Graphics Reference
In-Depth Information
MPEG-2 transport streams use fixed-length 188-byte packets, which do not
directly align with variable length HEVC NAL units. The start of NAL units in
MPEG-2 systems can be identified using unique start codes along with the optional
byte stream format. Avoidance of start code emulation within NAL units has been
part of the NAL unit designs in both AVC and HEVC.
At the time of this writing, a proposed real-time protocol (RTP) payload format
for HEVC [ 11 ] is in Internet Draft status in the Internet Engineering Task Force
(IETF). RTP packets are of variable size, enabling alignment between variable
length NAL units and RTP packets. The aggregation packet allows multiple NAL
units to be placed within a single RTP packet. For example, parameter sets and/or
SEI messages may be placed within the same packet as coded slice segments. The
fragmentation unit packet allows a coded slice segment to be split into two or more
RTP packets. However, the use of fragmentation units can negatively impact error
resiliency, because loss of one packet of a fragmented slice renders the other packets
in the slice undecodable. Slices are designed to be individually decodable, with no
parsing dependencies between slices. However, parsing of slices does depend upon
parameter sets.
In HEVC, like in AVC, parameter sets may be sent in-band or out-of-band, and
may be sent repeatedly for improved error resiliency. Many systems specifications
provide special handling of parameter sets, to benefit from advance availability
of sequence and bitstream level information. For example, when RTP is used,
parameter sets may be carried in the Session Description Protocol (SDP), which
is used for describing streaming media parameters for session announcement and
capabilities exchange/negotiation. In ISOBMFF, parameter sets may be carried in
the sample entry of a track.
A Media Aware Network Element (MANE), located in a network between an
encoder and decoder, may perform adaptation upon packets of a video transmission,
based upon examination of the NAL unit header. As described in Sect. 2.2 ,allNAL
units contain the NAL unit header followed by the NAL unit payload data. The
NAL unit header is of fixed length, and contains a temporal identifier value, and
reserved bits for a layer identifier to be used in the extensions to HEVC. A MANE
may easily access the temporal ID of each packet, and discard those packets whose
temporal ID value exceeds a target value, creating a compliant sub-bitstream at a
lower frame rate. For systems like MPEG-2 which do not align NAL units with
packets, each temporal sub-layer may be placed in a separate Packetized Elementary
Streams (PES) to enable sub-bitstream extraction based upon the PES.
The NAL unit header also contains a NAL unit type, which describes the coded
picture type. As described in Sect. 2.2.2.1 , certain NAL unit type values correspond
to random access points. Identification of random access points is particularly
important for ISOBMFF. In ISOBMFF, the location of random access points within
a bitstream can be identified in multiple ways, such as by the sync sample table
and/or some random access points related sample groups.
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