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Satellite Workshop

"Scarting out Right"

This is twelfth in a series of articles which will help you to save money on repairs and installation.

To some installers, a Scart lead is a means of earning an extra tenner on each installation; to others it's a nightmare. Scart leads come in various lengths and types but the main two subdivisions are "fully wired" and "part wired."

The scart system was developed some years ago before satellite receivers and decoders were considered. For this reason, just one connection (pin 8) is used for signalling purposes. The intention was that a video recorder (for example) would put a "here I am" voltage on pin 8 when it was switched to "play." The TV would respond by showing the video signal (from the Scart lead) instead of the terrestrial picture from the aerial input. The main advantages of the Scart connection are that the picture is "cleaner", because it doesn't go through a modulator then a demodulator, and stereo audio is possible.

The Scart system gives a problem if, for example, the Scart lead is used to connect a D2Mac decoder to the TV. Now each time the D2Mac decoder recognises a D2Mac signal, it puts a voltage on pin 8, which makes the TV switch the decoder picture in, even if you don't want it to! Since it is possible for some decoders to recognise a D2Mac signal even in standby, the result can be a blank screen on the TV. The only answer is to disconnect the wire from Scart pin 8 and use the TV "AV" button to select the decoder picture only when it's wanted.

For connecting anything to a TV, I would *always* recommend using a "partly wired" scart lead. This won't eliminate the "pin 8" problem but it will avoid several other problems. For example, Some satellite Amstrad receivers use pins 10 and 12 for a data connection which is used *only* for factory programming at the time of manufacture. These pins are connected directly to the internal microcontroller. Putting a voltage onto them can either confuse the micro or permanently damage it. Since some TVs and Videos do have a connection to pins 10 and 12 you should always ensure that they are disconnected in the Scart lead.

Some Pace receivers also use connections in the Decoder Scart socket for the same purpose. So any Scart lead used to connect a decoder should also have these wires cut, unless you are specifically using the Scart connection to download channel information from one receiver to another.

In the case of some (later model) Churchill D2Mac decoders, pins 10 and 12 are used to tell the decoder whether to output "PAL" or "MAC" deemphasised baseband signal. In this case, the wires must be cut inside the Scart plug and either shorted together or insulated (in accordance with the instructions on the label).

For Amstrad models from SRD510 onwards, pin 12 of the decoder Scart carries MAC deemphasised baseband which most D2Mac decoders prefer. You will have to pull out pin 12 in the Scart plug and insulate or discard it. Pull out pin 19 (video out) and push it into position 12. The pins are held by tiny spring tags which you must flatten with a sharp point. Mark the modified Scart plug "Receiver end."

Another problem which arises is that there can be signals or interference emanating from the TV or VCR on the Scart wires. Pin 19 (at the TV end) is a likely candidate and should always be cut, otherwise picture interference may occur.

Picture distortion may occur if the satellite receiver or decoder is incompatible with the TV or VCR. Occasionally the video level from the Scart connection is too high for the TV or VCR to accept. The answer is to modify a Scart lead in advance and to keep it in the van. The last thing you want is to have to solder in the customer's living room! You will need two 1/4 watt resistor of value 27 Ohms and 47 Ohms. Disconnect the wire from Scart pin 20 (video in) and solder the 47 Ohm resistor from pin 20 to pin 17 (video ground). Solder the 27 Ohm resistor from pin 20 to the end of the wire which you cut. The resistors will reduce the video signal. Mark the modified end "TV". You can now use this modified Scart lead for testing purposes and, if it cures a problem, leave it connected and make another to put in the van later.

Some D2Mac decoders go into standby automatically if no video is detected. If the receiver is switched off, the decoder goes into standby. However, some receivers feed a video signal to the decoder socket even in standby. In this case, the decoder will not go into standby unless you physically disconnect the Scart lead to remove the video signal.

General rules:
You should use only a part-wired Scart lead for connecting a decoder to a satellite receiver. Pins 19 (video out) and 20 (video in); pins 1 and 3 (L & R audio out) and 2 & 6 (L & R audio in); pin 8 (the "here I am" signal connection which may need to be removed) and the video and audio ground connections.

For connecting to the TV, the wire to pin 19 (at the TV end - label the plug!) should be removed to avoid possible interference.

An exception to the general rule is that a D2Mac decoder is usually capable of providing Red/Green/Blue signals which can be fed to a suitable monitor or TV to give (in theory) better picture quality. However, a side effect is that satellite on-screen menu graphics might roll. The other exception, as mentioned, is for downloading channel information between receivers.

You can pay more for a gold-plated Scart lead but what is the advantage? Well, unless the plug pins *and* the socket pins are gold plated, there is unlikely to be any advantage at all.

Some equipment requires the use of "phono" ("Cinch") plugs instead of a Scart plug. In this case there is no point in using a fully wired lead since only video and audio connections are used. Some receivers require the use of both Scart and phono plug at one end. In the case of the Nokia SAT1800, the wire to pin 20 is cut and extended to a phono plug at the receiver end.