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A comparison of bitrates between FiOS and Comcast reveals signals compressed up to 38%.  (Source: AV Science Forum)

Red Hot Chili Peppers live: crisp and clean on one side, a blocky mess on the other.  (Source: DViCE/AV Science Forum)
Comcast tries to fit three HDTV channels in the space of two

HDTV aficionados with Comcast service might be in for a rude awakening: the nation’s largest cable provider seems to have ratcheted up the compression on its cable HDTV signals.

A thread at AV Science Forum updated last Monday details what appears to be compression of up to 38%, allowing Comcast to deliver more HDTV channels per line while using the same amount of bandwidth. A side effect of this, however, means that HDTV’s pristine video is now jagged and muddy for Comcast customers, full of MPEG-style compression artifacts and stuttered movement:

For the most part, fine detail remains very good on static (non-moving) images with Comcast's added compression, but you do see reduced contrast, with more dithering artifacts (banding) between colors and objects. With some channels, it looks a bit like Comcast is taking a 24-bit image and reducing it to 18-20 bit. This tends to reduce the 'pop' effect in some images. The difference in 'pop' was quite noticeable on Food HD, despite the relatively small bitrate reduction.

The greatest differences are seen with movement. With slow movement on Comcast, the first thing you notice is added noise and a softer image, as fine detail is filtered from the picture signal. The greater the rate of movement, the more detail you lose and the more noise you see. With intense movement, you see more blocking and skipped frames. In VideoRedo, I noticed that a number of frames in the FiOS signal simply did not exist in the Comcast signal during motion intensive scenes. This may be responsible for the stutter and excessive motion blur seen with some video sequences on Comcast.

Still images comparing Verizon’s FiOS HDTV service with Comcast’s HDTV service, taken at the exact same time in the exact same broadcast, show Comcast’s images losing much of the legendary detail that HDTV is so well known for – in a screenshot  of the Red Hot Chili Peppers playing live in Milan, the Comcast image was almost completely stripped of all fine-grained detail; lead singer Anthony Kiedis’ textured wristband becomes flat and blocky, and the tattoo on his left arm made pixellated and blurry.

A request for comment was received by Comcast, but not replied to.

The purpose of Comcast’s increase in compression is unclear; however it would appear that the company is attempting to fit three HDTV video streams inside of one QAM (Quadrature Amplitude Modulation, Comcast’s DTV broadcast format) signal, as opposed to the previous two. In a bitrate comparison between each provider’s broadcast of the same show, the Verizon signal was recorded at 17.73 Mbps, while the Comcast signal recorded at 13.21 Mbps, a 34% reduction in size.

According to Ken Fowler, the A/V buff known as “bfdtv” at AV Science Forum, Comcast’s compression increase currently affects most customers that were not originally in Adelphia’s cable system, which Comcast purchased in 2005. Further, the increased compression only affects national networks like A&E or HBO; local TV signals are rebroadcast at whatever bitrate they were originally sent in.



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By raytseng on 4/1/2008 7:51:42 PM , Rating: 2
Did people really read the article closely?...

quote:
Further, the increased compression only affects national networks like A&E or HBO; local TV signals are rebroadcast at whatever bitrate they were originally sent in.


It's not as bad as the article makes it out to be. I know everyone would rather have the higher quality picture, but really how often do you seriously watch those other HDTV channels other then when you first got your HDTV? Does anyone seriously watch those concerts they have on the HDTV demo channel?

Doesn't say what the compression was for ESPN or ESPN2, and a shame about DiscoveryHD and SciFi, but other then that, the majority of my HDTV watching is still on the local network TV feeds(fox/nbc/cbs/abc) which comcast can not recompress (due to licensing or some other agreement/law that escapes me).




By KristopherKubicki (blog) on 4/2/2008 12:54:53 AM , Rating: 2
Well in my area (Chicago), HD channels are more the norm than not. History Channel, TNT, TBS and Science channel are pretty much 90% of my viewing, and the broadcast networks you can always get OTA anyway without satellite or cable.

But yeah the movie channels are a big letdown not in HD-lite.h


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