Memotech Video & Graphics Output

Introduction

Before we get into the specifics of Memotech video, it might help to make a few comments about the various video standards that you'll have seen. I won't be able to go into too much detail here, you'll find a wealth of information on the internet if you need to know more. Rather, I'll just give a brief overview of the most common connectors that you'll see on your Audio-Visual (AV) equipment, and how they relate to the Memotech.

Type	Connector		Notes
Composite Video   PAL (720 x 576i) NTSC (720 x 480i)	RCA		All of the required video information, chrominance, luminance and synchronisation signals, are encoded on a single channel.
Composite Video With Audio	RCA x 3		Your TV will probably have three inputs for AV. Composite Video - Yellow Stereo Audio Right - Red Stereo Audio Left - White
S-Video	Mini DIN		The luminance (Y; grey-scale) signal and chrominance (C; colour) information are carried on separate, synchronised signal and ground pairs.
Component Video	RCA x 3		The signals are referred to as YPbPr Y carries luminance (luma) and sync Pb carries the difference between blue and luma (B − Y). Pr carries the difference between red and luma (R − Y).
This image from the PCMag Online Encyclopedia shows how brightness/luma (Y) and colours/chroma (U and V) are transmitted by the different techniques. In Europe, the most common connector these days, at least until HDMI takes over, is the SCART connector.
SCART	21-pin EuroSCART		Signals carried include composite and RGB (with composite synch) video, stereo audio input/output and digital signalling. The standard was later extended  to support S-Video.
If your TV does not have a Composite Video input free, you may be able to use a SCART converter such as those shown here.			Note: not all SCART sockets are created equal, not all inputs may be present on a given TV socket, you will need to consult the manual for your TV.

PAL / NTSC

The table below compares the main parameters of the PAL (UK) and NTSC (US) broadcast standards in widespread use before the advent of Digital TV. The SECAM-L system was used in France.

	PAL	NTSC	SECAM
TV Lines	625	525	625
Visible lines	576	483	576
Video mode	576i	480i	576i
Fields /second	50	60	50
Frames / second	25	30	25
Aspect ratio	4:3	4:3	4:3
Sub-carrier clock frequency (Mhz)	4.43361875	3.579545	4.41 & 4.25 MHz

Video Synchronisation & Scan Frequencies

Again, this will be a gross simplification of the process, but it should hopefully be enough to explain the basics.

The basis of this information is analogue television, but for our purposes, the principles apply equally well to computer image generation. The "picture" on a screen is typically generated by a process called raster scanning, where the image is split into horizontal strips called scan lines. One line of picture elements (pixels) is drawn from left to right in turn, the display is built up as subsequent lines are drawn from the top to the bottom of the screen.

The horizontal synchronisation pulse (HSYNC) separates the scan lines. The horizontal sync signal is a single short pulse that indicates the start of the line and the rest of the is drawn after it.

The vertical synchronisation pulse (VSYNC) separates the video fields (frames). The frequency of the vertical sync can therefore be much lower than that of the horizontal sync. Usually, the frame rate is matched to the power line frequency (50Hz or 60Hz) which prevents the display from "weaving".

Probably the most common sync systems are separate sync and composite sync. As its name suggests, separate sync uses separate wires for horizontal and vertical synchronization. When used in RGB connections, five separate signals are sent (Red, Green, Blue, H-Sync, V-Sync). Composite sync combines the horizontal and vertical synchronization signals onto one pair of wires. When used in RGB connections, four separate signals are sent (Red, Green, Blue, Sync).

The maximum rate that a monitor can refresh the screen is measured in Hertz (cycles/second) and is called the vertical refresh rate (or vertical scan rate). The horizontal scan rate is the number of times that the monitor can move the electron beam horizontally across the screen, then back to the beginning of the next scan line in one second.

Most early monitors were fixed frequency e.g, the IBM CGA 5153 monitor had a horizontal sync rate of 15.85 kHz and a vertical refresh rate of 60.5 Hz. Later monitors were capable of synchronising over a number of frequencies, such monitors were pioneered by NEC who have a trademark on the word MultiSync. As display resolutions increased, scanning frequencies also increased, leading to the term "double scan" where the horizontal scan rate is ~31kHz. Most PC monitors today have a minimum horizontal scan rate of ~30Khz making them incompatible with CGA or broadcast TV (PAL/NTSC) type outputs.

 

Video Output from the MTX Video Board

The Video output from the MTX computer was generated by either a Texas Instruments TMS 9929A (UK/PAL) or TMS 9918 (US/NTSC) Video Display Processor on the system board through pins 35, 36 and 38. The relevant pin-outs for the different processors are shown below :-

It can be seen that a TMS9918A could have driven a composite video (NTSC) monitor directly from pin 36 without the need for an additional video board. Video output from the TMS9918A is at 60Hz.

The TMS9929A provided a direct component video output (YPbPr), at 50Hz. This was desirable for machines being sold into Europe where both PAL and SECAM standards were used. (See here for a detailed discussed of PAL and a comparison with SECAM.) It would have been cheaper to have a single video board and make region based adjustments with the TV modulator.

[The Reflex Controller in my Video Wall equipment uses a MTX 4000-05 Computer board with a TMS9929A but with no video board installed. The "Y" signal is taken directly off J3 (which usually connected to the Video Board) and used to drive a mono monitor output using the Component video Luminance (grey scale) and Composite sync signals.

This is a potentially useful piece of information, should you have problems with the TV modulator or the Video board itself, it would be possible to connect a monitor as described and get a useable, although Black and White display, not very good for playing games, but at least rendering the MTX useable. Using all three component outputs, a colour picture could be directly generated on a compatible TV.] 

The the circuit diagrams in the manuals that I have show the PAL and NTSC video boards both driven by a TMS9929A. Not having seen a US model of the MTX, or a schematic for the US version, I can't verify this and it does not sound right. In any event, the video board would still have been required to generate the TV output for the US. 

I will just describe the output from the TMS9929A here. If there is a demand for more detail on the output from the TMS9918A, let me know and I will add them.

Output from the TMS9929A VDP is in the form of "R-Y", "B-Y" and "Y" signals. The "Y" output signal contains the horizontal and vertical synch components as well as luminance. The "B-Y" and "R-Y" colour difference signals contain the unmodulated chrominance information. These signals require external encoder circuitry to drive a video monitor or TV.

Output from the VDP is fed to the MTX video board via J13 pins 3 (B-Y), 4 (R-Y), & 5 (Y) on the video board. This is conditioned by the LM1889N TV Video Modulator installed on the video board to produce the monitor output. Identification of the oscillator frequency on the video board is a quick way to confirm which video board is installed - if you didn't know already!

Sound output from the SN76489AN sound generator is fed to J13 pin 1 on the video board. The sound output is not modified by the video board, but the signal is split to feed the modulator and the Hi-Fi connection on the rear of the case.

J11 on the video board takes the video and sound signals to the RF modulator where they are combined and output from the TV phono socket on the rear of the MTX. For the UK, the modulator was tuned to TV Channel 36 (UHF).

J12 on the video board takes the BNC Monitor and Phono Hi-Fi outputs to the connectors on the rear of the case. Many TVs have AV inputs for Composite Video and sound, these are compatible with the MTX outputs. You may have an old Hi-Fi phono cable with 3 RCA connectors like the one pictured at the top of the page from one of your pieces of AV kit, this is the easiest way to make a connection to your TV / composite video monitor.

For the monitor out, you will need a BNC to RCA convertor like this one. These should be available at your local electronics store or are readily available on eBay.
The MTX outputs audio in mono, if you want to use the Left and Right audio inputs of your TV/Monitor, you will need to split the signal, using something like this :
Or this, again, these items are readily available at low cost.

The circuit diagram from the PAL video board is here.

The French Connection

The TMS9918/29 manual shows the raw output from the VDP being conditioned by an external RGB encoder to drive an RGB monitor as an alternative to the video encoder to drive a composite video monitor.

Until 2016, I thought that the video output from all Memotech MTX computers was limited to composite video, generated by the LM1889 encoder on the video board. However, Gilles Bronchain has sent me details of what appears to have been an add-on board developed by the French distributor that generated RGB SCART from the YUV signals off the VDP. (Photos here)

It would be interesting to try and reverse engineer the board and see whether we could produce a cheap RGB adapter for the MTX, but it appears that an "off-the-shelf"It would be interesting to try and reverse engineer the board and see whether we could produce a cheap RGB adapter for the MTX, but it appears that an "off-the-shelf" converter might be a good option. JS Technology in the UK sells a range of video converters, including YUV to SCART, and their website is a useful source of information on video conversion.

Further details to follow  . . . . . .

Video Output from the 80 Column Video Board (FDX/SDX)

The connections to the 80 Column Board installed in the FDX are shown below :-

The 80 column Board provides two video outputs, monochrome and colour. The monochrome output on the FDX is output through the RCA/Phono socket and is Composite Video. It is therefore compatible with the monitor used by the MTX video output, though obviously not in colour. Additional details on the 6845P and it's use in the FDX can be found in this technical note.

In contrast (no pun intended) to the MTX Video and TV analogue outputs, the colour output from the 80 Column Board is digital, using TTL (Transistor-Transistor Logic) levels to drive an RGB monitor via the 9-pin "D" type connector. This is similar to the IBM CGA adapter, which used the same CRT controller chip - a Mototola 6845P

An aside - the Light Pen Interface

I used to think that the "Sync" signal on pin-7 was a composite sync signal for an RGB monitor, (for RGBS), as well as the separate horizontal and vertical sync signals (for RGBHV), however, whilst that may be true, I suspect that it was a combined sync signal for the Light Pen. I have not seen any information from Memotech on the Light Pen input, but having read up a little on the design of 1980s light pen technology, I am pretty confident that this is the case.

Basically, a light pen consists of a photo detector at the tip which detects the light emitting from a CRT screen, the detector produces an output current proportional to the light received. As well as receiving video and button signals, the light pen interface also used a sync signal to determine the position of the light pen. I'm not very sure of the detail, but if we know the time that, say, frame scanning started at the top left corner of the screen, using perhaps the vertical blanking signal, then length of time before the light pen "saw" the scanning beam could be used to calculate the position of the light pen on the screen.

 

"What's the Frequency, Kenneth?"

- (or why won't a VGA conversion cable work with my MTX?)

Suitable colour monitors for the 80 Column colour output would have been RGB TTL monitors such as the IBM 5153 or a Microvitec Cub as shown in the Memotech adverts.

So, what about today? You'd like to use the video output from your MTX or 80 Column Board with your VGA monitor and you've seen one of those cheap cable kits that claim to convert AV to VGA etc.? Will it work?

Well, it might, but it is unlikely, if it does, it will only do so with certain monitors.

When first introduced, the most common VGA mode was 640x480 pixels at a scanning frequency of 31.5kHz with 16 displayable colours out of a colour palette of 64. As described above, some VGA monitors, mainly older models, had the ability to take input from a wide range of frequencies, most more modern ones, have horizontal refresh rates of 30 kHz or higher, which is double that of composite video (15.7 kHz) and will be unable to synch at the lower frequency. VGA also uses analogue RGB, the voltage varies between about 0 and 0.7VDC.

Conversion of the 80 column output to VGA is therefore somewhat problematic, you would need to convert the digital colour information into analogue and you would need a scan converter (upscaler) to change the frequency. This requires additional active electronics, rather than just a cable with passive components.

Whilst it may be possible to build an interface such this yourself, it is probably better value, and certainly much quicker, to get something like this from eBay :-

Look for "CGA/EGA/YUV/RGB TO 1 VGA for Arcade Game Monitor to CRT LCD PDP Projector ,in UK". As of October 2012, these were available on eBay UK for £18.95 + £5.00 P&P.

As my FDX is not working, I can't try this myself, so can't vouch for the quality of this item, but I am pretty confident that it would allow you to connect the output from the 80 column card to a VGA monitor - with a suitable cable of course.

Update : I was a bit concerned about the way that the advert was worded and worried that this board would only support analogue RGB. I had a few e-mails back and forth with the vendor that did not make things any clearer and did a bit of "Googling" to find an answer. There is some good information on the use of this board on Atari and Acorn forums, Mark (who provided the information on the MTX power supply) has had good results using this board with Acorn Electron and BBC micros (which also use TTL RGB).

There is also a later revision of the board currently being advertised on eBay, the HD9800 Version 5.

As you can see, these boards look identical and there does not seem to be any functional difference between them. Mark has had good results with both so I am now very confident that these will work with the Memotech 80 column board output and have now bought one, but have not been able to test it yet.

Update: Inaki (Luis on the MTX-500 Facebook group) has used the HD9800 Version 5 board with his FDX with good results.

I have now been able to test my HD9800 with my FDX - my experience with the converter are on this page.

 

80 Column Board -  Output to TV?

The first thing to say is that this is likely to give very poor results, I have not tried it, but I think that 80 column text on a TV would probably be barely legible. However, if you have an 80 column board and no monitor, it might be worth a try. The 56 column version of NewWord (N56.COM) may just be OK.

The BBC Micro also used TTL level RGB and was usually used with a Microvitec Cub monitor, I found an article on the mdfs.net website which described how to connect a BBC Micro to a TV through a SCART socket. I was  not aware of this site, but as well as information on the BBC, it hosts a range of information including Z80, CP/M, etc. and is well worth a look.

 

Here is a schematic of the design shown on the mdfs.net page. If you plan on trying this, please read the full article on the mdfs webpage. You should check whether your SCART socket accepts RGB inputs before you start down this route. If you have more than 1, then one is likely to be RGB compatible.

As you can see, the modification is very simple and only requires the addition of a few resistors. No upscaling is going on here, the horizontal scanning frequency of the TV is compatible with that of the original RGB monitor. The resistors are converting the 0-5V TTL signals into 0-1V required by the SCART RGB inputs.

I have not tried this myself, and as ever, you'll be doing this at your own risk!

 

80 Column Board Graphics Capabilities

The Memotech 80 Column display adapter contains two character generator PROMs, one for alpha characters and one for bit-mapped graphics characters. That is, all characters that could be displayed on the screen were predefined in the PROMs. Each character was made up of 8x8 pixels, giving a total pixel count of 640 (80x8) by 192 (24x8) for the 1920 character positions. Control codes allowed individual points to be plotted in coordinates ranges of 160x96 "dot" positions.

Want to see a 1984 monitor review?

After I'd finished this page, in amongst my Memotech stuff, I found an article ripped from the December 1984 edition of "Your Computer" - a "Buyer's Guide" to monitors available at the time. The MTX does not rate a mention, but the article discusses the advantages of monitors over TVs, which we all know, but also contains brief specifications of a number of period monitors which you may find useful if you ever try to pick up an old one from the likes of eBay.

I have scanned the article in colour which makes it about 12 Meg, but you may find it useful, or at least, a reminder of the technology of the time !

And Finally . . .

As I mentioned above, the information on this page provides a very simplistic view of how the MTX video signals are generated and how they can be used to drive various displays. Although basic, I believe that the information is accurate, however, if you spot any mistakes or require clarification on any of the points, please let me know.