SuperGraphs for the Sinclair ZX-81 Computer

Help Wanted - ZX-81 Output resists Capture!

October 2006

This page is intended to present the development of SuperGraphs, a charting application written in assembly language for the tiny Sinclair ZX-81 computer of the early 1980s.  The original SuperGraphs Users Guide in Acrobat is available here

To see a larger picture, click any image

Sinclair ZX-81 with 64k MemoPak Sinclair ZX-81 Hardware System Overview SuperScope C-190 Cassette Deck
Q-Save Amplifier/Filter Unit Sinclair ZX-81 AC Power Supply Sinclair ZX-81 TV Adapter

We intended to use a digital video capture card to capture the animated demo for SuperGraphs, played on the ZX-81,to a video available on this page.  After a month of excellent input from some of the great minds on the Web, it appears that the only way to get the funky ZX-81 output to be recognized by a digital video capture card is to use a Time Base Corrector.  Alas, access to such a device has proven daunting.

As a result, the demo has been made Blair Witch style, using a Canon Elura 100 Camcorder on a tripod, pointed at the flat screen TV screen, recorded to a Mini-DV tape in the camera, firewired into the computer to a file, and mixed with the original audio soundtrack using Windows Movie Maker.

There is a black band that rolls through the video caused by the sync differences.  Think of this as "artistic dramatic enhancement".  :-)  

Without further ado, here is the SuperGraphs Demo Video

 

Waking up the Sinclair ZX-81 from years of storage

After 20 years in the garage, the old ZX-81 with it's trusty 64k MemoPak and QSave hardware was connected to an old 12" black and white TV through it's TV channel 3 RF interface, and an old SuperScope cassette deck.

After a bit of fussing and cassette head cleaning, the ZX-81 booted up and was running SuperGraphs once again!

Ready to start the video capture, the TV channel 3 RF interface from the ZX-81 was connected to the Leadtek WinFast TV2000 XP Expert Composite input.  After initially showing just a few frames of the computer output, the captured video faded to black.  The capture card is in a 2.8Ghz P4 Intel D865PERL motherboard running Windows XP in 1G memory with 760 GB storage in 3 drives.  Here is the AV Connections diagram for the system.  

The RF inputs of several old VCRs connected to TVs were tried with the same results.  However, when connected to the RF input of a Daewoo DTQ27U4SC 27" flat screen TV, the ZX-81 picture was perfect!

The Daewoo 27" TV has a composite video monitor out jack that provides an output signal for the built in TV tuner and all composite video sources connected to the back of the TV. We connected that monitor out jack directly to the WinFast capture card composite input and also tried it through a Jensen ISO-MAX video isolation transformer.  

Whenever any video from the Daewoo not from the ZX-81 was selected for viewing and monitor output, the WinFast card captured it perfectly! Using or not using the ISO-MAX made no difference in any of the results.  

Here is a screen cap from VirtualDub showing a normal TV signal and its associated histogram from the Daewoo monitor jack being captured by the WinFast card being monitored in VirtualDub's capture mode:

 

Here is a screen cap from VirtualDub showing the ZX-81 signal and its associated histogram from the Daewoo monitor jack being captured by the WinFast card being monitored in VirtualDub's capture mode:

Click here to see a Windows Media video of what is seen through the capture card in the first second when the Daewoo is switched to the ZX-81 signal.  After initially showing just a few frames of the computer output, the captured video fades to black.

Finally, here is a screen cap of one frame of that captured video, when a ZX-81 SuperGraphs menu screen appears in pretty good shape!

We tried every filter and setting in both VirtualDub's capture interface and in dScaler's capture interface.  Absolutely nothing made any difference.

We are hoping an expert out there can tell us why the ZX-81 signal can be seen on a couple TVs, yet the 75 ohm composite out of this TV image cannot be seen on a capture card except for a few frames.

This page is posted as a resource for discussion on the Doom9 web site Capturing Video Forum in the thread How to capture old computer and games RF output. A Doom9 forum registration is required to post in the forums.