By Mukta L. Kar, Cable Television Laboratories, Inc., Sam Narasimhan, Motorola Broadband Communication, Richard S. Prodan, Terayon Communication Systems
Existing ad insertion systems enable cable headends and broadcast affiliates to insert locally generated commercials and short programs in a channel seamlessly before delivery to the home. The revenue generated by local ads and short programs is very significant. Current ad insertion systems are hybrid systems, where commercials are stored in the ad server in digitally compressed form, while splicing is accomplished using analog techniques. The compressed commercial is decompressed and converted to analog format before insertion into an analog channel. The complete digital technology for delivery of compressed digital audio, video, and data is superior to existing analog methods. However, dealing with compressed video poses several challenges. One such challenge is splicing into a compressed digital bitstream compliant to the MPEG-2 standard seamlessly, that is to say, without adversely affecting the display due to decoding discontinuities at the insertion point. These applications involve insertion of locally generated compressed digital commercials and short programs into a digital channel containing previously multiplexed and digitally compressed programs. The Society of Motion Picture and Television Engineers (SMPTE) has defined standard SMPTE 312M “Splice Points and Cue Messages for MPEG-2 Transport Streams” to promote interoperability of compressed digital video splicing equipment. SMPTE 312M is aimed at studio broadcast and requires a constrained (a synonym for preconditioned) MPEG-2 transport stream. For use in deployed digital cable systems, this will require significant changes to encoding methodologies to enforce predetermined conditions at the splice points. The Society of Cable Telecommunications Engineers (SCTE) Digital Video Subcommittee (DVS) has established a splicing standard DVS 253 “Digital Program Insertion Cueing Message for Cable” to provide cue messaging and splicing in a more digital headend friendly manner, which does not require restrictions or constraints on MPEG-2-compliant transport streams. This paper will present a brief overview of existing analog/hybrid program insertion systems. The limitations of SMPTE 312M will be discussed. The solutions employed in DVS 253 will be described including digital headend friendly features. The implementation of DVS 253 to insert compressed commercials at the headend, including the issue of invisibility from commercial killers, will be addressed. As cost is related to complexity and flexibility of implementation, complexity issues and network operational constraints will also be examined for various implementations of digital program insertion systems and constituent components.