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ZR36050

JPEG IMAGE COMPRESSION PROCESSOR

Unspecified 

ADVANCE INFORMATION

ZR36050

CODE VOLUME CONTROL

The JPEG Baseline encoding algorithm does not inherently

provide a means of achieving a target code volume for a com-

pressed image (or an equivalent target average coding rate in

bits per pixel, or target compression ratio). In fact, since the

standard specifies that the whole image frame is encoded using

the same set of quantization tables and Huffman coding tables,

it provides no built-in mechanism to modify the coding rate once

encoding of an image has started.

One way to achieve a target code volume in compression of a

particular image is to iterate the compression, using different

quantization tables each time, until the desired target is

achieved. The Scale Factor (SF register in the Internal Memory)

allows the host to modify the quantization tables by changing a

single parameter. In operation, the ZR36050 multiples the quan-

tization tables, specified in the DQT segment of Internal Memory,

by the Scale Factor before storing them in the Quantization

Tables Store.

When it is activated with appropriate values of the Scale Factor

and an Allocation Factor (AF register), the unique Bit Rate

Control feature of the ZR36050 ensures that the actual code

volume achieved is equal to or slightly less than the target code

volume. Moreover, if the Scale Factor is close enough to the

correct value, the Bit Rate Control operates without affecting the

compressed image quality. The ZR36050 has a Statistical Pass

mode of operation, in which it performs a preliminary pass over

the image data, and determines appropriate values of the Scale

Factor and Allocation Factor for use in a Compression Pass with

Bit Rate Control. The host specifies the target code volume to be

used in the computation of the Scale Factor and Allocation

Factor, in the TCV_DATA register. There is also a mode of oper-

ation in which the ZR36050 performs a Statistical Pass followed

automatically by a Compression Pass with Bit Rate Control. For

more details on these modes of operation, see the “ENCODING

MODES” section.

The code volume overflow detection option (activated by the

OVF bit of the OPTIONS register) is useful if Bit Rate Control is

not used, or if the Scale Factor and Allocation Factor contain

inappropriate values. With this option selected, the ZR36050

continuously compares the accumulated code volume with a

target limit specified in the TCV_NET register. If the code volume

exceeds the limit at any time during a compression pass, the

TCVOVF status register bit is set, and the interrupt pin is activat-

ed to notify the host. Since this is considered a destructive event,

the ZR36050 aborts the compression pass and goes Idle.

COMPRESSED DATA FORMAT

The ZR36050 supports encoding and decoding of all three com-

pressed data format classes defined by the JPEG standard:

s The full interchange format, in which the specifications of all

the tables used for encoding the image are included with the

compressed image data, in their marker segments. This

format is used when the compressed image must be

decoded by a JPEG decoder that has no knowledge of the

tables used.

s The abbreviated format for compressed data. This format

contains the compressed image data and the frame and scan

headers, but where some or all of the tables are omitted.

s The abbreviated tables-only format. This format contains

marker segments with table specifications but no

compressed image data.

The two abbreviated formats complement each other in applica-

tions based on the ZR36050. For example, when a sequence of

image frames, such as a video clip, are compressed, the same

Huffman tables are used for all the images in the sequence. If the

compressed sequence is to be sent to a JPEG decoder that has

no prior knowledge of the tables, it is most economical for the

encoder to create initially, an abbreviated format compressed

data-less image, containing only the Huffman tables, followed by

a sequence of compressed images in which the Huffman tables

are omitted. To expand the image sequence, the JPEG decoder

first decodes the tables-only data, thus installing the Huffman

tables, which are subsequently used in the decoding of all the

images in the sequence.

Compressed data generated by the ZR36050 fully complies with

the JPEG standard and includes all headers and marker

segments necessary to allow it to be decoded by a compliant

JPEG decoder. The JPEG standard allows virtually unlimited

permutations in the order and repetition of optional marker seg-

ments. As a practical matter, there are some restrictions on the

permutations and repetitions in compressed data generated by

the ZR36050:

s The order of optional marker segments is fixed, as follows:

APP, COM, DRI, DQT, DHT.

s Only one instance of each optional marker segment can be

inserted before each SOS, or the SOF, marker.

s The frame header can specify up to eight image

components.

s A compressed image can contain up to eight scans.

A compressed image to be decoded by the ZR36050 must be in

JPEG standard format. The ZR36050 recognizes and decodes

all the following marker types: SOI, SOF (SOF0 or SOF3), SOS,

APP, COM, DRI, RST, DQT, DHT, DNL, EOI. All marker

segments other than JPEG Baseline marker segments, i.e.,

markers followed by a length parameter, are disregarded and do

not cause any error. Markers without a following length parame-

ter, except for SOI, RST and EOI, will cause unpredictable

behavior. A marker can be prefixed by any number of FF bytes.

In its decoding modes, the ZR36050 can expand any JPEG

Baseline compressed image. The finite size of the Internal

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