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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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RE: Another sad move....
By djc208 on 4/1/2008 3:37:53 PM , Rating: 5
They have to make more room for the internet traffic now that they're not going to sensor bit torrents.

You didn't think they would actually improve their service by adding more bandwith did you?

Sad part is Comcast isn't unique in this, they're just the first to get caught.

RE: Another sad move....
By Heph on 4/1/2008 6:55:51 PM , Rating: 2
They have to make more room for the internet traffic now that they're not going to sensor bit torrents.

Actually this is not the case. Docsis 3.0 will be launching at the end of the year in most Comcast systems. This will allow channel bonding and enable Comcast customers to obtain faster Internet speeds on par with FIOS.

As of February of next year even though Comcast by law does not have to stop Analog broadcast on their closed system they will do so with the exception of basic cable (just local channels). So I am personally hoping this is just a stop gap to allowing them to broadcast more HDTV channels until some of the Analog spectrum can be reclaimed for digital services, but this is Comcast so you never know.

RE: Another sad move....
By mindless1 on 4/1/2008 8:09:52 PM , Rating: 2
The launch of Docsis 3 support does not mean they can just immediately switch everyone over.

Do you have a source for your claim Comcast will stop analog transmission in February? That would be surprising, as other cable companies are not currently planning to do so AFAIK (many have been asked by customers but are taking a position of waiting to announce any changes which seems like a delay beyond that timeframe considering all the customers who will need to switch equipment.

Further, many people overlook institutional customers. Schools, hospitals, nursing homes, etc - these people can't just switch on a whim, it is a massive undertaking which some simply won't be able to do at all.

RE: Another sad move....
By Heph on 4/1/2008 11:57:48 PM , Rating: 2
Let’s say I know they will be without giving to much info. Boston is already an all digital system. If you were to just plug any analog TV into a wall jack without a box you only get approximately 22 channels.

RE: Another sad move....
By theapparition on 4/2/2008 8:59:21 AM , Rating: 3
Boston is already an all digital system. If you were to just plug any analog TV into a wall jack without a box you only get approximately 22 channels.

Well, that wouldn't be all digital now, would it?

RE: Another sad move....
By djc208 on 4/2/2008 3:29:11 PM , Rating: 2
I keep hearing this, and I don't doubt it's truth but one thing bothers me about it, cable boxes.

If they took all the "normal" analog channels and made them clearQAM it wouldn't be so bad, but chances are they'll all be scrambled, so now I have to use their boxes at $6/month for each TV, oh and if I want to record from my HTPC that's more complexity/trouble/money, or I have to rent their crappy DVR for an additional $11/month.

One of the reasons I stuck with cable is because I can split it and connect it anywhere I want. They make it just like satalite and I'll probably just get a satalite.

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