In almost every culture in the world a red light means 'stop'. And so it is in sound engineering and recording culture too - if you see a red light on your meters, it's telling you that you are doing something wrong. And whatever it is, it would be a good idea to stop right now.
In analog audio, which includes analog mixing consoles of course, a red light on the meters generally means a warning, rather than indicating that something disastrous has already happened. This also applies to digital mixing consoles and signal processors.
Mixing consoles use the concept of 'headroom'. It's like driving a tall vehicle under a bridge. If the bridge has an opening 3 meters high and your vehicle is 2.5 meters high, then you have plenty of headroom. Drive under the bridge as fast as you like.
But if the bridge is 3 meters and your vehicle is 2.99 meters, then you have shaved your headroom right down to the bone. Drive carefully and if there happens to be a pebble in the road, the worst you will do is skim the paint.
In a mixing console, headroom is the margin between the normal operating level and the absolute maximum level, above which the signal will be clipped and distorted. Headroom doesn't cost anything, apart from a tiny little bit of extra noise, and it means that you never have to worry about clipping the mixer.
In a recorder however, tape, disk or computer, then it is considered good practice in a studio situation to use every last bit of dynamic range, thus optimizing the noise performance. This means that headroom is shaved down to zero. When you see that red light on the recorder, it means you have clipped. Distortion has occurred and you need to back off before you go for the final mix.
Oddly enough, some digital audio workstations don't as default meter the output. So you could be clipping without being aware of it. The channels may show entirely green, but added together they make a deeper shade of red.
So in conclusion, in mixing consoles and signal processors, a red light is generally a warning. In a recorder, a red light means more than stop, it means stop and back off!
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