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Working with older computers is often difficult because they are not compatible with modern hardware. If you’ve noticed that the prices of old CRT (cathode ray tube) TVs and monitors have skyrocketed recently, you can thank the retro gaming and retro computer community. Not only do low-resolution graphics look better on CRTs, but many older systems simply can’t reproduce video that’s acceptable on modern monitors. One solution is to use an adapter to convert the old RF or composite video signal to a more modern signal. To assist in the development of such adapters, dmcintyre has created this video launcher for oscilloscopes.
While converting video, dmcintyre encountered an issue where the TMS9918 video chip did not trigger the scope reliably. This makes it almost impossible to analyze the video signals, which would be necessary for those trying to convert them. The Texas Instruments TMS9918 VDC (Video Display Controller) series chips are very popular and are used in older systems such as ColecoVision, MSX computers, Texas Instruments TI-99/4, etc. This video trigger provides composite video bandwidth and interface USB for oscilloscopes. The USB connection allows you to quickly capture waveforms on many oscilloscopes, including dmcintyre’s Hantek oscilloscopes.
The video trigger circuit is mostly discrete and requires only a few integrated circuits: a Microchip ATmega328P microcontroller, a 74HC109 flip-flop, and an LM1881 video sync splitter. All components are soldered to a standard breadboard. Once the dmcintyre code has been ported to the ATmega328P, it is very easy to use. Connect the cable from the system to the Video Trigger input and the cable from the Video Trigger output to a compatible monitor. Then connect the USB cable to the input of the oscilloscope. Set the scope to trigger on a trailing edge with a threshold of about 0.5V.
With this setup, you can now see the video signal on the oscilloscope. Pressing the rotary encoder on the video trigger device toggles between the rising and falling edge of the trigger signal. Turn the encoder to move the trigger line, press and hold the encoder to reset the trigger line to zero.
It doesn’t actually do any video conversion, it just allows the user to analyze the video signal coming from the TMS9918 chip. But the analysis should help people develop compatible video converters to connect older computers to modern monitors.