Cable's Quality Question16 May, 2008 By: Holly J. Wagner
Cable companies have been among high-definition disc's most aggressive competitors.
If you can get movies and TV shows in high-definition from your cable company, especially on-demand, why buy Blu-ray at all?
But cable HD may not be all it's cracked up to be. The problem is highly compressed digital channel signals transmitting multiple layers of programming, which maximizes cable company bandwidth, but sometimes at the expense of picture quality. The high-def miracle looks less than miraculous on some cable channels, while others come in crisp and clean.
Not so with Blu-ray, which may present a marketing opportunity for its proponents.
Cablers “send standard-def channels on the same signal as the high-def channel, crowding and sometimes choking the pipe,” said video guru Joe Kane, video engineer and author of Video Essentials, Digital Video Essentials and his most recent tutorial, DVE: HD Basics, available on HD DVD and Blu-ray.
What cable viewers see is largely dependent on how the signal is encoded, and how much it's compressed. The average cable channel may be compressed by a 2-to-1 ratio, but some channels are compressed at a 3-to-1 ratio, which “steps on” or waters down the quality of the high-def signal layered between standard signals that appear on other channels.
Some of the quality loss may come from changes in how the signals are encoded. The bitrate — think of the rate at which water comes out of a hose — also can be a huge factor in signal quality.
“It's potentially a combination of two things: doing a poor job of decoding and re-encoding, or trying to reduce the bitrate to squeeze more in,” Kane said. “They could be compromising the bitrate of MPEG-4 so that no matter what the signal, it would not look as good.”
Bitrate is not the only factor in signal quality, said Comcast spokeswoman Jenni Moyers.
“Signals with higher bitrates do not always result in a higher-quality picture,” she said. “The quality of the source signal can vary from program to program. Was it shot in high-def or upconverted to high-def? And picture quality is fairly subjective. People can set things like color and contrast [at home].”
AVScience Forum poster bfdtv found wide disparities in bitrates on Comcast and Verizon FiOS signals. He posted his information online, including screen captures.
He found a Discovery Channel HD program transmitted at 14.16 Mbps on FiOS, but only 10.43 Mbps on Comcast. Programming on Starz HD was sent at 11.93 Mbps on FiOS, but 9.76 Mbps on Comcast. Among the high-def channels he listed as suffering signal degradation on Comcast are Sci Fi, Discovery Channel, USA, Animal Planet, Discover HD Theater, A&E, Starz and Universal HD. Other channels, including HBO HD and Cinemax HD, may be victims in some markets.
That led another forum poster, Rakesh.S, to respond: “Outside of sports on ESPN, I have no use for cable television — TV shows on OTA (over-the-air) are available from the local affiliates. I'm glad we have HD discs for movies now … don't need to worry about stuff looking like ass on cable.”
Others in the blogosphere have noted improvements in Comcast's transmissions since an Associated Press article in late April called attention to the problem.
Moyer attributed some of the problems noted earlier to working bugs out of a new technology the company is using, called “second-pass encoding.”
“What that lets us do is it looks at it two times [and] removes redundant data that can't be seen by the human eye but could decrease signal quality,” she said. “Second-pass encoding increases the processing of the incoming signal. The reason we use that is that it optimizes the MPEG video without decompressing and recompressing it.”
Comcast isn't the only cabler with issues, but a Time Warner Cable spokesman did not provide information by press time.
To make matters more confusing, Kane said the studios may dictate the quality of copies of their movies in contracts with the various cable stations.
“In PPV, the DirecTV system is having the AVC encoding done by the studio or a post-production studio in Hollywood,” Kane said. “The bitrate is being dictated by someone in Hollywood, rather than someone at the service.” Only a large cable company could pull off the same feat, he added.
“What the average viewer is seeing in difference in picture quality has much more to do with the preparation of the master than the encoding,” Kane said.