Yes, but audio records don’t have to encode frequencies below the human hearing range, whereas a video one should be capable of showing multiple consecutive bl

Yes, but audio records don’t have to encode frequencies below the human hearing range, whereas a video one should be capable of showing multiple consecutive black or white frames. The pickup doesn’t detect groove depth, it detects change in groove depth. My guess is that they used AM since all you’d have to do was amplify it and send it straight to the lightbulb, no demodulation necessary.