When there is temporal or spacial or

spectral redundancy in the signals, we can actually

predict what the next frame or the next sample is going to be.

And then we can subtract this from the incoming data

to reduce the bit rate further.

This is what we did in the CELP case, that is the Code-excited linear prediction.

And there are similar things that we can do for the other types of signals.

So just to recap, here is the spirit of processing we do at the end quarter.

The time-frequency decomposition puts the signal in a different space,

so that we can take advantage of the redundancy in the signals and

ignore the irrelevant parts.

These are the two operative words.

Redundancy and irrelevancy using our prior knowledge of the signals.

For redundancy, think of a pure tone.

It has all the samples in the time domain but you need only

two samples to represent it very compactly in the frequency domain.

Similarly if you have short transients in the time domain

you get to have very wide spectral distribution.

[COUGH] So you can have very compact representation in the time domain or

transient and very compact representation in the frequency domain for

tonal sounds [COUGH].

For the irrelevancy part, the more you know about the signals,

the source and the sync, the more irrelevant parts that we can

identify and take advantage of them as we do this process.

And as we discussed before, the quantization step implements

a lossy bitrate reduction scheme.

We simply assign fewer bits to the parts that are less

relevant based on what we already know.

Then comes the Entropy Coding.

There are many types of entropy coding systems that we can leverage

in the pipeline.

But, for now, I want you to realize, recognize, that the entropy coding

is a loss-less compression and quantization is a lossy compression.

[COUGH] So let's look at some of the options we have for

the time frequency decomposition.

We already saw LPC in the previous course.

Other than that, the short-time fourier transform is the basic work hours

because it doesn't make any assumption about the source or the sync.

The picture at the bottom left shows the tiling of components in the STFT.