of the time -- is: not being tuned on-frequency, which means the Sync Pulse is out of range and the entrie process never gets started right.

Actually, the RED/GREEN/BLUE lines are not transmitted at the same time. They are sent sequentially-- one after the other. That's why color pix take longer. Due to being mistuned, the lines are shifted slightly (an out-of-phase condition) -- creating the appearance of a 3-D pix. In a way, these staggered colored panels are shifted by mistiming very similar to the mistiming that causes slanted pictures and zigzag edges. One error is a technical anomaly. The other is operator error.

SPLIT PICTURES   (see Example #2 above)

In addition to displaying "mis-registration" of colors, example #2 shows a "split" picture where part of the right side is displayed on the left side. Such a "split" picture -- most of the time -- is an extreme case of the receiving station being off frequency. Mistiming, due to operator negligence or error, causes this problem.

In summary, the SSVideo software and the PC must be calibrated so they receive and transmit the picture formats (S1,S2,M1,M2, etc) as those formats were designed. Finally, the operator must tune his receiver to the Sending station's frequency.

Regardless of the fact that most software has Automatic Frequency Control, tune to the CENTER of the Sync Pulse segment (an area or line 50 Hz on each side of 1200 Hz Center). When you do not do that, your transmit frequency can be outside the Sync range of one or more Receiving stations. The reason for this is that very few receivers whose Frequency Readout shows, for example, 14.233 MHz, are really on that exact frequency!

So you paid $5,000 for your rig! Only if you re-calibrate your "radio" every three months against a frequency standard certified by the National Bureau of Standards can you claim to be on-frequency. Even then, you can't be flat sure!

FINAL COMMENT

Finally, please, OM, observe this little courtesy: when you join an ongoing SSTV session: tune (using the Sync Pulse & Histogram) to the frequency of the Sending station -- preferably the Net Control station. If everyone follows that procedure, everyone will be on the same frequency. PLEASE: Center the Sync pulse!

Far too many operators punch up their Memory button for 14.230 MHz and assume they are on 14.230 MHz. Not exactly! Very few of our transceivers agree on the readout display. You are NOT on-frequency until you have received a pix on that channel and the Sync Pulse is in the middle of the aforementioned 100 Hz segment in the Spectrum. If you are joining an on-going QSO, tune to the center of the channel (as indicated by the Sync Pulse) being used by Net Control or those already there.

Enjoy the Mode and let's show respect for this wonderful Hobby by exchanging pictures that the entire family can look at and appreciate!!

de Cholly (W3OHV)

(Editor's Note: