PREPROCESSING METHOD FOR ADAPTING MPEG-4 DATA STREAMS TO THE INTERNET NETWORK

Abstract: The invention relates to a preprocessing method for adapting digital data streams such as MPEG-4 ones to the so-called real time protocol (RTP) used by the "Internet" network. A synchronization layer SL being the interface defined between MPEG-4 media layers and the RTP stack, a reduced SL header format is specified in order to fit to the RTP header. Considering that the RTP header fields that are common to the SL header specifications are: padding, marker, sequence number, and time stamp, the semantic of the RTP fields according to the MPEG-4 SL data that are carried in the packets is redefined.

Description: Preprocessing method for adapting MPEG-4 data streams to the internet network. The present invention relates to a preprocessing method for adapting digital data streams such as MPEG-4 ones to the so-called real time protocol (RTP) used by the "Internet" network. Multimedia streaming over the internet network is now a daily reality. Services related to this domain are numerous : electronic commerce, interactive games, video on demand, on so on. MPEG-4, described for instance in "Overview of the MPEG-4 Version 1 Standard", ISO/IEC-JTC1/SC29/WG11-N1909, October 1997, and in "MPEG-4 Systems", ISO/IEC-JTC1/SC29/WG11-N1901, November 1997, is a standard for the coding of natural and synthetic audio- visual data in the form of audiovisual objects that are arranged into an audiovisual scene by means of a scene description. The advantages of this MPEG-4 standard in the context of the Internet network are various (excellent picture quality at low bitrates, high interactivity possibilities, ability to mix bi- and tri-dimensional representations,...). The protocol RTP is adapted to the transmission of multimedia data, especially data that have real-time constrains such as audio or video. MPEG has also defined an interface to underling network technologies : the Synchronisation Layer (SL). However, direct mapping from SL-packets to RTP packets is not easy, mainly due to the difference of complexity between the two formats. It is therefore the object of the invention to propose a very simple way to adapt SL-packet to RTP packets by selecting in the SL-header the information that can be used by the RTP protocol. To this end the invention relates to a preprocessing method such as defined in the introduction of the description and which is moreover characterized in that, a synchronization layer SL being the interface defined between MPEG-4 media layers and the RTP stack, a reduced SL header format is specified in order to fit to the RTP header. The invention will now be described in a more detailed manner, with reference to the accompanying drawings in which : Fig.1 shows a header for an RTP data packet ; Fig.2 illustrates the protocol stack for using MPEG-4 over RTP structure. According to the general RTP specification, RTP data packets consist of a 12-byte fixed header (see Fig.l). This header is followed by a variable optional part and the payload (video frames, audio samples). On the other hand, the MPEG-4 SL packet header format can be found in MPEG-4 specifications. If one considers an MPEG-4 implementation for IP networks, the system structure can be presented as indicated in Fig.2. The SL layer is the interface between MPEG-4 media layers and the RTP stack. Then, the SL layer has to be configured to convey properly the SL information through the network. Different propositions have already been made. What is proposed here is to specify a reduced SL header format that will fit to the RTP header. The RTP header fields that can be common to the SL header specifications are : a) Padding (1) b) Marker (1) c) Sequence Number (16) d) Time Stamp (32) Then, using the fields that are common in RTP and SL headers as mentioned above, and regarding the protocol stack of Fig.2, the semantic would be as in Table 1 , giving the semantic structure for SL/RTP mapping : This very simple and efficient way to map MPEG-4 SL packets to RTP packets is based on the fact that one has defined a SL_Config_Descriptor structure that forces to reduce the SL header to what values can be stored in RTP header fields, thus defining the semantic of the RTP fields according to the MPEG-4 SL data that are carried in the packets : a) Padding flag will mean that the packet is only made of padding data ; b) Marker bit is indicating the start of an Access Unit ; c) Sequence Number is coded on 16 bits to respect RTP header specification and according to MPEG-4 that allows variable length of the SL fields ; d) Time Stamp field of the RTP packet conveys the two SL header time stamps (decoding and compositions). Each SL time stamp is coded on 16 bits and concatenated to form the TimeStamp of RTP which is coded on 32 bits. What is additional, in the case of the present invention, is the need for the definition of an RTP payload type to refer to MPEG-4 streams (as usual for other data types to be transported by RTP). To preserve SL full header, the optional "extension fields" of the RTP header could be used to carry the information that are not present in the classic RTP header, with a main drawback : overhead bytes.

Claims: CLAIMS: 1. A preprocessing method for adapting digital data streams such as MPEG-4 ones to the so-called real time protocol (RTP) used by the "Internet" network, wherein, a synchronization layer SL being the interface defined between MPEG-4 media layers and the RTP stack, a reduced SL header format is specified in order to fit to the RTP header. 2. A method according to claim 1, wherein the RTP header fields that are common to the SL header specifications are : padding, marker, sequence number, and time stamp. 3. A method according to claim 2, wherein the semantic of the RTP fields according to the MPEG-4 SL data that are carried in the packets is defined as follows : a) Padding flag means that the packet is only made of padding data ; b) Marker bit is indicating the start of an Access Unit ; c) Sequence Number is coded on 16 bits in order to respect RTP header specification and according to MPEG-4 that allows variable length of the SL fields ; d) Time Stamp field of the RTP packet conveys the two SL header time stamps "decoding" and "compositions", each SL time stamp being coded on 16 bits and concatenated to form the TimeStamp of RTP which is coded on 32 bits.

Inventors: HERRMANN LAURENT, HERRMANN, LAURENT
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H04L29
IPC codes: H04J3, H04L12, H04L29, H04N7