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ZR36050

JPEG IMAGE COMPRESSION PROCESSOR

Unspecified 

ADVANCE INFORMATION

ZR36050

Memory, however, does impose some restrictions, that have

only a very minor effect on the applicability of the device:

s The number of image components specified in the frame

header must be eight or fewer.

s A DHT marker segment must have a length of 420 or fewer

bytes (excluding the DHT marker), otherwise the tables may

be decoded incorrectly into the Huffman Table Store.

A JPEG Baseline DHT segment has a length of only 418

bytes, so this is not normally a problem. The restriction, how-

ever, stems from the fact that the standard does not explicitly

disallow the repetition of the same tables in a DHT segment.

s A DQT marker segment of any length is decoded correctly

into the Quantization Tables Store. If, however, its length

(excluding the DQT marker) is greater than 262 bytes, it may

overwrite the DHT, APP, and COM segments in Internal

Memory, affecting the ability of the host to read out the

contents of these segments correctly.

A JPEG Baseline DQT segment has a maximum length of

262 bytes. The restriction again stems from the fact that the

standard does not explicitly disallow the repetition of the

same tables in a DQT segment.

In addition to the above restrictions, to be decoded correctly, the

frame header of a JPEG Lossless compressed image must

specify a precision of 12 or fewer bits, and only horizontal sub-

sampling. The scan header must select a type 1 predictor (one-

dimensional horizontal), and can specify at most two different

Huffman tables.

OPERATING MODES

The host sets the operating mode of the ZR36050 by program-

ming the MODE register. Nine distinct modes can be selected,

falling into two categories:

s six encoding modes, involving compression and associated

functions. The encoding modes are: JPEG Baseline

Compression Pass, Auto Bit Rate Control, Statistical Pass,

Compression Pass with Bit Rate Control, Tables-only Pass,

and Tables Preload.

s three decoding modes, involving expansion: JPEG

Baseline Expansion, Fast Preview, and Tables Preload.

Two additional modes, the Lossless compression and expansion

modes of operation, are not distinguished from the JPEG

Baseline Compression Pass and Expansion, respectively, in the

programming of the MODE register. Rather, the ZR36050 enters

the JPEG Baseline or Lossless mode based on the SOF (Start

Of Frame) marker. If the marker found in internal memory (com-

pression) or the compressed data (expansion) is SOF0 (FFC0),

the ZR36050 configures itself for JPEG Baseline operation. Oth-

erwise, if the marker is SOF3 (FFC3), it configures itself for

Lossless operation.

Auto Bit Rate Control, Statistical Pass, and Compression Pass

with Bit Rate Control are relevant only to JPEG Baseline com-

pression, and Fast Preview is useful only with JPEG Baseline

compressed data. These modes have no meaning for Lossless

operation.

ENCODING MODES

JPEG Baseline Compression Pass

The Compression Pass performs the Baseline encoding opera-

tion on the input image component samples. During a

Compression Pass, the ZR36050 reads the JPEG marker

segment information written by the host in the Marker Segments

Section of Internal Memory, and uses it to determine the MCU

configuration, and includes the compulsory and optional marker

segments (selected by the MARKERS_EN register) in the com-

pressed data stream. Note that, if the DQT marker segment is

enabled, the ZR36050 first multiplies the quantization tables

specified in the DQT segment of Internal Memory by the Scale

Factor, and stores the scaled tables in the Quantization Table

Store. The quantization tables included in the compressed data

are the same as the stored (scaled) tables. At the completion of

the JPEG Baseline Compression Pass, the ZR36050 calculates

a New Scale Factor (NSF) and saves it in the SF Internal

Memory register. The NSF can be used in the next encoding

operation or the host can overwrite it by its own Scale Factor.

Statistical Pass

In the Statistical Pass, the ZR36050 performs the computations

for JPEG Baseline encoding of the image, with the initially spec-

ified Scale Factor, but without transferring any data to the Code

Buffer. It accumulates the code volume and a total activity

measure. Based on the Target Code Volume (TCV_DATA regis-

ter), it calculates the Allocation Factor and a new Scale Factor at

the end of the pass. It writes the new Scale Factor in the SF reg-

ister, in place of the initial Scale Factor, and the Allocation

Factor, Accumulated Code Volume, and Total Activity measure,

in their respective registers (SF, AF, ACV, and ACT), where the

host can access them if needed.

Compression Pass with Bit Rate Control

This mode allows the user to ensure a compressed data volume

equal to or slightly less than the Target Code Volume. Before

encoding each block, the ZR36050 computes a measure of the

block activity, and allocates a code volume to the block based on

the activity and the Allocation Factor (AF register). During

encoding of the block, if the accumulated code volume for the

block exceeds the allocation, the ZR36050 truncates the code

for the block. The code is also truncated if it exceeds the

Maximum Block Code Volume specified in the MBCV register.

Aside from the bit rate control, this mode is the same as a JPEG

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