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How is lossless compression possible using non-rectangular windows?, [was “Question about windowing”/TOS #6]
evoj2
post Nov 17 2013, 01:41
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Hello.

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.
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C.R.Helmrich
post Nov 17 2013, 02:24
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QUOTE (evoj2 @ Nov 17 2013, 02:41) *
I wonder how may a codec be lossless if the original signal is multiplied by a window which is not rectangular.

I thought in FLAC the signal is only windowed for the LPC filter computation, but a rectangular window is used for framing and coding?

In HD-AAC (SLS), where overlapping integer time-frequency transformation is used, the windows satisfy certain constraints, which allow perfect signal reconstruction in the absence of signal quantization (which of course is what we aim for in lossless coding).

Chris

This post has been edited by C.R.Helmrich: Nov 17 2013, 02:25


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evoj2
post Nov 17 2013, 03:02
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QUOTE (C.R.Helmrich @ Nov 17 2013, 02:24) *
QUOTE (evoj2 @ Nov 17 2013, 02:41) *
I wonder how may a codec be lossless if the original signal is multiplied by a window which is not rectangular.

I thought in FLAC the signal is only windowed for the LPC filter computation, but a rectangular window is used for framing and coding?


Thanks for answering.

It seems that I was confused.

I've studied that the first stage in most encoders is blocking/windowing. In FLAC, this stage is followed by interchannel decorrelation, prediction and residual encoding stages. So, which is the window used in the blocking stage? Rectangular one?

If that's true, then I suppose that Tukey window is used in linear prediction. Right?

Thank you.

This post has been edited by evoj2: Nov 17 2013, 03:02
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C.R.Helmrich
post Nov 17 2013, 14:39
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QUOTE (evoj2 @ Nov 17 2013, 04:02) *
... So, which is the window used in the blocking stage? Rectangular one?

If that's true, then I suppose that Tukey window is used in linear prediction. Right?

Thank you.

Yes and yes (although you could use other windows than Tukey, e.g. Hamming, in the linear prediction to possibly improve the prediction gain on some signals).

Chris


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evoj2
post Nov 17 2013, 15:28
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QUOTE (C.R.Helmrich @ Nov 17 2013, 14:39) *
QUOTE (evoj2 @ Nov 17 2013, 04:02) *
... So, which is the window used in the blocking stage? Rectangular one?

If that's true, then I suppose that Tukey window is used in linear prediction. Right?

Thank you.

Yes and yes (although you could use other windows than Tukey, e.g. Hamming, in the linear prediction to possibly improve the prediction gain on some signals).

Chris


Thank you.
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