Local ad insertion has long been a method used by TV networks and service providers of all stripes to supplement revenues. It allows them to combine national programming with local advertising, thus profiting from airing commercials for local businesses.

The growth of digital TV content has brought about the need for ad insertion into digital programs. Digital program insertion (DPI) can enhance the potential for additional income if done effectively. On top of performing ad insertion, DPI allows network operators to switch programs seamlessly, as in moving from a national feed to a local feed, for example. Ad insertion techniques Ad insertion products are comprehensive systems that integrate several different components including storage devices, a compression system, scheduling programs, distribution networks and a "switcher/splicer." Figure 1 details the components of a basic ad insertion system. Storage is the central component in an ad insertion system. Ads are stored in videotape banks in analog format or in video servers in digital form after being compressed using, for example, the MPEG standard. The server receives new programming and advertising content from sources such as tapes, encoders, compressed files on DVDs or from the distribution network.

The digital compression system consists of encoders that compress the ads to high-quality constant bit rate (CBR) MPEG files for storage and playback. Scheduling systems create the time controls for the ad or clip database, arrange the ads on the server, build ad packages, and remap the outputs from the video server. Billing and management functions may also be included with scheduling. Contribution is performed through the master headend by file transfer over an existing network. This process is necessary for distributing the compressed ads or programs to several sites.

When both the original program and stored ads are in analog TV format, a standard switch can function as the splicer. Selecting between the incoming program and the stored ads is predetermined. The switch becomes a multiplexing unit with splicing capabilities for digital TV services. The splicing of MPEG streams allows local ads to be inserted into digital programming seamlessly and ensures smooth and uninterrupted delivery of the content. Analog ad insertion Original ad insertion systems operated solely on analog programs and ads. National feeds would sometimes include gaps into which local ads were to be dubbed. The splicer would switch, at predetermined times, from the national content to local ads or programming. Automated analog playout systems started including "cue tones" in the original content, such as a movie, to indicate the points of insertion for ads. These audio cue tones were used to trigger insertion into the network feed with a video switch.

When TV networks and service providers started transmitting digital compressed programming, local content insertion still utilized analog methods. In the analog workflow, incoming MPEG-2 content is decoded back to analog formats (NTSC, PAL). The local content, if stored digitally, is also converted to analog and inserted into the output stream. Systems that transmit digital content to the user’s home would require that the modified program be encoded again to MPEG. Most ad insertion solutions available today use analog insertion techniques. Figure 2 illustrates an analog ad insertion system. Though this process is effective for local content replacement, it is problematic. The process of two encoding/decoding passes can degrade overall video quality. MPEG is a "lossy" compression system that deletes unnecessary information that the human eye does not see. Multiple encoding and decoding may remove too much information, causing visual artifacts.

A more important issue to the provider is that analog insertion in a digital system is a very expensive process. Multiple decoders and an encoder may be required to create the system, which thus requires increased time and labor for system configuration and management. Digital program insertion DPI is a completely digital version of local content insertion. DPI requires that original content include ad insertion markers in the MPEG-2 transport stream as part of the service information tables and according to the SCTE 351 standard. Special encoders enable creation of the program insertion markers in the transport stream during the contribution process. These digital markers are similar to the "cue tones" used in analog ad insertion. At the local station or headend, the splicer detects insertion markers in the transport stream. The insertion markers trigger the video server to insert the relevant ads automatically. Communication between the splicer and the video server is based on the SCTE 302 protocol. Benefits of DPI Digital satellite and cable systems are delivering an ever-growing number of channels to their viewers. Increasingly, this content is in compressed form to digital set-top boxes in customers’ homes. These systems require broadcasters to use digital turn-around (DTA) solutions, which remove a digital program from a national source and add it to a multichannel, digital multiplexed stream – digital in to digital out. DTA-based systems can benefit from using DPI.

Because programming remains in MPEG throughout the whole process, multiple encoding and decoding is not required, and thus image quality is preserved. The quality of the content is based solely on original encoding parameters that are controlled by the broadcaster.

Equipment costs for DPI are greatly reduced compared to those associated with analog insertion. There is no need for decoders to convert the original content and local ads to analog formats. Likewise, there is no encoder required to recompress the modified content back to digital format. The savings in equipment costs is multiplied by the number of digital channels the local service provider offers on its system.

Another issue that must be considered with DPI systems is dealing with different bit rates of the original and local content. Because of system data rate limitations and the ads being stored in CBR format, the DPI system must be able to adjust the bit rate of the local content to match the original program. This process is known as "transrating" and is performed in the digital domain. Transrating may also be required on the modified content to be combined properly into a multichannel multiplex that has a limited aggregate bit rate. A single device may now perform both the transrating and splicing functions. (See Figure 3.) Taking targeted ads further Continually evolving technologies for targeted content insertion hold the promise of reaching consumers on an individual basis. The increasing adoption of digital set-top boxes by consumers and the wholesale migration of the broadcast industry to digital operations, in conjunction with advances in the leveraging of purchasing and marketing data, mean that DPI has the potential to bring tailored advertising to specific viewers at a household level. Robust encoding and multiplexing solutions allow all players in the digital video food chain to maximize local ad dollars for a valuable bottom-line boost. Michael Acer is a manager of customer engineering for Scopus Video Networks. Reach him at macer@scopus.net.

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