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The Memotech MTX Series


     Multi-Effect Video Wall    


Decoder / Distribution Amplifier


Each DDFS has two IDC connections - one for each Frame Store Controller Board (see below) and two RGBS connections to the Decoder/Distribution Amplifier (DDA), again, one for each Frame Store Controller.

The DDA accepted a PAL or NTSC composite video signal and decoder it to generate RGB video signals for the DDFS.

(See my Video Wall Overview page more information on how the components of the Memotech Video Wall system are assembled.)

The major components in the DDA are :-
My selection of Video Wall hardware has 4 DDA modules, these are unlikely to have come from a single system - a typical Video Wall system would have used a single DDA, taking one or two video inputs for display on the Video Wall. Additional DDAs could be added to facilitate the use of additional video feeds.


Video Decoder / Distribution Amplifier

One of my PAL Video Decoder / Distribution Amplifier (DDA) modules.


This is a Memotech branded unit and would have been produced after the Memotech exclusive distribution agreement with Cameron  had ended.

This DDA has two composite and one RGB video input, an external Decoder Adjust connection and eight RGB video outputs for use by up to four Video Wall DDFS controllers

The rear panel of another of my DDA, this one has two composite and one RGB video input, no Decoder Adjust connection and eight RGB video outputs.


This DDA is a Cameron branded unit.

Another model of DDA, this one has a single composite video input, no RGB input and four RGB video outputs. The inverted "RGB input" connector is actually for a Decoder Adjust dongle.


This DDA is a Cameron branded unit.

Another DDA, with the same I/O configuration as the one above.

This unit is labelled as a Memotech PAL DEC 15

Although they are all slightly different, each of my DDAs includes the same type of PCBs - a PAL Decoder and a Distribution Amplifier.

Some of the units have power supplied from an AC transformer with DC regulation done on the Distribution Amplifier board and others have separate DC power supplies.


Video Decoder Board

The video decoder boards were one of the very few circuit boards in the whole Memotech range that were not designed and built by Memotech at the Witney factory. Although the decoder boards do not have any markings identifying the manufacturer or model number, Geoff (Boyd) has advised that they were "a standard part manufactured by Hantarex - a CRT monitor company out of Italy with strong operations in UK catering to the pub video games machine market".


The Hantarex decoder board is based around a ST TDA3562A "PAL/NTSC One-Chip Decoder".
What follows is my attempt to understand the operation of the decoder board, it is based on my reading of the TDA3562A datasheet and various web searches intended to give me enough information to reconstruct the circuit diagram. It it more than likely to contain errors due to my lack of knowledge in this area - if you spot any, please let me know.
Geoff also recalls that "decoder design follows the TDA3562A data sheet with the addition of a flywheel sync circuit which was very useful for stabilizing sync separation from the composite input signal. Hence although the MTX Video Wall worked with composite sync (CSYNC) we preferred to regenerate that CSYNC from the Hantarex HSYNC & VSYNC which were flywheel stabilized."

[Random pulses due to noise and other interference are sometimes present on a composite video signal. The principle of flywheel synchronisation is similar to that of a mechanical flywheel, which, due to its large momentum, maintains an average speed unaffected by random changes. A flywheel sync circuit maintains an average frequency of the sync pulses by monitoring and taking the average frequency of a number of incoming line pulses so that a random pulse will have little effect on the frequency.1]

The three large variable resistors on the lower left hand corner of the board are variable resistors for adjustment of brightness, contrast and colour.


In the example above, you can see a number of wires attached to the variable resistors, these go to the "Decoder Adjust" connection on the rear panel. This allowed connection of a fine tuning "dongle", enabling colour matching to be done without opening up the case.

As well as a phono plug connection for a composite video input, the decoder board has additional terminals for an RGBS video input.

The same connector has Horizontal (HS) and Vertical (HS) sync outputs, generated on the board from the composite sync input and the flywheel sync circuit described earlier..

The decoder board has a 6 pin connector taking the R, G, B signals to the distribution amplifier board, this connector also has HS and VS output signals - which are not used in the DDAs.

You can see that the variable resistors on this board do not have the wiring for connection of the external decoder adjustment tool.

The RGB outputs from the decoder board are derived directly from the TDA3562A on pins 13, 15 and 17.


An external RGB signal, if used, would be fed to pins 12, 14 and 16 of the TDA3562A.

To be continued . . . . . .





Distribution Amplifier Board



To be continued . . . . . .



The other board in the DDA is a Distribution Amplifier - it takes the RGB signal from the decoder, amplifies it, and, depending on the DDA model, splits the signal into 4 or 8 outputs. Some of my DDAs have two composite inputs connected from the rear panel, with a single composite output to the decoder board, on others, the single composite input connects directly to the decoder board.
This DDA is powered using a mains transformer which supplies low voltage AC to the DA board. At the right hand side of the board you can see the large capacitors and voltage regulators which provide stabilised DC voltages (+12,+5, -5) to the DA and Decoder boards. Other DDAs use a 30W Skynet SNP-3032 to provide the DC voltages and the DA board has the power supply components omitted.
Another version of the DA has the power supply components, but not the composite video input connections or associated electronics.


Power Supply



1. Newnes "Guide to Television & Radio Technology", by K.F. Ibrahim, Elsevier, 2007, ISBN : 978-075068-165-0




(Some of the Video wall information comes courtesy of Geoff Boyd, 2012/2013, from his recollection of the Memotech Video Wall)


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