ASUS V1/V2 Codecs

Table of Contents

        1 Introduction
        2 Terms and definitions
        3 Highlevel Description
            3.1 Picture
            3.2 Macroblock
            3.3 Block
            3.4 Coefficient group
            3.5 DC coefficient Dequantization
            3.6 AC coefficient Dequantization
                    c_{xy}'=\frac{c_{xy}}{16}\left\lfloor \frac{Dq_{xy}}{QP}\right\rfloor 
        4 Bitstream
            4.1 Fourcc
            4.2 Global (per file) Header
            4.3 Macroblock
            4.4 ASV1 Block
            4.5 ASV2 Block
        5 VLC Codes
            5.1 ASV1
                5.1.1 Level
                5.1.2 Coded coefficient pattern
            5.2 ASV2
                5.2.1 First Coded coefficient pattern
                5.2.2 Coded coefficient pattern
                5.2.3 Level
        6 Example ASV1 decoder 
        7 Changelog
        8 Copyright


1 Introduction

ASV1/2 are purely intracoded DCT-based codecs, with a per file 
quantization parameter (QP), use 4:2:0 YCbCr colorspace and 
operate on macroblocks.
The latest version of this document is available at 
http://www.mplayerhq.hu/~michael/asv1.{lyx,txt,html,ps}

This document assumes familiarity with mathematical and coding 
concepts such as the discrete cosine transform, quantization, YUV 
(YCbCr) colorspaces, macroblocks, and variable length codes 
(VLCs).

2 Terms and definitions

AC Any DCT coefficient for which the frequency in one or both 
both dimensions in non-zero.

DC The DCT coefficient for which the frequency is zero in both 
dimensions

QP Quantization Parameter

(I)DCT (Inverse) Discrete Cosine Transform

VLC Variable Length Code

3 Highlevel Description

3.1 Picture

The picture is split into macroblocks which are coded 
left->right, top->bottom, as long as MBs are completely within 
the width/height area that is divisible by 16. If width is not 
divisible by 16, the right column is coded after the main body. 
If height is not divisible by 16, the bottom strip is coded after 
the right column.

Example: 56x56 sized image


+-----+-----+-----+----+
| 0   | 1   | 2   | 9  |
+-----+-----+-----+----+
| 3   | 4   | 5   | 10 |
+-----+-----+-----+----+
| 6   | 7   | 8   | 11 |
+-----+-----+-----+----+
| 12  | 13  | 14  | 15 |
+-----+-----+-----+----+


3.2 Macroblock

16x16 luma + 8x8 chroma as 4 8x8 luma blocks and 2 8x8 chroma 
blocks 
Y:
+----+---+
| 0  | 1 |
+----+---+
| 2  | 3 |
+----+---+
 Cb:
+---+
| 4 |
+---+
 Cr:
+---+
| 5 |
+---+


3.3 Block

Contains 64(=8x8) coefficients in 16(=4x4) coefficient groups 
with 4(=2x2) coefficients each

+----+-----+-----+----+
| 0  | 1   | 5   | 9  |
+----+-----+-----+----+
| 2  | 3   | 8   | 12 |
+----+-----+-----+----+
| 4  | 7   | 11  | 14 |
+----+-----+-----+----+
| 6  | 10  | 13  | 15 |
+----+-----+-----+----+


Coefficients in the coefficient groups 10..15 cannot be coded 
(they must be 0) in ASV1.
The first coefficient (DC coefficient) in the first coefficient 
group must be coded as 0. Note it is coded seperately in the 8 
bits prior to the AC coefficients.

3.4 Coefficient group


+----+---+
| 0  | 1 |
+----+---+
| 2  | 3 |
+----+---+


3.5 DC coefficient Dequantization

c_{00}'=8c_{00}


c_{00}
 quantized and coded DC coefficient

c_{00}'
 unquantized DC coefficient

3.6 AC coefficient Dequantization

  c_{xy}'=\frac{c_{xy}}{16}\left\lfloor \frac{Dq_{xy}}{QP}\right\rfloor 


QP Quantization Parameter from the global header

D 64 for ASV1, 128 forASV2

c quantized and coded coefficient

c' unquantized coefficient

q ISO MPEG1 intra quantization matrix

4 Bitstream

The bitstream in ASV1 is stored with byte-swapped 32bit words 
(24..31, 16..23, 8..15, 0..7, 56..63, 48..55, 40..47, 32..39, 
...)

The bitstream in ASV2 is stored with the bits in each byte 
reversed so (7..0, 15..8, 23..16, 31..24, 39..32, ...)

4.1 Fourcc

ASV1 / ASV2

4.2 Global (per file) Header


+-----------------+--------------------+
| Length in bits  |       Value        |
+-----------------+--------------------+
+-----------------+--------------------+
|       8         |         QP         |
+-----------------+--------------------+
|       24        | unkown, 0 for ASV1 |
+-----------------+--------------------+
|       8         |        'A'         |
+-----------------+--------------------+
|       8         |        'S'         |
+-----------------+--------------------+
|       8         |        'U'         |
+-----------------+--------------------+
|       8         |        'S'         |
+-----------------+--------------------+


Note, the biSize parameter (in the case of storage in AVI) is 48

4.3 Macroblock

Simply the bitstream of the 6 blocks

4.4 ASV1 Block

8-bit DC coefficient

. for each coefficient group

  2-5bit Coded coefficient pattern

  . for each coded coefficient

    2-11bit level

5bit End Of Block

4.5 ASV2 Block

4-bit coefficient group count

8-bit DC coefficient

2-4bit First Coded coefficient pattern

. for each coded coefficient

  2-13bit level

. for each coefficient group

  2-6bit Coded coefficient pattern

  . for each coded coefficient

    2-13bit level

5 VLC Codes

5.1 ASV1

5.1.1 Level


+--------------+----------+
|    0011      |    -3    |
+--------------+----------+
|     011      |    -2    |
+--------------+----------+
|     11       |    -1    |
+--------------+----------+
| 000xxxxxxxx  | xxxxxxxx |
+--------------+----------+
|     10       |    1     |
+--------------+----------+
|     010      |    2     |
+--------------+----------+
|    0010      |    3     |
+--------------+----------+


xxxxxxxx is a twos complement signed 8 bit nubmer (=int8_t)

5.1.2 Coded coefficient pattern


+--------+-----+
| 00001  | 14  |
+--------+-----+
| 00010  | 13  |
+--------+-----+
|  ...   | ... |
+--------+-----+
| 01101  |  2  |
+--------+-----+
| 01110  |  1  |
+--------+-----+
| 01111  | EOB |
+--------+-----+
|  10    |  0  |
+--------+-----+
|  11    | 15  |
+--------+-----+


5.2 ASV2

5.2.1 First Coded coefficient pattern


+-------+------+
| VLC   | CCP  |
+-------+------+
|  00   | 0111 |
+-------+------+
|  01   | 0000 |
+-------+------+
| 100   | 0110 |
+-------+------+
| 101   | 0100 |
+-------+------+
| 1100  | 0011 |
+-------+------+
| 1101  | 0001 |
+-------+------+
| 1110  | 0101 |
+-------+------+
| 1111  | 0010 |
+-------+------+


5.2.2 Coded coefficient pattern


+---------+------+
|  VLC    | CCP  |
+---------+------+
|   00    | 0000 |
+---------+------+
|  010    | 0100 |
+---------+------+
|  011    | 1000 |
+---------+------+
|  1000   | 1010 |
+---------+------+
|  1001   | 1100 |
+---------+------+
|  1010   | 0010 |
+---------+------+
|  1011   | 1101 |
+---------+------+
|  1100   | 1111 |
+---------+------+
|  1101   | 1110 |
+---------+------+
| 111000  | 0111 |
+---------+------+
| 111001  | 0101 |
+---------+------+
| 111010  | 0011 |
+---------+------+
| 111011  | 0001 |
+---------+------+
| 111100  | 0110 |
+---------+------+
| 111101  | 1001 |
+---------+------+
| 11111   | 1011 |
+---------+------+


5.2.3 Level


+----------------+----------+
|  0000111111    |   -31    |
+----------------+----------+
|      ...       |   ...    |
+----------------+----------+
|    001111      |    -7    |
+----------------+----------+
|    001011      |    -6    |
+----------------+----------+
|    001101      |    -5    |
+----------------+----------+
|    001001      |    -4    |
+----------------+----------+
|     0111       |    -3    |
+----------------+----------+
|     0101       |    -2    |
+----------------+----------+
|      11        |    -1    |
+----------------+----------+
| 00000xxxxxxxx  | xxxxxxxx |
+----------------+----------+
|      10        |    1     |
+----------------+----------+
|     0100       |    2     |
+----------------+----------+
|     0110       |    3     |
+----------------+----------+
|    001000      |    4     |
+----------------+----------+
|    001100      |    5     |
+----------------+----------+
|    001010      |    6     |
+----------------+----------+
|    001110      |    7     |
+----------------+----------+
|      ...       |   ...    |
+----------------+----------+
|  0000111110    |    31    |
+----------------+----------+


xxxxxxxx is a twos complement signed 8 bit nubmer (=int8_t)

6 Example ASV1 decoder 

• foreach macroblock, decode Y0 Y1 Y2 Y3 U V blocks, Y blocks are 
  arranged as: 
Y0 Y1 
Y2 Y3 

• foreach block: 

  – DC coeff is the next 8 bits in the stream * 8 (this gives a 
    range of 0..2040)

  – foreach of up to 11 coefficient groups: 

    ∗ get vlc from ccp table, value should range from 0..16

    ∗ if value is 16, coefficient decode is done

    ∗ if 11th iteration, 16 should occur, else error

    ∗ ccp contains 4 bits at this point (3-0)

    ∗ each bit corresponds to a quantized coeff. #(iter. * 4 + 
      bit) 

    ∗ if 0, coeff. is 0 else get_level():

      ∙ get vlc from level vlc table

      ∙ if vlc is 3, use the next 8 bits (as a signed number) as 
        level

      ∙ else, use vlc - 3

      ∙ store coefficient

  – dequantize, idct

7 Changelog

0.01 2003-05-21
initial version by Michael Niedermayer with some stuff from Mike 
Melanson

0.02 2003-05-21
spelling, punctuation, example decoder, various minor changes by 
Mike Melanson
various minor changes by Michael

0.03 2003-09-01
minor fixes and ASV2 by Michael

0.04 2004-08-24
clarify bitstream order by Michael

0.05 2016-03-12
Add biSize value used in avi

8 Copyright

Copyright 2003 Michael Niedermayer <michaelni@gmx.at> 
This text can be used under the GNU Free Documentation License or 
GNU General Public License
see http://www.gnu.org/licenses/fdl.txt