5. Video

5. Video

The following sections describe the video encodings that are defined in this memo and give their abbreviated names used for identification. These video encodings and their payload types are listed in Table 5.

All of these video encodings use an RTP timestamp frequency of 90,000 Hz, the same as the MPEG presentation time stamp frequency. This frequency yields exact integer timestamp increments for the typical 24 (HDTV), 25 (PAL), and 29.97 (NTSC) and 30 Hz (HDTV) frame rates and 50, 59.94 and 60 Hz field rates. While 90 kHz is the RECOMMENDED rate for future video encodings used within this profile, other rates MAY be used. However, it is not sufficient to use the video frame rate (typically between 15 and 30 Hz) because that does not provide adequate resolution for typical synchronization requirements when calculating the RTP timestamp corresponding to the NTP timestamp in an RTCP SR packet. The timestamp resolution MUST also be sufficient for the jitter estimate contained in the receiver reports.

For most of these video encodings, the RTP timestamp encodes the sampling instant of the video image contained in the RTP data packet. If a video image occupies more than one packet, the timestamp is the same on all of those packets. Packets from different video images are distinguished by their different timestamps.

Most of these video encodings also specify that the marker bit of the RTP header SHOULD be set to one in the last packet of a video frame and otherwise set to zero. Thus, it is not necessary to wait for a following packet with a different timestamp to detect that a new frame should be displayed.

5.1 CelB

The CELL-B encoding is a proprietary encoding proposed by Sun Microsystems. The byte stream format is described in RFC 2029 [18].

5.2 JPEG

The encoding is specified in ISO Standards 10918-1 and 10918-2. The RTP payload format is as specified in RFC 2435 [19].

5.3 H261

The encoding is specified in ITU-T Recommendation H.261, "Video codec for audiovisual services at p x 64 kbit/s". The packetization and RTP-specific properties are described in RFC 2032 [20].

5.4 H263

The encoding is specified in the 1996 version of ITU-T Recommendation H.263, "Video coding for low bit rate communication". The packetization and RTP-specific properties are described in RFC 2190 [21]. The H263-1998 payload format is RECOMMENDED over this one for use by new implementations.

5.5 H263-1998

The encoding is specified in the 1998 version of ITU-T Recommendation H.263, "Video coding for low bit rate communication". The packetization and RTP-specific properties are described in RFC 2429 [22]. Because the 1998 version of H.263 is a superset of the 1996 syntax, this payload format can also be used with the 1996 version of H.263, and is RECOMMENDED for this use by new implementations. This payload format does not replace RFC 2190, which continues to be used by existing implementations, and may be required for backward compatibility in new implementations. Implementations using the new features of the 1998 version of H.263 MUST use the payload format described in RFC 2429.

5.6 MPV

MPV designates the use of MPEG-1 and MPEG-2 video encoding elementary streams as specified in ISO Standards ISO/IEC 11172 and 13818-2, respectively. The RTP payload format is as specified in RFC 2250 [14], Section 3.

The MIME registration for MPV in RFC 3555 [7] specifies a parameter that MAY be used with MIME or SDP to restrict the selection of the type of MPEG video.

5.7 MP2T

MP2T designates the use of MPEG-2 transport streams, for either audio or video. The RTP payload format is described in RFC 2250 [14], Section 2.

5.8 nv

The encoding is implemented in the program `nv', version 4, developed at Xerox PARC by Ron Frederick. Further information is available from the author:

Ron Frederick Blue Coat Systems Inc. 650 Almanor Avenue Sunnyvale, CA 94085 United States EMail: [email protected]