80-Bus News


July–August 1983, Volume 2, Issue 4

Page 7 of 55

shadow mask of the tube and tube size. The bigger the tube, the more lines it should resolve. Some manufacturers quote the vertical resolution at the centre of the tube, meaning that the total resolution across the face of the tube would be this figure if the tube was good enough. It is reasonable to expect some roll off towards the edges, but not too drastic roll off. Checking the deflection angle can be of some help here. 90 degree tubes do not roll off so much at the edges as 110 degree tubes. Most monitors are fitted with 90 degree tubes for this reason.

Don’t fall for the old trick of looking for a high input bandwidth, all this means is that the electronics are up to it, but says nothing about the tube characteristics. 24MHz bandwidth might imply that the monitor is capable of 800 line resolution, but what is to stop the manufacturer fitting a low res. tube, whereas, a quoted resolution of 800 lines means that both the electronics and the tube must be capable of the job.

A low res. monitor would typically have a 12″ or 14″ tube and resolve 400 lines at the centre of the display (less at the edges, although this shouldn’t be too much less, perhaps 350 lines). This resolution is about the same as a 14″ colour TV (and is probably where the tube came from in the first place) and is adequate for the Climax and just about for the Nascom card. These are typically in the £200 to £300 range.

The next are the medium res. monitors with about 600 lines resolution for a 12″ or 14″ tube. Entirely suitable for the Nascom card in the normal eight colour mode, and the Pluto at a push. Typical prices are in the £300 to £400 range. The higher price is dictated by the purpose made tube with its finer shadow mask. It is highly probable that the electronics are identical to the equivalent low res. version. This is certainly true between the KAGA Vision – I and Vision – II where the difference in price is solely down to the tube.

Lastly are the real high res. monitors with 800 line or better resolution. These are very costly, between £450 and £2000 depending upon the size of the tube, in fact the major part of the cost is the purpose made tube, so tube guarantees and insurance are well worth exploring.

Monitors will most probably have one of two types of input, logic, where the input is either a colour at full saturation or off (black), or analogue where the intensity of the colour is related to the level of the input voltage. Some monitors are switchable in this respect. Logic inputs are suitable for the Nascom card and the Pluto card (if expansion to the Pluto Palette is not envisaged). The Climax requires an analogue input as does the Pluto Palette.

Various monitors and an old Philips TV were used with the colour cards. Those used were the:

KAGA Vision – I12″ tube400 line resolutionanalogue input
KAGA Vision – III12″ tube800 line resolutionanalogue input
Microvitec Cub14″ tube400 line resolutionlogic input
Luxor14″ tube800 line resolutionlogic input
Luxor14″ tube800 line resolutionlogic/​analogue
Philips portable TV18″ tube600? line resolutionRF input

All were found to satisfactory within their specifications (except the TV, which was awful). The analogue input on the Luxor was found to be quite linear and could double as a TV repeater monitor. The KAGA inputs were not quoted as being analogue, but were found to be reasonably linear although not as good as the Luxor.

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