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November 30, 2012
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The once and future efficiencies of VBR

Service providers deploying IPTV have tended to use constant bit rate (CBR) as opposed to variable bit rate (VBR) encoding. Several emerging use cases, however, could bring VBR back into play. It also is playing a surreptitious role in making adaptive bit rate (ABR) streaming even more efficient.

“Most IP video deployments today in general are CBR,” said Tom Lattie, VP Product Management at Harmonic. But he pointed to two potential use cases that leverage the efficiencies of varying the amount of output data over time. One is capped VBR, which sets a limit for VBR streams and allows for the re-harvesting of fallow capacity when streams drop below that rate. It has been discussed in the context of bandwidth constraints, for instance, in a VDSL circuit with a 20Mbps maximum downstream. “The idea is you have three HDs, and not totally kill the data service,” Lattie said.

The other use case is progressive download, which uses VBR encoding within a file and enables playback at high or low rates. This would enable “a high level of video quality without wasting bits,” he said.

Lattie said the benefits of VBR are less obvious in an advanced cable environment, where bandwidth may be more abundant than in a DSL network. But it remains an option for cable operators extending their DOCSIS 3.0 platforms and plotting respective IPTV strategies.

“Service providers from emerging markets are very interested in the work we did on self-averaging of VBR,” said ARRIS VP of Advanced Technology Mark Bugajski, who first discussed the benefits of transmitting VBR-formatted IP video streams over multiple bonded channels at an industry event in London nearly five years ago. These service providers today are daunted, however, by the lack of VBR standards. At the same time, Bugajski noted that operators are burdened by “ton of legacy equipment … which has been designed for CBR.”

But the technology has taken off elsewhere. “Netflix and all the OTT providers use not only VBR but also adaptive bit rate (ABR),” Bugajski said. And that raises an innovative application.

Not a stream per se, ABR is actually a collection of synchronized files, typically encoded into CBR, explained Lattie. “But to achieve better video quality, you could say, ‘I’m going to actually encode that sequence as a VBR.’ Which means that across say, a 10-second chunk, I may have 5 seconds at 4 Mbps, and 5 seconds at 1 Mbps, so the average for the file is still about 2 Mbps. But for the hard bits it’s better video quality, because I’m not spending them on the easy bits.”

Lattie described this use of VBR as simply another instance of the basic encoding task: deciding where to spend bits. He said Harmonic can execute the technique, but client support depends upon which version of HTTP Live Streaming (HLS) is being used.