monitors for use with old computers.

From: A.R. Duell <>
Date: Fri May 30 08:57:46 1997

> I'm fairly new to the collecting scene and I'm looking for a monitor I
> can use with some of my systems. I'm mainly interested in the 8bit home
> computers, spectrum, c64, atari, dragon, bbc etc
> I'd like to know if there is a particular type of monitor that can be
> used on the above machines. I'd like to buy, say one, I can use with all
> of the above.

Right. The most important thing to match up when selecting a monitor is
the horizontal scan rate - since (in 99% of cases) the EHT (and other
voltages) are produced by the horizontal output stage, changing this scan
rate is almost always non-trivial. The vertical scan rate is much easier
to fiddle with, since nothing else is produced by the vertical deflection

Fortunately, almost all home computers (and all the ones you mentioned)
use TV-like scan rates. In the UK, they're 15625Hz horizontal, 50Hz
vertical, while in the States they're 15750Hz horizontal (I think), 60Hz
vertical. Those 2 horizontal scan rates are close enough that most
monitors for one can be tweaked to work at the other.

Now, as to the video signal itself. It will typically be one of a small
number of possibilities :

1) Digital RGB colour (BBC, for example). This has 1 TTL level signal for
each of R,G,B, and hence you're limited to 8 colours. The Syncs are
separate TTL level signals, either separate (HSync and VSync) or combined
(Composite Sync). Amstrad used a varient of this system where each of the
R,G,B signals could be in one of 3 states (high, low, floating) giving 27

2) Analogue RGB colour. This has separate analogue signals for each of
R,G,B, giving essentially unlimited colours, but is otherwise as in (1).
Sometimes the Sync signals are combined with the green video signal

3) Monochrome with separate syncs (very rare!). This system has a single
signal (either analogue or digital) that carries the intensity
information, together with 1 or 2 sync signals as above. I can't think of
a home computer that used this system

4) Composite monochrome. This combines the syncs and intensity information
into a single signal that is almost identical to a TV video signal (i.e.
what you'd find in a TV after the video detector, or what you'd get from
the video output socket on a VCR). These signals are almost always
compatable with the TV standards of the appropriate country.

5) Composite PAL or NTSC. PAL and NTSC are the systems used in the UK or
the States for colour television. These signals are in general compatable
with the TV standards of those countries, and combine all colour,
intensity and sync information into a single signal.

6) 'Y&C' (Commodore, in general). The colour TV signal consists of 2 parts
- the 'Y' (or luminance) signal which caries the intenstity information
and the 'C' (or chrominance) signal which caries the colour information.
Some CBM machines have separate outputs for the 2 signals, which gives
slightly better quality than combining them and them separating them again
in the monitor.

7) VHF/UHF TV signals. These are essentially very low power TV
transimissions, on Channel 3/4 (VHF) in the States, and Channel 36 (UHF)
in the UK. You link them to the aerial input of at TV set, which then
converts them to Composite PAL/NTSC.

Now, as to what converts to what easily :

Composite PAL/NTSC and Composite monochrome are compatible in both
directions with no problems. The TV standards were designed that way.

VHF/YHF TV can be converted to composite video using a TV tuner/IF strip,
either as part of a TV, or extracted from a dead VCR.

There are single-chip solutions to convert composite PAL or NTSC (and even
some chips that do both) to analogue RGB. Be warned that _building_ a
circuit using one of these chips is often hard, since there are a number
of critical adjustments to get right. There are also single chips that go
the other way

Digital RGB outputs will often drive analogue RGB inputs, sometimes a
resistor network is needed. The other way needs a few high-speed
comparator chips (although _sometimes_ a direct connection works!)

There are chips that will separate the syncs from a composite signal, and
produce video + syncs. Combining syncs and video to make a composite
signal is generally a simple circuit as well.

> I know there seems to have been a few different methods used in
> producing the video signals and, from reading newsgroups, I get the
> impression that it is sometimes possible to select video outputs/monitor
> inputs such that, even if the monitor is not directly compatible, a
> reasonable result can be obtained.

Well, there have been monitors (Philips, Barco, a few others) that will
accept :

Composite PAL (and hence mono). A few do NTSC as well
Digital RGB
Analogue RGB

Take one of those, add a TV tuner, and you can handle just about all the

Some _good_ portable TV's have a SCART socket with RGB inputs. The
bandwidth there is often high enough to enable the set to be used as a
monitor for (at least) 80 column text. A SCART socket will also have a
composite PAL input on it. And of course a TV will accept UHF or VHF
signals as appropriate.

Apart from special monitors, I make do with 3 things :

1) A portable (9") NEC monochrome monitor. Composite inputs
2) A digital RGB monitor (again NEC, although I'd prefer a Microvitec)
3) An analogue RGB monitor (Barco, big, heave, expensive!)

> Any pointers would be greatly appreciated.
> --
> Pete Robinson
> - faqs, emulators, links, web utilities.

The gates in my computer are AND,OR and NOT, not Bill
Received on Fri May 30 1997 - 08:57:46 BST

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