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.