It's not really a restriction, it probably is because the device handling the inputs triggers on a video signal, and won't turn on that input without a video signal. That's so if you get noise on the audio line caused by a bad cable or connection or other unwanted signal source, it won't try to activate the input because audio without video is simply ignored.
After all, it is primarily a video device. Imagine your television auto-switching inputs every time you got a crackle on the audio input.
I see.
Yes, it is mainly a video device.
But I meant that if the A/V input is manually selected, I would want the audio to work without needing video.
Yeah, that is a frustrating thing. But it's probably the same deal - the input chip triggers on a video input, and doesn't pass anything until it sees that video signal. It's a cheap way of keeping the rest of the system blanked until the main component, the video signal, is present. It probably originally was related to power saving, but now is just built in because that stuff gets copied over and over without the manufacturer understanding why it was put there in the first place.
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