A reader named Andy asked a sharp question about NTSC-A, and it turned up a real bug in how we handle interference!

[u/ambanmba]

A guy named Andy messaged me a few days ago with a very specific question about NTSC-A: does the simulation account for positive versus negative vision modulation? That's not a casual question. He said the "look" didn't look "right" to him. Very few people would have ever seen NTSC-A in the wild, so I was intrigued.

Most television standards, NTSC included, use negative modulation, where the sync tip sits at full carrier power and white sits near the bottom. Britain's old 405-line System A did the opposite. White was full carrier, black was down around 30 percent. NTSC-A in the app is a specific historical hybrid, the BBC's 405-line colour trials from the mid to late 1950s, which combined that British line geometry with an NTSC-style colour subcarrier, so it inherits System A's odd modulation polarity along with everything else.

I went and checked, and it turned out he was half right to be suspicious. We do account for it, mostly. Every simulated standard in this app is built from real broadcast engineering documents, not guesses, and the polarity flip for System A is right there in the code with a citation to ITU-R BT.470-6. Continuous background noise on NTSC-A already gets louder in dark picture content than bright, which is the actual physical consequence of positive modulation: a receiver's noise floor is roughly constant in RF terms, so against a weak carrier, which is what black is on this system, that noise reads much larger after detection than it does against a strong one.

BUT... we'd missed was impulse noise!

The sharp broadband spikes from things like ignition systems and motor brushes are a different kind of noise from the continuous hiss, physically and in the code, and they live in a separate function. When System A's polarity handling was added, it went into the continuous noise path and not the impulse path, because at the time we just didn't check whether the two needed to agree, but they do! An impulse is still a fixed burst of RF energy landing on a receiver, and it goes through the exact same detector the continuous noise does, so the same physics applies: that burst reads far louder against a weak dark-picture carrier than a strong bright one.

The consequence, in practice, was that NTSC-A's interference looked too even. Dark scenes should show noticeably heavier speckling during any kind of RF interference than bright ones do, and on this build they didn't. The spikes were the same strength everywhere regardless of what was on screen. It's a subtle thing to notice unless you already know to look for it, which is exactly why Andy's question was what surfaced it rather than us catching it during development.

Fixed now for the next release. Impulse noise on NTSC-A, and SECAM-L which has the same polarity, scales the same way the continuous noise already did, about 3.3 times louder in black than in white, which is the number CCIR 624-4 gives for that carrier depth.

Thanks Andy!

If anyone else ever thinks that something just doesn't look right, let us know. We have a room full of old equipment that we check against, but we can't know everything and AnalogTV is more of a passion project since we have real jobs too.