Question about Quantization in the DCT domain

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2 total messages Started by Saad Bedros 625 7542 Fri, 06 Nov 1992 15:11

#96

Author: Saad Bedros 625 7542
Date: Fri, 06 Nov 1992 15:11

18 lines
1277 bytes

working in quantization for the DCT coefficients, I know I can apply
MAX quantizer for the ac coefficients with a laplacian source,
But for the DC term, a uniform quantization is best.
What is the best approach for quantizing the DC term:
 1- calculate the min and max, then quantizing using 8 bits after
     normalization. in this case, the min and max should be transmitted.
 2- find a global max for the DC, normalize then quantize. I know in JPEG
    they use 16 as a quantization level.

What is the advantage to set the range between -128,128 instead 0,255
before taking the DCT ? ( I think that is what they do in JPEG)

By the way, I was looking for a DCT in matlab, and I got one, not fast
but not slow if somebody is intersted.

Saad Bedros
U of Minnesota

Re: Question about Quantization in the DCT domain

#101

Author: Alex Mou
Date: Wed, 11 Nov 1992 05:41

29 lines
1758 bytes

In article <1992Nov6.151148.7943@news2.cis.umn.edu> bedros@loon.cs.umn.edu (Saad Bedros 625 7542) writes:
>
>working in quantization for the DCT coefficients, I know I can apply
>MAX quantizer for the ac coefficients with a laplacian source,
>But for the DC term, a uniform quantization is best.
>What is the best approach for quantizing the DC term:
> 1- calculate the min and max, then quantizing using 8 bits after
>     normalization. in this case, the min and max should be transmitted.
> 2- find a global max for the DC, normalize then quantize. I know in JPEG
>    they use 16 as a quantization level.

From a hardware implementation point of view, finding a global max may
need quite an amount of memories to accomodate all transformed blocks.
This is the drawback I see in the second method.

>
>What is the advantage to set the range between -128,128 instead 0,255
>before taking the DCT ? ( I think that is what they do in JPEG)
>

The advantage is to simplify H/W implementation.
Hardware implementation usually uses 2's complement representation of 
numbers. That is the cause.

>By the way, I was looking for a DCT in matlab, and I got one, not fast
>but not slow if somebody is intersted.
>
>Saad Bedros
>U of Minnesota

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