The transmitter consists of a Worthing group TX + P.A. Block (M57762 Brick) feeding an Alford slot antenna through an inter digital filter. The receiver is a Wood and Douglas 24cms down converter + VDIF fed via a second Alford slot and inter digital filter the antennas are separated in height by about 6 feet RX being the heighest.
The logic control of GV consists of a conventional video sync detector circuit (NE556 IC) which is used to control a video / audio switching IC. Selecting the off air signal or beacon mode images. It is also used to flag the status of the repeater's receiver to a 386 PC that simultaneously generates pictures / captions, provides Morse ident, continuous tone and logic control. A video converter card installed in the PC provides all Pictures and captions seen on GV.
The PC controls many other features of GV including the access caption and Morse "K". When engineering work is taking place on site it is also used to generate announcements etc.
How the PC is used to control GB3GV
When power is first applied to the PC a self-executing programme is started. The picture magazine seen on GV's output is loaded from the hard disk to a RAM disk. (Changes to the picture magazine normally taking place on the first day of each month, or when there is a special event, rallies Christmas etc). Using this method pictures are stored on the hard disk months in advance of them being required. Once every twenty four hours during the early morning the PC is auto reset ( rebooted ). The control programme is reinstalled, the PC's internal date is then checked and the appropriate picture magazine is loaded. Having a reboot each day has the advantage that should the PC control programme crash or the PC locks up then a restart and reload of the control programme will rectify this. Since the introduction of a PC at the repeater site, and with the severe fluctuations in the mains supply that have been noted, I do not think there has been a single case of a programme crash, just reboots due to total loss of power.
When all the pictures have been loaded to RAM disk then the hard disk is stopped. It will not be required again until the next reboot running the hard disk continuously is not necessary as it's only required at boot-up.
During loading the Video output is blanked to stop the loading process being seen on GB3GV's output. When loading is complete and GV is running in beacon mode, each picture is displayed for approximately 1 minute, the current magazine consisting of approximately 16 pictures. This number is limited only by the size of the RAM disk currently set at about 7Mhz.
All pictures and captions seen on GV are obtained from standard image file types. They are obtained from photos through a picture scanner or by a video frame grabber, if not already in a 640 x 480 resolution then they are converted to this standard. A higher resolution is not required for high quality 625-line colour television.
The pictures can then be edited and GB3GV's call sign is inserted and any other text added as necessary, each image is then turned into self-executing file.
With the above method it is possible to have new pictures installed on GV on a regular basis. Even a change of magazine could be initiated on a day to day basis, but for me to visit the repeater site requires a round trip of about 20 miles, consequently visits to site are limited. The time taken to collect suitable pictures, edit them, prepare for the repeater and to visit the site once a month is a lot to do. Updating of the picture magazine could be done over the air but suitable safeguards would be required to stop unauthorised access etc.
The above method of controlling GV has proved very reliable.
I have many ideas for future addition's ie a live site camera / video recorder a 10 GHz link or even video frame capture through the PC. All this should be possible, but it takes time and money. As stated above, with so few users it is already taking proportionately more of the groups' resources to run then any of the other devices on site. Many ATV stations in surrounding counties have commented on the lack of activity.
So please anyone who is active with ATV and can access GV, members of the group or not. Please use it or lose it!
73 de G8OBP
From David