INTERFACING THE SLOW SCAN CONVERTER WITH YOUR TRANSCEIVER

Many Hams running Slow Scan Video are transmitting hash, noise and other kinds of illegal emissions when they switch from Voice Mode to Video Mode. If the other operator does not point this out, the sending operator does not know how bad his signal sounds. Some may not care, but the rule for many years in amateur radio has been that a signal report should be given the sending station. It has always been a matter of courtesy in the past. The fact that some Hams seem offended by a poor quality report should not change this practice. After all, he doesn't hear himself in most cases. Few, if any, want to send a bad signal.

Five years ago before the computer made SSVideo cheap and easy and newcomers began to pour into the Video channels, it was noteworthy that Video stations had better sounding audio than the typical DXer. Reason: you don't or should NOT use a Speech Processor when sending Video. Also, each operator told the newcomer how his video operation sounded. It is time that operations on 14.230 MHz get back to that practice.

Today's most prevalent problem with newcomers -- and some old timers -- is the presence of  1) RF in the signal,  2) intermittent hash  3) and clicks, pops and other forms of  audio distortion. The latter two occur when the operator switches from using his microphone to sending Video and back to Voice. At least, a third of the stations sending appear to have this problem. Most of them are never told about it. Some probably don't care as long as their buddies say nothing about it.

Technically speaking, if every converter had its own PTT feature, they would not have to use VOX -- part of the problem usually. The sound card is the source of the trouble since it was never designed for SSTV use. If manufacturers designed their radios properly with a well isolated input circuit, this problem would not exist. But it does exist and it has to be dealt with!

If radio manufacturers would correct this input problem as ICOM has, the use of RIGBLASTER, SIGNALink and similar interfaces would be unnecessary. The IC-746 and most ICOM radios and YAESU appear to have no distortion problem when operating SSTV! If they can do it, then all makers of radios can do it! The problem is that no one in Amateur radio -- like the ARRL -- uses its influence to have such anomalies corrected! They have a relationship with all the manufacturers of Ham equipment via advertising in QST, hence they could work this problem out easily. Unfortunately, ARRL is dominated by officers who have been an enemy of SSTV for many years !

For the Newcomer to Video who wants to do the right thing, we offer the following guidance. It is offered in the spirit of the tradition of Ham Radio: individuals helping others to promote the enjoyment of the hobby for everyone. Meeting regulations is also a worthwhile objective.

THE PROBLEM AND AN EXAMPLE

Recently, I started using a YAESU FT-920. I thought that such a recent design would address the problems of interfacing with converters for Digital Modes. After several weeks, I concluded that YAESU probably had no "hands-on" expertise in running Slow Scan, hence their "radios" would be no better than any of the others. My Kenwood TS-450S/AT did not have a problem but the 13-pin DIN connector was an irritation. My tubed rigs like the Drake Twins and the TR4-C were relatively easy to interface. My TEN-TEC Omni V was impossible to use on Video without distortion. Now, I had an FT-920. I was determined to deal with the interface problem once and for all.

FT-920

This rig is a little brother to the FT-1000MP in many respects. It has a separate Accessory socket wired to match the standard TNC. It is identified as the DATA section. This was a very appealing arrangement to me since it meant when I went to DATA, I was operating in a part of the 920 that has no connection to the Microphone circuit. Great!

After studying the Manual for several days, it became clear that the only reference to Slow Scan suggested you can put the Xmit signal in the AF IN of the Phone Patch. Of course, the reference stated, you will have to remove the mike when sending the picture! That was the clue that they didn't care much about Video.  I focused on the DATA circuitry that used the Accessory connections. While operating flawlessly (i.e., no distortion, clicks or pops) the transmit signal was great, however, it transmitted a couple of kilocycles above or below the operating or Voice frequency. Hardly a cool way to go!

After a day of experimentation, I solved the offset problem by going into the Menu to U-45 and setting the Offset to 0.000 from the Default 2.215 kc. Going to U-46, that Default value was 2125-L (I was operating in the 40 Meter band). This arrangement put the picture transmission exactly on the Voice frequency. Success!

CAUTION: To initiate picture transmission, you press the DATA key. It will display LSB, first -- which is okay for FSK. For SSTV/AFSK press DATA once more to display USB. USB is for AFSK. Seems like they could have used FSK and AFSK. Yaesu tech support people do not know why such a confusing identification was chosen.)

Actually, success was not quite what we got! We had to learn a little more.

The following day, I went up on 20 Meters and tried sending Video on 14.223 MHz. Sadly, I was told the picture was coming in about 2 kc away. This time, going to U-46 , I changed the reading to 2125-U and in U-45, set the offset to 0.000 again. Each time you change U-46, U-45 has to be reset to zero offset!

This procedure worked perfectly!!

As you can see, to use the Accessory designed for DATA (specifically, for AFSK or Slow Scan Video)  if you want to run Slow Scan, you must make changes in the Menu when going from a band that uses LSB to one using USB.

Once I had learned as much as I could, I contacted YAESU Tech Support. After a week of exchanges via e-mail, it was clear YAESU engineers did not address the specific needs of Slow Scan. The interaction between the SSVideo converters (e.g. ROBOT 1200C, Creative Sound Boards) and the TEN-TEC transceivers (except the new PEGASUS) also existed in the FT-920.

Obviously, at this point,  it was necessary to consider a "work-around"-- the very thing I feel should not be necessary in equipment costing thousands of dollars. The AFSK or DATA input in the FT-920, however, is a step forward. It works quite well as long as you remember to make the Menu changes noted above.

The alternative (if we use the Phone Patch) is to control the XMIT signal from the Sound Board by using a relay so the LINE OUT in the Sound Board is disconnected when receiving.  Either PTT, MOX or the VOX line can be used with this arrangement. This same basic arrangement works with the TEN-TEC "radios".

RELAY SYSTEM

In the Ten-Tec transceivers, you must control both Receive and Xmit lines from the Sound Board. This means you need a relay with a Normally Open (NO) connection (for the XMIT function) and a Normally Closed (NC) connection for Receive. We built such a relay system in a plastic box. All material was obtained at Radio Shack. The 13.4 vdc required by the relay is available on the rear panel of the "radio". To control it, I used a switch up-front connected via an RCA phono connection into the plastic box. The relay worked very well with the FT-920 and the OMNI V. Go to
RELAY SOLUTION  for details for building this unit.

CONCLUSION

All the hash, pops, clicks, and whistles heard on the Slow Scan channels can be cleaned up. But it will take cooperation between all parties to clean up the Video channels. Those with problem signals should be informed of their problem first, however. It should be done in a tactful, friendly way. Then move off the main channel to another spot where you can help the other fellow solve his problem under relaxed conditions.

END