DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 14.7 Example 9-frame
GOP without B-pictures
I0P1P2P3P4P5P6P7P8 Figure 14.8 GOP reordered
for transmission
I0P3B1B2P6B4B5(I9)B7B8 ISO/IEC 11172-1, Coding of
moving pictures and associated
audio for digital storage media at up
to about 1,5 Mbit/s – Part 1:
Systems [MPEG-1].
encoding delay. (If the sequence were coded without
B-pictures, as depicted in Figure 14.7, transmission of
the coded information for P 1 would not be subject to
this two-picture delay.) Coding delay can make MPEG
with B-pictures unsuitable for realtime two-way applications
such as teleconferencing.
If the coded 9-picture GOP of Figure 14.6 were transmitted
in that order, then the decoder would have to
hold the coded B 1 and B 2 data in a buffer while
receiving and decoding P 3 ; only when decoding of P 3
was complete could decoding of B 1 start. The encoder
must buffer the B 1 and B 2 pictures no matter what;
however, to avoid the corresponding consumption of
buffer memory at the decoder, MPEG-2 specifies that
coded B-picture information is reordered so as to be
transmitted after the coded anchor picture. Figure 14.8
indicates the pictures as reordered for transmission.
I have placed I 9 in parentheses because it belongs to
the next GOP; the GOP header precedes it. Here, B 7
and B 8 follow the GOP header.
MPEG-1
The original MPEG effort resulted in a standard now
called MPEG-1; it comprises five parts. In the margin,
I cite Part 1: Systems. There are additional parts –
Part 2: Video; Part 3: Audio; Part 4: Compliance testing;
and Part 5: Software simulation. MPEG-1 was used in
consumer systems such as CD-V, and has been
deployed in multimedia applications. MPEG-1 was optimized
for the coding of progressive 352×240 images at
30 frames per second. MPEG-1 has no provision for
interlace. When 480i29.97 or 576i25 video is coded
with MPEG-1 at typical data rates, the first field of each
frame is coded as if it were progressive; the second field
is dropped. At its intended data rate of about 1.5 Mb/s,
MPEG-1 delivers VHS-quality images.
For video broadcast, MPEG-1 has been superseded by
MPEG-2. An MPEG-2 decoder must decode MPEG-1
constrained-parameter bitstream (CPB) sequences – to
be discussed in the caption to Table 40.1, on
page 475 – so I will not discuss MPEG-1 further.
CHAPTER 14 INTRODUCTION TO VIDEO COMPRESSION 125