Page 212 - DCAP303_MULTIMEDIA_SYSTEMS

Multimedia Systems

notes start of scan (sos) segment
This segment contains header information as well as actual scan data. Multiple SOS Segments can
be found if there is more than one scan. The number in brackets indicates the number of bytes
the information is stored in.

• Segment marker (2). Is always FFDAH
• Section length (2). Does not include the segment marker. This also only includes header
information; it does not include compressed image data.
• Number of components in scan (1).
• Component ID (1). This was defined is the SOF segment, step 7.
• Huffman Table used for component in step 4 (1). High nibble is DC Table ID and Low nibble
is AC Table ID.
Steps 4 and 5 are repeated for each component defined in the SOF segment

• Start of spectral selection (1). Not used in JPEG Baseline. Always 00H
• End of spectral selection (1). Not used in JPEG Baseline. Always 3FH
• Spectral Selection high and low bits (1). Not used in JPEG Baseline. Always 00H

Appended to this is the actual compressed image data.
Each consecutive MCU data is added bit by bit with no padding i.e. there are no byte boundaries
from one MCUs data to the next. The only padding that occurs is at the end of the scan when the
remaining bits in the last byte to be filled with 1 are if the byte is incomplete.
If ever, throughout the scan, a situation occurs where a byte contains an FFH then a 00H must be
inserted after it to ensure the decoder does not mistake it for a new segment.
end of image (eoi) segment

This segment is always 2 bytes long and always contains the marker FFD9H.

Select an image in JPEF format and convert it into PNG format with help of
multimedia tools.

12.5 Moving picture experts Group (MpeG)

The MPEG is not actually a format but short for Moving Picture Experts Group, a working group
of ISO/IEC that develops video and audio encoding standards. But in most cases MPEG is taken
for the name of the standard formats introduced by that group.
There are several MPEG formats, all standardized:
• MPEG-1 is initial video and audio compression standard. The video quality of this standard
is almost as good as a VHS tape.
• MPEG-2 compared with MPEG-1, it has better quality of coding, multi-channel sound and
higher image resolution. Due to all this MPEG-2 became standard in digital television and
DVD industry.

• MPEG Audio Layer-3 is used for audio compression and creates almost CD quality sound.
Previous versions were MPEG Audio Layer 1 and 2. Today most people know this format
as MP3.

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