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     Introduction - Live Pictures

      G4AYT and G6PKS are experimenting with narrow band television transmission. Having had plenty of experience over many years with standard 625 line television, narrow band offers new challenges and projects to experiment with. We have joined the Narrow Bandwidth Television Association (NBTVA) as an excellent source of information and ideas. The aim is to send a viewable moving picture in a narrow bandwidth, such as may be used for 'phone transmission. To do this the video content of the signal has to be restricted to an audible bandwidth. A common standard in current use is just 32 lines and 12.5 frames per second, this might seem too little to make sense, but in fact surprisingly recognizable content is possible even which such a low line rate - and it can be transmitted on topband!! The sample picture on the right consists of just 32 vertical lines. Other higher line rates are in use, but 32 makes a good starting point, especially with the readily available literature/circuits etc. There are several sources of computer software, using a normal soundcard to generate the 32 line picture from a webcam or other source, or to decode and display the received picture (some soundcards are better than others). An excellent source is the website of Gary Millard, software from Gary's site was used to produce the sample picture. It is a little known fact that the BBCs first ever TV pictures transmitted in the 1930s used this line standard. In those days much of the equipment was mechanical and some today still replicate and attempt to improve such mechanical systems, however, for now at G4AYT only electronic techniques are being investigated. Some digital systems currently available for NBTV appear to use a record and playback approach, rather than live pictures, somewhat like sequential slowscan frames, a method less appealing. However, computer programs are now available for low resolution digital TV in a narrow bandwidth, running around one frame per second, and have been successfully used at G4AYT on 2m FM. Some improvement is anticipated in the future.

     Project 1

      The first consruction project is a waveform generator, PCB (undrilled) and ICs from the NBTVA. The generator is capable of producing a grey scale, pulse and bar, checkerboard, as well as just black or white, at 32 lines. The small PCB is easily constructed and fitted in a case with an 8V regulated supply, line frequency and video output level pots have been brought out to the front panel.

        

     More than one of these boards have been built and all show some picture disturbance, curiously this does not show up on an oscilloscope. However, in the 'scope' display included in Gary Millard's Big Picture some spiking and jitter is clearly visible, suggesting an incompatibility issue with the software/computer. 

                                                            

      Above is the grey scale as seen on an oscilloscope (the verticals in the steps and sync are the 'scope graticules) and the three main patterns. Although the pulse and bar looks fine, careful observation of the checkerboard and grey scale indicate minor problems. It should be noted that the line frequency control setting is quite critical, with some small but annoying signs of drift.

     Project 2

      The second project is to enable the 32 line picture to be displayed on a standard 625 line TV monitor. Two versions are being tried and tested, the MK1 which has a lower resolution and the superior MK11 developed two years later with a higher resolution and slightly larger picture. Both display the picture in the middle of a 625 line monitor screen with a black surround. Both Mk1 and MK11 PCBs (undrilled) and PIC are available from the NBTVA. A flat screen monitor, as ever, gives the best result, although both converters suffer from quite severe flicker. The flicker makes photographing off screen difficult due to the camera shutter speed.

                                                 

      The above photos show the MK1 board on the left with an off screen shot of the checkerboard from the triple waveform generator, with the MK11 board on the right with a similar image. There is some unwanted screen reflection corrupting both checkerboard images. The improved definition of the MK11 is visible in the block sizes. Careful observation shows how the picture is generated by four combined lots of 8 vertical lines. Pins from each board connect to external components such as input/output sockets, 'on' LED and pots for input level and fine frequency adjustment. The stability is excellent, the picture remaining locked for hours on end.

     Project 3

      Next is the opposite way round to project 2, i.e. to convert a 625 line video signal, from a camera for instance, to 32 lines.

     TX/RX Experiments

      Although it was stated in the introduction that an audio bandwidth is required for the 32 line picture, it is obvious that many amateur TX/RXs have restricted audio bandwidth for speech, presenting some challenges and making homebrew equipment perhaps a better option. There is no reason, however, why old amateur analogue AM transmitters for 70cm 625 line TV cannot be pressed into 32 line service. Initial experiments with 'normal' transceivers have proved quite disappointing, probably for the reason just explained.

     SSTV

      Slow Scan Television or SSTV involves the transmission of static pictures. Several stations in Kent, Essex and Suffolk are readily received on 2m at G4AYT, and well beyond when conditions allow. Analogue or digital transmission using programs such as MMSSTV or EasyPal respectively are in common use. G4AYT uses both and most stations are set up for both, including MB7TV replay station near Frinton-on-Sea. The main channel is 144.500MHz with vertical polarisation and a relay at G4AYT is available when required between 144.500MHz><432.750MHz either way. Standard AM, FM or SSB equipment is generally suited to the mode, as are the HF/LF bands, not just VHF. 40m (7.058MHz) is quite popular with local stations for digital but groundwave propagation at this frequency is not good, although it is well suited for much of Europe.

                   

                     Vertical 8 Element 2m J-Beam used for SSTV at G4AYT                                    Analogue Reception by PD3F East of Amsterdam (226 miles)

      Replay stations, such as MB7TV as already mentioned, operate on a single channel by receiving the picture and on completion, re-transmitting it. Analogue replay stations indicate they are ready by sending a 'K' on receipt of a brief 1750Hz tone from the station wishing to use the facility. Digital replay is similar, but no tone is required, the transmitting station sends a request for replay from within the program. Additional features are available with digital EasyPal, such as the ability to send a text message in the waterfall, or to 'interrogate' any replay stations on channel.

      The off screen photo on the right above, taken by Frank PD3F, shows reception of G4AYT at his QTH east of Amsterdam using MMSSTV during lift conditions. Frank was using an X300 vertical GP to RX and reception was direct, not via a replay. At over 200 miles and considering the QRM that resulted from the lift conditions, this is a remarkably clean picture. G4AYT was using the 2m yagi photographed above and 75W FM on 144.500MHz.