48K Spectrum and VGA monitors...

A place to discuss the ZX80's and ZX81's younger brother - the Sinclair ZX Spectrum
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quigbert
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48K Spectrum and VGA monitors...

Post by quigbert » Sun Jan 18, 2015 1:47 pm

Hello everyone. My name is Steve, I live in London and this is my first post on the SZXW site. :)
Anyone on facebook might like to be drawn to this page as well if they aren't already... https://www.facebook.com/profile.php?id ... 50&fref=ts
I recently won a 48K Spectrum on ebay (with a LO Profile keyboard :) ) Its nice to have a spectrum again after all these years (I'm 39 by the way..)
I would like to know if it is possible to use a VGA monitor with the 48 in any way?
Has anyone had any success with this?
Does the GBS8200 work?
Any help appreciated, Thanks.
Steve (quigbert)

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stefano
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Re: 48K Spectrum and VGA monitors...

Post by stefano » Fri Jan 23, 2015 2:47 pm

As far as I know it is not a starightforward thing.
Getting standard RGB (SCART) signal is tricky already; to get a valid VGA output you also need to adapt the scan frequency and generate few more signals, they seem to be doing it here:

http://velesoft.speccy.cz/zx/zx-vga/

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RetroTechie
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Re: 48K Spectrum and VGA monitors...

Post by RetroTechie » Fri Jan 23, 2015 7:10 pm

Well you'd need:

* Some circuit to take composite sync (in the Y signal?) from the ULA, and pull separate horizontal + vertical syncs from it. Such circuits exist (LM1881 for example), but how to wire those into a Spectrum 48K...??
* Some circuit to convert color signals from the ULA (= some flavor of YUV) into RGB.
* VGA monitor that can handle ~15 kHz horizontal sync. Such monitors exist, but which ones? I've never seen a good list.
* Wiring up all of the above.

In other words: lots of work including some circuit design, with uncertain outcome. Instead, I'd choose between 1 of 2 routes:

1) Find a Spectrum 128K or later model, and use with a monitor/TV that has a SCART input. Or:
2) Use any monitor/TV that includes a composite video input (apparently even some VGA monitors have this!). Such a monitor would surely have to support the horizontal sync frequencies that are common for devices with composite video output.

Hint: in case you lack a 128K or later model: these tend to be expensive, and chances are you can buy a looooottt of monitor for the same money. ;)
Bitcycle.org - just flipping bits

gozzo
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Re: 48K Spectrum and VGA monitors...

Post by gozzo » Fri Jan 23, 2015 8:44 pm

there is supposed to be a 'composite video' connection on the expansion slot of a standard 48k machine.....as for VGA monitors handling 15.625khz, most likely any suitable ones would be LCD types as the line output stages in CRT types are 'tuned' to run at a specific frequency (or narrow range), unless complicated circuitry is used.. trying to persuade a normal VGA one (31khz - ish) to run at 15khz or so would probably result in line output transistor failure quite quickly..converting YUV to RGB(which could then feed a 15khz capable monitor?!) ... there are fairly simple circuits around on the net that do this...can't remember where though at the moment!

gozzo
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Re: 48K Spectrum and VGA monitors...

Post by gozzo » Fri Jan 23, 2015 8:47 pm

and there ARE composite to VGA converters on ebay at various prices, some dirt cheap, bought direct from china...

Kiwisek
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Re: 48K Spectrum and VGA monitors...

Post by Kiwisek » Sat Jan 24, 2015 11:31 pm

Use this http://www.8bit.8u.cz/Files/Faudraj/ or Velesoft's slightly adopted version of this design. It works great.

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-B-
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Re: 48K Spectrum and VGA monitors...

Post by -B- » Wed Mar 30, 2016 10:43 pm

I have done the composite mod on mine then put it through a SCART to HDMI converter (£20 eBay) then through a HDMI to VGA converter (£5 eBay)

The composite mod consists of removing the 5v power to the RF module, desoldering the resistor connected to the back of the RCA connector and feeding the input to the RF modulator straight to the centre pin of the RCA connector.
image.jpeg
Handles composite, RGB and Y/C inputs.
image.jpeg (1.33 MiB) Viewed 4141 times
image.jpeg
No lag detected
image.jpeg (2.24 MiB) Viewed 4141 times
-B-

pniklas
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Re: 48K Spectrum and VGA monitors...

Post by pniklas » Sun Jan 01, 2017 9:54 pm

Hi,

Could you tell me in which pin of the Scart have you connected the composite video?

Thanks.

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-B-
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Re: 48K Spectrum and VGA monitors...

Post by -B- » Mon Jan 02, 2017 12:46 am

I used an A/V to SCART connector in the photo above but pin 20 is composite in and pin 17 is ground.

https://en.m.wikipedia.org/wiki/SCART

-B-

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