MPEG-4 in Broadband Streaming Applications, Part II

Editor’s Note: We present part II of an article authored by Dr. Mukta L. Kar, CableLabs Senior Advisor, Digital Network Architecture, on MPEG-4 use in broadband streaming applications. The emerging MPEG-4 standard will provide new opportunities to broadband environments. Streaming media, enabled through MPEG-4 technologies, can create personal viewing and interactive applications, such as video-on-demand (VOD), targeted advertising, and hot-button interactive product purchasing. This article describes some of the newer capabilities added to the MPEG-4 standard that are advantageous to broadband environments. Please see Specs News & Technology from CableLabs®, Vol. 13, Number 2 for part I of this article.

In a digital environment, statistical multiplexing of compressed video streams helps to utilize digital channels better than constant bitrate-encoded streams, assuming that the peak demand for bits from all video encoders does not coincide. The important constraint here is that all video needs to be encoded while being multiplexed. A previously compressed stream typically had to be decompressed and re-encoded before it could be inserted in a statistical multiplex. This is a detriment to local ad insertion because of equipment cost of the stream decompression and recompression, as well as visual quality degradation. As an alternative, a remultiplexer can be used to manipulate or "groom" the multiplexed stream in the compressed domain. By definition, a remultiplexer receives one or more multiplexed streams as input and creates a new output multiplexed stream from local operator-selected programs, such as local ads. Nominally, a remultiplexer does not alter bitrates while constructing a new multiplex out of the input streams. The technology that deals with multiplexing compressed video streams and trims the resulting multiplex to match an assigned constant total transmission channel bitrate, is known as rate-remultiplexing. Rate remultiplexing meets the latter constraint by transcoding individual video streams within the output multiplex. Transcoding is the technique by which a compressed video stream is translated to a lower bitrate strictly within the compressed domain. It can reduce the bitrate of MPEG-2-compressed video without fully decoding and re-encoding a bitstream. Thus, without cascaded compression, degradation in picture quality is not noticeable with occasional or moderate reductions in the average bitrate of individual video streams.

In addition, through rate-remultiplexing techniques, storage requirements for commercials can be reduced by storing only one high-quality version in the server and using rate-remultiplexing to adjust its bitrate. Rate-remultiplexing technology provides the capability to insert compressed digital commercials into digital channels at the headend and removes the need to match the bitrate of the locally compressed commercial with that of the remotely transmitted program, or creating and storing different bitrate versions of the commercials in the ad server.

While insertion of MPEG-2-coded advertisements into a channel with stat-muxed video requires bitrate transcoding of the advertisement stream, MPEG-4 coding schemes not only allow use of coding tools that are more attractive for advertisements (such as natural video merged with synthetic video with lots of scene changes), they allow for lowering of bit rates to values that are far lower than MPEG-2 stat-mux stream rates. Using a combination of synthetic and natural hybrid tools, advertisements could be authored at rates lower than 500 Kbps with quality that matches that of the MPEG-2-coded video. MPEG-4 provides tools, such as simple, advanced simple, and FGS profiles (visual) capability, and animated 2d mesh, basic animated texture, scalable texture and simple face for coding advertisements on television today.

As advertisements are typically not coded in real-time, MPEG-4’s combination of natural video (including sprites) and synthetic video tools provide a very efficient coding scheme for these applications, even without the use of MPEG-4 system tools, such as BIFS. With the use of MPEG-4 coding, multiple advertisement streams can now be inserted into a stat-mux channel instead of a bitrate-transcoded, single advertisement stream using MPEG-2. Currently, standards have been completed in both WG11 for carriage of MPEG-4 streams in networks that carry MPEG-2 transport [1], and the Advanced Television Systems Committee (ATSC) that provide for implementation of "targeted advertisements" at the consumer premises equipment by use of appropriate signaling in broadcast streams [11].

Amendment 7 to MPEG-2 systems [1] specifications specifies the insertion of MPEG-4 audio and visual elementary streams into an MPEG-2 transport multiplex with synchronization using the MPEG-2 STC (System Time Clock). In addition, the amendment specifies the use of a MPEG-4 system layer, such as Synchronization Layer (that duplicates some of the PES layer functions). Applications such as AICI (Advanced Interactive Content Initiative) use this amendment for insertion and synchronization of MPEG-4 content with MPEG-2 content. In addition, they also have used BIFS streams to generate advanced compositions on the display screen with user interaction enabled even in a broadcast environment.

The ATSC specification defines signaling based on the system information part of the standard (called Program and System Information Protocol PSIP) for a function called "Directed Channel Change DCC." This is a "virtual" re-tuning of the viewer channel to another part of the transport multiplex at specified timing based upon events, as well as user preference settings in the receiver. The switch from the channel being watched to a "directed" channel can occur for criterion such as program identifier, demographic category, postal codes, content subject category, authorization level (premium subscribers) and content advisory values. This information is sent as an MPEG private_section at regular intervals and at the activation time (which could be the cue time of ad insertion), the receiver can switch to the audio, video, and data PIDs based upon the directed channel change table and switch criterion. Switching back to the original viewed program occurs at the end of the ad-insertion event.

There are two methods of "targeted" ad-insertion that can occur using the above two standards. In the first method, the multiple MPEG-4-based ad streams are inserted into the stat-mux channel during the ad-insertion period and the DCC directs the receiver to the appropriate ad channel based upon user preferences. In the second method, several MPEG-4-based ad streams can be generated in a multi-program transport stream that can be shared between several stat-mux channels and the DCC for each of the stat-mux channel can direct the user to one of the ad streams in the large multiplex. The second method allows for larger targeting of streams and for more user preference categories.

This article discussed some of the advanced features of MPEG-4 standards, which can be implemented to realize advanced A/V content delivery system and targeted commercial insertion in an all-digital environment. Streaming media applications utilize network capability effectively and deliver viewer-preferred content on a one-to-one basis. Streaming media, which delivers content using IP transport, provides unlimited choice to consumers in terms of content available anywhere in the world. This article also discussed the enabling of low bitrate "effective" advertisement authoring by emerging MPEG-4 natural and synthetic video coding tools, as well as the use of two additional standards that enable "targeted" ad-insertion.

1] ISO/IEC 13818-1 (Systems), ISO/IEC 13818-2 (Video) and ISO/IEC 13818-1 Amendment 7, December 2000, ISO/IEC 14696-1 (Systems), ISO/IEC 14696-2 (Video).

2] Hyder M. Radha, Mihaela van der Schaar, and Yingwei Chen, " The MPEG-4 Fine-Grained Scalable Video Coding Method for Multimedia Streaming over IP," IEEE Trans. On Multimedia, Vol. 3, No.1, March 2001.

3] ISO/IEC 14696-2 MPEG-4 Video Amendment 4: Streaming Video profile, MPEG output document N3518.

4] MPEG-4 Overview & Profiles, MPEG home page (www.cselt.it/mpeg/).

5] Mukta l. Kar, and Sam Narasimhan, "Targeted advertisement insertion using MPEG-4 coding and SCTE standards for cue-messaging (DVS 253) and API (DVS 380)."

6] Mukta L, Kar, Bill Kostka, Majid Chelehmal, and Munsi Haque, " Streaming Over HFC-MPEG-2 or IP or Both?"

7] Julien Signes, "Binary Format for Scene (BIFS): Combining MPEG-4 media to build rich multimedia services", France Telecom R&D Document, CA, USA 1(650) 875-1516.

8] Mukta Kar, Majid Chelehmal, and Richard S. Prodan, "Digital Program Insertion for Local Advertising," NCTA Technical Paper, 1998.

9] Mukta Kar, Sam Narasimhan, and Richard S. Prodan, "Local Commercial Insertion in the Digital Headend," NCTA Technical Paper, 2000.

10] Richard S. Prodan, Mukta Kar, and Majid Chelehmal, "Rate-remultiplexing: An Optimum Bandwidth Utilization Technology," NCTA Technical Paper, 1999.

11] Amendment 1A to ATSC standard A/65A, "Program and System Information Protocol for Terrestrial and Broadcast and Cable", May 31, 2000.

12] Digital Program Insertion Cueing Message for Cable, SCTE Standard DVS 253, December, 1999. Digital Program Insertion Splicing API, SCTE Standard DVS 380 (committee draft).