WEBM Quantization

[deepthi shridhar]

Hi

I was going through the source code of encoder and I couldnt get the
logic of quantization module. Can anyone help me with this. Appreciate
any help.

Thanks
Deepthi

[Rajesh Pamula]

Hi,

As you might have seen the quantization parameter 0-127 is mapped to something called as Qstep and these Qsteps are different for the DC coeffs and AC coeffs. Both encoder and decoder maintain a look up table, an array which maps Qp to Qstep. Once Qstep is known, quantization is roughly the division process, like k*coeff/Qstep, where coeff is the transformed coeff value and k is a scale factor.

To maintain higher precision, you may increase the bit depth prior to divison process by doing a left shift. I guess this is what is implemented in the reference encoder.

Thanks and regards,

Rajesh

Deepthi

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Rajesh Pamula
Post Graduate Student
Analogue and Digital Integrated Circuit Design
Imperial College London.
Residence Phone: +44-20-78522056