Windowing and lossless compression

Question

I'm studying how FLAC works, although my question is valid for any lossless codec.

I wonder how may a codec be lossless if the original signal is multiplied by a window which is not rectangular.

I think this operation will modify the stream that we don't want to change.

I know a rectangular window has a terrible spectral response (sinc, with many lobes), but, what's the problem? We don't want to disturb the audio stream, and by multiplying by something different to 1, we will.

Thank you.

Answer 1

A window function can be applied when you want to transform your signal from the time domain to the frequency domain. If you are working with chunks of data then a window can be applied to minimise the effects of spectral leakage.

You can use a (symmetrical) window and apply it to chunks of audio if you also introduce whats known as overlap. Usually 50% overlap is used. This means that the last 50% of your previous chunk is added to the first 50% of your next chunk. This is a lossless operation.