參數(shù)資料
型號(hào)： ZR36050
廠商： Zoran Corporation
英文描述： JPEG Image Compression Processor(JPEG 圖象壓縮處理器)
中文描述： JPEG圖像壓縮處理器（JPEG格式圖象壓縮處理器）
文件頁(yè)數(shù)： 11/52頁(yè)
文件大?。?/td> 262K
代理商： ZR36050
ZR36050
11
ADVANCE INFORMATION
Memory, however, does impose some restrictions, that have
only a very minor effect on the applicability of the device:
I
The number of image components specified in the frame
header must be eight or fewer.
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.
I
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
OPERATING MODES
I
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.
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:
I
six
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.
three
decoding modes
, involving expansion: JPEG
Baseline Expansion, Fast Preview, and Tables Preload.
encoding modes
, involving compression and associated
I
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
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