MPEG-4 in Broadband Streaming Applications Part I

Streaming media adds a new dimension by allowing a personally designed viewing experience. The viewer’s choice is not limited to local headend offerings. With streaming media, a viewer can request content at a time when they wish to see it, and from a source that may reside anywhere in the world. This content can be delivered through a headend-managed, DOCSIS™-enabled IP transport stream or through a traditional MPEG-2 delivery system to a set-top box (STB) [6]. It also adds in the ability to target content to a specific type of viewer rather than broadcasting the same content to the entire audience. A streaming media session adds increased value by capturing viewers who are definitely interested in the program. Revenue can come from the customer actually purchasing the streaming media content or through the use of targeted advertisements that are more relevant to the viewers [5].

Through streaming media, customers can actually purchase content that they are willing to watch at their convenience. One of the most visible applications of this technology is video-on-demand (VoD) service where movies can be streamed to customers at their request. The application can be extended to other types of content, such as "how to" shows, news programs, and weekly serials. This service can be enhanced by adding in VCR capabilities, such as pause, fast-forward, and rewind. However, there would be an increased amount of downstream data generated by each customer request [4], and some upstream traffic due to interactive VCR features. The additional traffic in both directions would be an added strain to a two-way cable plant. For services like these to exist on a large scale in a two-way cable infrastructure, a technology like MPEG-4 must be implemented to permit high-quality interactive video to stream at low bit rates [4].

Targeted Advertising

Another revenue source opened up by streaming media is targeted advertising. In the existing method of broadcast delivery, national commercials are inserted into the program stream by the networks before delivery to cable headends and broadcast affiliates. However, advertisements and short programs originated locally are inserted (overwritten on top of selected national ads) per cue tones delivered by the networks. The existing method of ad delivery is not very efficient. Technically, existing approaches splice local ads into the nationally distributed program stream using analog techniques by uncompressing and converting digital streams into an analog format. The output is then delivered over the local cable network as an analog signal with a resulting degradation in visual quality as well as a loss in bandwidth revenue. In terms of scope, these commercials are broadcast to the entire regional viewing audience, but fail to be relevant to many viewers. With streaming combined with digital statistical remultiplexing techniques, these slots can contain different local digitally inserted commercials that can be targeted to individual viewers [5]. For instance, a car company could put out a different commercial for each of their product lines based upon a viewer’s profile for the same commercial time slot. This way the most relevant commercial reaches the viewer. As techniques become developed further, streaming also can enhance broadcast commercials by combining additional data to create a higher visual quality commercial instead of having the commercial constrained to the encoding bit rate of the channel. Lastly, the commercial can be made more interactive by providing a way for viewers to stream back their responses. An example of this would be the ability to immediately purchase an advertised product. Similar to VoD, these types of services will increase traffic loads in the cable network to accommodate individualized information generated by the streaming application. Technology, such as MPEG-4, will be necessary to allow these services to exist on a large scale by reducing the bandwidth required for low bit rate transmissions and allowing interactivity [1][4].

The MPEG-1 standard was created to satisfy the need for storage applications, such as CD-ROMs, and adopted frame-based video compression methods. Later MPEG-2 standards were developed to meet the requirements of the broadcast industry. MPEG-2 included field/frame-based video coding to deal with interlaced video as well. This standard was optimized primarily for one-way broadcast delivery of television content [1]. The MPEG-4 standard initially differed by its focus on low bit rate coding applications over IP connections. Later its scope was increased to cover a wider range of multimedia applications including videophone, interactive TV, and streaming video [4]. Additionally, to include interactive capability at various levels, the coding paradigm changed from frame-based to content-based or object-based coding. One of the more important advantages of MPEG-4 is that it is well optimized for scalable low bit rate (LBR) transmissions (less than 2 Mbps) and its ability to selectively incorporate natural and synthetic objects into a scene. These advantages will allow streaming media to exist on a large scale by economizing the available bandwidth while allowing interactivity to be integrated [3].

There are five general categories of coders (Figure 1): 1) video, 2) audio, 3) graphics, 4) text, and 5) scene. MPEG-4 adapts each of its codecs to conform to multiple profiles and levels of transmission to accommodate different delivery formats. The video and audio codecs are used on natural video and audio objects optimized for good quality at low bit rates. In addition, MPEG-4 adds in a spatial and temporal scalability factor to traditional A/V coding and provides graceful degradation of objects during times of congestion [3]. The graphics coder provides a means to animate and render synthetic objects. Synthetic objects can be computer-generated objects with interactive components. An example of this would be a "hot-button" to purchase an advertised product. The text coder provides an efficient way to code text. The scene coder—binary format for scene (BIFS)—is responsible for scene composition and rendering. This coder can manipulate (spatially, temporally), layer and even edit out objects from the scene [7]. It also can add or delete streams such that a directed channel change (DCC) can occur to allow for targeted advertising [12].

This base/enhancement layer partition provides several advantages. In an IP-congested environment, a graceful degradation of the image (by sacrificing the quality of certain visual objects) can occur during times of net congestion through the use of an embedded bit stream property. Also, multiple consumer premises equipment (CPE) with different bandwidths, quality-of-service (QoS) parameters, or processing restrictions, can view the video stream at different resolutions or qualities due to the stream’s scalability and embedded properties. For lower bandwidth connections, selective enhancements through bitplane shifting and coefficient weighting can improve visual quality by prioritizing enhancements of certain regions of the video first [2][3]. This approach also can allow for a picture-in-picture format without doubling the bandwidth. In VoD or personal video recording (PVR) applications, the base layer can provide a quick, continuous search capability within the bandwidth constraints of the connection. Since elementary streams do not have to come from the same source (this is determined by the BIFS information), enhancement layers can be added to increase the visual quality of an advertisement without a corresponding increase in bandwidth.

Current Streaming Media Technology

Existing media streaming codecs largely used on today’s PCs do not take advantage of all the interactive services enabled by a sequence mix of synthetic and natural objects. Current implementation by several equally popular proprietary codecs limit network streaming applications to video- and audio-focused material delivered to the PC. As coverage extends to other CPE, and implementation develops more personal interactive applications, the complexity in codec and system development will either lead to a limit in growth due to the support of too many proprietary formats or will drive the acceptance of a single format [4]. This single format may arise from a proprietary implementation or from the MPEG-4 standardization process. In either case, support of the full scope of streaming media on a large scale will require the adoption of MPEG-4 features, if not the exact approaches to implement them.

Ad Insertion

The hybrid method is fine if content is delivered from a local headend in analog format. But in an all-digital delivery environment, a standardized digital-insertion method [9], where a compressed commercial is inserted into a compressed network program in the headend, is the most desirable. With this objective, the SCTE digital video subcommittee (DVS) developed a standard [11] entitled "Digital Program Insertion Cueing Message for Cable (DVS 253)." This standard defines just the cue-message and does not impose any constraints on insertion/splicing equipment. The cue message carries timing information using the coordinated universal time (GPS UTC) for scheduling, and MPEG PTS time for frame-accurate insertion, which the splicing device may use to perform the splice. Cue messaging, if required, may be passed on to authorized downstream equipment, such as a pass-through via a remultiplexer to a set-top box. The timing correction needed after remultiplexing also may be transmitted to maintain timing accuracy of the cue signal.

In addition to these two message components, each splice command enables splicing of complete programs or individual components of a program (such as video or audio or data) through the use of component_tags enabled by the stream_id_descriptor. Today’s systems use only program-level splicing, where all the components of a program are replaced at the splice point. In the future, programs and advertisements will be "enhanced" using data broadcast and interactive elements. Typical enhanced commercials could include delivery of discount coupons, prize drawings, or free software. These systems will use component-level splicing, where only selected components of a program are replaced at the splice point. This splicing enables pre-loading of data streams that are part of the same program by inserting the data component ahead of A/V content. This may be done to load and run the data enhancements in the receiver’s application engine.

Figure 3 shows the functional block diagram of a digital headend where local commercials are inserted into a program utilizing the cue message multiplexed in the transmitted program. The satellite-integrated receiver demodulates the RF signal to its baseband MPEG-2 transport stream and decrypts it. The cue message, multiplexed in the transport stream is detected and is processed by the splicer and ad server. At the splice_insert time, the splicer switches from the network program to the local ad. At the end of the avail, the splicer switches back to the network program. A headend may have multiple splicers and ad servers interconnected among them so that the same commercial need not be stored in more than one server. The cue message standard, DVS 253, does not specify how to splice between two bitstreams. The techniques and resultant constraints for splicing compressed streams have been left to splicing equipment manufacturers. This adaptability also allows for incorporation of advancements in knowledge, such as MPEG-4-enabled technologies.

Figure 3. Typical Commercial Insertion System per DVS 253

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.

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).

MPEG-4 in Broadband Streaming Applications will be continued in Specs News & Technology from CableLabs®, Vol. 13, Number 3.