Windowing and lossless compression
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.
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.