STV0974 datasheet(18/69 Pages) STMICROELECTRONICS

18 / 69 page

18/61

STV0974

Functional description

4.4.5

Entropy coder

This block performs the following functions:

● insertion of JPEG Markers

● runlength encoding

● Huffman encoding

4.4.5.1 JPEG markers

These markers are compliant with ISO DIS 10918-1 Annex B.

The output JPEG file includes markers defined in

Table 7, in order of appearance.

4.4.5.2 Runlength and Huffman encoding

Encoding of DC coefficient

The so-called DC coefficient is the first coefficient of each DCT data block. This DC coefficient is

coded through its DPCM difference with its previous value, which is huffman encoded. This is

described in ISO DIS 10918-1 Annex A - Section F.1.2.1. The DC Huffman tables are described in

ISO DIS 10918-1 Annex A - Section K.3.

In the example from

Figure 10, the DC coefficient in Block Y2 is equal to 4, the previous Luminance

DC coefficient is 12 (DC coefficient of Block Y1). The DPCM value is 4-12 = -8 and the encoded

value will be Huffman (-8). The code that is generated is Code = DC Huffman (-8).

Encoding of AC coefficients

The 63 left coefficients of each DCT block are called AC coefficients. They are encoded using run-

length and Huffman encoder. The run-length encoding consists in counting the number of zero

values between each non-zero coefficient. When a non zero coefficient is found, the Huffman code

of the pair (number of preceding zero, Number value) is Huffman encoded. If a run contains more

than 15 zeros, a specific number called ZRL is Huffman encoded.

If all the values up to the end of the block are equal to zero, a specific code called EOB is Huffman

encoded.

Table 7: JPEG markers included in

STV0974 output data stream

Marker Function

Name

Value

Start of image

SOI

FFD8

Define Quantization Table

DQT

FFDB

Start of frame for baseline DCT

SOF0

FFC0

Define Huffman Tables

DHT

FFC4

Start of Scan

SOS

FFDA

End of image

FFD9

Figure 10: Encoding of DC coefficient

Block Y1 data values

234

DCT coefficient numbers in zigzag order

234

Block Y2 data values

416

STV0974 Datasheet(PDF) 18 Page - STMicroelectronics

STV0974 Datasheet(HTML) 18 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69

Datasheet Download

Link URL