Digital splicing
May 1, 2007 12:00 PM, BY PETER CHAVE
DPI platforms create ad insertion growth and revenue for broadcasters.
Splicer mechanics
A digital splicer must complete a sequence of actions within a short time frame to achieve a seamless, frame-accurate splice. As with most things that seem simple on the surface, there's considerable complexity underneath.
The completion of a seamless, frame-accurate splice, for example, isn't a matter of merely switching from one stream to another, as with analog or uncompressed digital video. Compression encoders generally process and buffer MPEG frames along two parallel domains: presentation order and decode order.
Figure 2. This group of pictures (GOP) show the typical digital-into-digital splice environment. A) First, the splicer searches for the out-point splice within the primary channel (at intra-frames, labeled “I,” or predictive frames, labeled “P”) as well as the in-point splice within the insertion channel (at I-frames). B) Next is the temporal alignment for splice in and out points and the close of the GOP in the primary channel for a seamless splice. C) The splice is completed and insert channel frames are placed into the output stream. Buffer management is required on the output buffer to ensure the video buffer verifi er model is not violated in the new spliced output stream.
A group of pictures (GOP) representing the primary video channel may enter an encoder's buffer in a B-P-B-B-I-B-B-P order, for example, as shown in Figure 2. Only an intra-frame (I-frame) can initiate digital splicing in, for splicing into the primary channel. Splicing out can only be performed on an anchor frame, which is an I-frame or predictive frame (P-frame).
At the onset of a scheduled avail, the splicer begins looking for an out-point splicing event within the primary stream and an in splice point in the insertion stream for an upcoming event. The output buffer provides the cushion of time necessary to apply as-needed transrating during a splice event.
Once the I-frame for the in splice and anchor frame for the out splice have been identified, the GOP in the primary channel closes to allow a clean transition to the insert channel. Then the splice is initiated.
At that moment, the insertion channel is placed into the output stream. When the ad avail finishes and a cue message indicates a return to network source, the splicer engine has to look for an out from the insertion and in back to the network to complete the seamless splice. All of this happens within the broadcast timeline.
Buffering plays a critical role in splicing. Likewise, an important part of a splicer's workload involves vigilantly managing its output buffers to prevent overflows and video buffer verifier (VBV) breaches.
What's next
DPI splicing for ad insertion continues to evolve, most notably in the direction of segmentation, targeting and addressability. One manifestation, for example, is the personalized or enhanced ad, which involves attaching an additional data stream within the digital advertisement. That data stream carries localized information, such as the address of the local store or a clickable coupon. A set-top box in the home receives and processes this information.
Also on the near-term horizon is the industry-wide shift to advanced video compression engines, such as H.264. The good news is that the increased flexibility of the H.264 encoding standard allows for longer and more complex GOP structures. The challenge that this benefit presents is the subsequent increase in processing complexity required.
Both cases — segmented advertising and advanced compression — require a powerful encoding and splicing platform. And the vendor community is responding.
Increased revenue
The intent of next-generation encoding and splicing platforms is to enable broadcasters and cable providers to increase splice density by as much as 20 times that achieved by legacy systems. In turn, increased splice density provides the room for revenue growth via new, segmented advertising models.
Again, in the overall digital transition, revenue-generating opportunities, or businesses poised for substantial revenue growth, tend to receive the most attention. DPI for advertising insertion is an early example. Other digital-in-digital splicing models will surely follow, particularly if the fundamentals are in place to enable more flexible splice opportunities.
Peter Chave is product manager for Scientific Atlanta, a Cisco company.
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