Re: Nabu video
Posted: Tue May 16, 2023 4:17 pm
An issue with DC bias on the composite video output has been reported by several Nabuers, but it really wasn't a nefarious attempt at copy protection or cost cutting as speculated.
Rather, the standard approach of the day was to capacitively couple the composite video signals between devices, conventionally placing the coupling capacitor and subsequent DC restoration circuitry on the input rather than output stage. There was solid rationale behind this approach and I'd refer the interested reader to the article here (https://www.analog.com/en/technical-art ... gnals.html). In this respect, the output of the TMS9918A was indeed deemed to be NTSC compliant. We simply added a simple emitter-follower stage to buffer the signal and provide impedance matching, allowing the monitors to provide the DC restoration. This worked pretty much without exception.
With the advent of digital signal processing, DC coupled video became more prevalent and the capacitive isolation provided on the input stages was dropped by a number of designers. Being "old-school", I think this has its drawbacks, one being the backward compatibility problem you are most likely experiencing today.
How to solve this? Well, you could look for a retro monitor. Alternatively, if you Google "NTSC DC restorer circuit" you'll find several IC based solutions of varying complexity. Before getting too elaborate, I'd suggest trying the simple sync tip clamping circuit in the attached figure. I've run a transient analysis on the design (using a very approximate "video" signal) that suggests it will achieve an effective sync level clamp while maintaining the signal amplitude. The green trace is the DC-biased input (from the NABU video output jack) and the blue trace is the DC-restored output (to the monitor).
Strictly speaking, this circuit should actually be powered from the video monitor's 5-volt supply, since one purpose of the coupling capacitor is to isolate the power supplies between connected devices and mitigate any possibility of fireworks. If you choose to power it from the NABU PC, please use caution. For testing it with your particular monitor, throw it together on a breadboard with suitable (presumably RCA phono) connectors on either end, and power it with a 9-volt battery. In this case, try a 1.8k resistor for R2. The value of R1 and R2 may have to be adjusted depending on your monitor.
Let us know how you make out.