Bit Rate

What Comcast's Crunched HD Looks Like

3/29/2008 12:27 AM

In the black art of video compression, the trick is to fool the human visual system into seeing things that aren’t there.

All digital video is compressed. The technology that does this removes a lot of data, stripping out visual information in clever ways so it can be packed down, sent over a wire or satellite, then unpacked on the viewing end to a TV set.

Without squeezing HD signals down, a distributor couldn’t feasibly distribute them — uncompressed 1080i video simply takes up too much room (around 1.5 Gbps). Even the HD DVD formats use compression.

The question is: How tightly do you twist the screws? The more you squeeze, the more video impairments you get. A rule of thumb for MPEG-2 broadcasts has been about 19 Mbps is needed to ensure good quality. That means two HDs will comfortably fit in a 6-MHz carrier on 256-QAM cable networks.

Cable needs to add more HD channels (i.e., to keep up with marketing from DirecTV and Dish). But the challenge is doing that in capacity-constrained cable systems.

There’s switched digital video, which sends down a channel only when someone tunes to it. Cablevision, for instance, will be offering the 15 Voom HD channels this way.

There is also "3-in-1" compression: Comcast is distributing some of its HD channels using a variable bit-rate encoding technique that fits three HDs into one QAM. One of Comcast’s technology suppliers for this project, startup Imagine Communications, has touted the ability of its system to deliver lower bit rates without harming quality.

But as Ken Fowler, an A/V buff in Virginia, claims to have found in an analysis he posted to AVSForum.com, the differences between some of Comcast’s more highly compressed channels and Verizon’s FiOS TV are indeed noticeable (see "Test Shows Comcast’s HD Squeeze In Virginia").

Dramatic, you might even say.

Below are cropped sections of images Fowler grabbed of MTV Networks’ MHD high-definition music channel, airing a Red Hot Chili Peppers concert, which he extracted using two TiVos with CableCards connected to both providers.

I cut out 480 x 270 sections showing the band’s bassist, Flea, because the full images won’t fit on this page. I used Microsoft’s Digital Image 2006 editor to crop the images and convert them to JPG from PNG format.

In addition to his screen captures, Fowler uploaded 11-second MPEG clips of MHD’s Chili Peppers program (click to download his FiOS and Comcast clips).

Now, it’s important to note that — to my eye, anyway — the differences in picture quality in the actual video aren’t as striking as in the still images. Furthermore, the images I’ve posted here are one-sixteenth screensize cutouts to show detail.

For example, the blockiness and blurriness evident in the Comcast detail below don’t jump out as much in the full-screen video. Then again, I was watching this on a PC screen rather than a 50-inch flat-panel display.

Click on the links below to see the full-screen (1920 x 1080) versions Fowler originally posted.

FiOS TV: MHD Red Hot Chili Peppers
Average bit rate = 17.73 Mbps

FiOS

Comcast: MHD Red Hot Chili Peppers
Average bit rate = 13.21 Mbps

Comcast

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