AVS Forum banner
Status
Not open for further replies.

Comcast HD Quality Reduction: Details, Screenshots

634K views 2K replies 335 participants last post by  Mike Wolf 
#1 ·
Last updated: April 3, 2008


Until recently, most Comcast systems passed all HD as is from the content provider, without any added compression or quality reduction. In response to competitive pressures from DirecTV and Verizon FiOS, Comcast recently decided to sacrifice some quality to improve quantity. By early April, most Comcast systems will recompress and degrade their HD, much like DirecTV and Dish Network do on their MPEG-2 channels. This creates room for new HD channels without the need to eliminate a significant number of analog channels.


Previously, Comcast allocated a maximum of two HD channels per 38.8Mbps QAM, so each channel had the full 19.4Mbps available if needed. Now, with the addition of new channels, Comcast is squeezing three HD channels into each 38.8Mbps QAM. Furthermore, some existing QAMs with two HD channels are being recompressed in preparation for new channel additions.


But what does that mean? How much difference is there, really?


To find out, I decided to compare the quality of the same programs on Comcast and Verizon FiOS. I recorded the same program from the same channel, at the same time, on both Comcast and Verizon FiOS in N. VA. I compared the size and bitrate of each MPEG-2 recording, as well as the subjective quality with video.


Note when I tested channels late last year, there were no differences between the two providers on HD. Any differences are new.


Background

Comcast is recompressing local HD channels in some areas and not others. Quality issues with local HD channels could be the fault of your area Comcast or the local affiliate; to determine that, you would need to compare the cable and off-air feed from an antenna. As of March 18, Comcast is not recompressing ESPN-HD or ESPN2-HD.


The Comcast cable channels with added compression are listed as follows, grouped by QAM.

Discovery Channel HD

SciFi HD

USA HD


Animal Planet HD

Discovery HD Theater

The History Channel HD


Food Network HD

National Geographic Channel HD

Universal HD


A&E HD

HGTV HD

Starz HD


Extra compression is also applied to other channels in many markets, including CNN-HD, MHD, TLC-HD, HBO-HD, and Cinemax-HD.


All of the channel groups above are re-compressed at the Comcast Media Center (CMC) in Denver and uplinked to the AMC18 satellite for distribution to Comcast systems around the country. If your Comcast system has all (or most) of the channels listed above, then the added compression is likely in effect on your system. Former Adelphia systems appear to be one exception to that rule; a number of former Adelphia systems are passing the original source feeds as is, and have not yet switched to the recompressed feeds from the CMC.


Bitrates

Average bitrates were obtained by comparing the size of each recording, in total bytes, and dividing by the total number of seconds reported by VideoRedo. Multiplied by 8 to convert MBps to Mbps.

Average Bitrates on FiOS v. Comcast
Code:
Code:
FiOS    Comcast        Difference

AETV HD                  18.66 Mbps      14.48 Mbps           -28.9%
Discovery HD             14.16 Mbps      10.43 Mbps           -35.8%
Discovery HD Theater     17.45 Mbps      12.60 Mbps           -38.5%
Food Network HD          14.32 Mbps      13.73 Mbps            -4.3%
HGTV HD                  14.76 Mbps      12.43 Mbps           -18.7%
MHD                      17.73 Mbps      13.21 Mbps           -34.2%
National Geographic HD   13.40 Mbps      11.92 Mbps           -12.4%
Universal HD             12.72 Mbps      11.01 Mbps           -15.5%

HBO HD                    8.87 Mbps       8.81 Mbps           -0.7%
Cinemax HD               11.40 Mbps      10.77 Mbps            -5.8%
Starz HD                 11.93 Mbps       9.76 Mbps           -22.2%

CNN HD                                   11.42 Mbps
History HD                               10.40 Mbps
SciFi HD                                 12.59 Mbps
USA HD                                   12.48 Mbps
See bottom of post for list of source recordings. Comparisons for more channels will be added soon.


Video Clips


By request, I've decided to devote a section to short, 10sec clips to compare high-definition video on Comcast and FiOS.

Comcast | FiOS - MHD - Red Hot Chili Peppers Live in Milan


Screenshots


I captured the same uncompressed frame from each program in full-resolution using MPC with Dscaler5-IVTC. These images were rescaled to half-resolution with XnView (Lanzos) and are shown below in lossless PNG format. Click an image to download the full-resolution version.


Discovery HD on FiOS (Comcast was the same until recently)



Discovery HD on Comcast



Discovery HD on FiOS (Comcast was the same until recently)



Discovery HD on Comcast



MHD on FiOS



MHD on Comcast



MHD on FiOS



MHD on Comcast



UHD on FiOS



UHD on Comcast


Links to More Captures
MHD
NGC
HGTV
A&E


Comparison screenshots of more channels coming next week.


Comments


It is obvious that the quality of the source signal plays a significant role in amount of degradation seen with Comcast's newly added compression. Sources like Discovery Channel (not Discovery Theater) and Universal HD are highly compressed to start with, and adding extra compression on top of that causes the picture to deteriorate rapidly with excessive noise and detail loss. In contrast, higher-quality sources like Discovery Theater still look very good, with the only obvious differences being some added noise and some minor loss of fine detail during motion.


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 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.


To Comcast's credit, I saw little to no difference on movie channels such as HBO, Cinemax, and Starz. I did see some blurring and reduced detail during fast movement on Starz, but the recordings from Cinemax and HBO were virtually identical, even on action movies such as 300 and Gladiator. When there was blocking on the Comcast feed of Cinemax, that blocking was also on the FiOS feed.

Update: April 3


I removed the comment on Food HD because I did not see that issue in my 3/30 recording of Paula's Home Cooking.


NGCHD on Comcast looks just as good as NGC-HD on FiOS, save for some blurring during fast motion, such as animals running across the screen.


The HGTVD source feed is clearly overcompressed. Blocking and blurring is common on this channel with both Comcast and FiOS. It is not specific to Comcast. Comcast is responsible for some added noise on both static and moving images , but the differences really pale in comparison to the problems with the original source feed.


Sources


Programs marked with a * were used in bitrate calculations.

Round #1: March 9-11

A&ETV HD - The Sopranos: All Due Respect*

Discovery HD - Mythbusters: James Bond Special*

Discovery HD - The Human Body Pushing the Limits: Sensation (recorded several hours apart)

Discovery HD Theater - Fantastic Festivals of the World*

Food Network HD - Challenge: Candy Castles*

National Geographic HD - Naked Science: Birth of America

National Geographic HD - Naked Science: Glacier Meltdown

Universal HD - Action Sports*

Starz HD - The Pursuit of Happiness*

Round #2: March 22-29 (in progress)

A&ETV HD - Matrix

Cinemax - 300 (different showings)

Cinemax - Gladiator*

HBO - The Good Shepherd*

HGTV HD - Find Your Style

HGTV HD - Hidden Potential

HGTV HD - Over Your Head, Artistic Bathroom*

History - Cities of the Underworld*

MHD - Red Hot Chili Peppers Live in Milan*

SciFi - Stargate Atlantis*

USA - Monk*

Round #3: March 30-31 (in progress)

CNNHD - Lou Dobbs Tonight*

FOODHD - Paula's Home Cooking

MHD - 2007 MTV Europe Music Awards

NGC-HD - Animal Oddities*
 
See less See more
10
#328 ·

Quote:
Originally Posted by davehancock /forum/post/13488962


In fact, the FCC is REQUIRING cable to provide analog signals to their customers for a minimum of 3 years past the cut-off.


But:
  • This only applies to qualified OTA stations that cable carries (does not mandate any approach for the typical cable channels like USA or CNN).
  • Cable can get around this by going 100% digital and providing digital boxes to the customer AT NO COST to the customer.
  • The FCC has made it clear that they may well extend this past 3 years.


That TerraPIX gateway, that I mentioned before would certainly help cable out here.

Yup, they couldn't make that TerraPIX fast enough!

That sounds like the win-win... we'll have to see if anything comes of it.


Still 20 or 30 analogs is much less of a burden than 60-70.
 
#330 ·
I currently pay Comcast close to $200 a month for the Triple Play option with the movie channels. As soon as AT&T gets the rights for television in Tennessee, I'm switching, that is if Comcast doesn't get the message on both quality of service as well as price. Then I will urge my family and friends to do the same. I have been a long time customer who feels like I am getting the shaft. Hope you are reading this, Comcast!
 
#331 ·

Quote:
Originally Posted by Wizio /forum/post/13489412


I currently pay Comcast close to $200 a month for the Triple Play option with the movie channels. As soon as AT&T gets the rights for television in Tennessee, I'm switching, that is if Comcast doesn't get the message on both quality of service as well as price. Then I will urge my family and friends to do the same. I have been a long time customer who feels like I am getting the shaft. Hope you are reading this, Comcast!

Also call your Comcast office and tell them this! They need to realize that they will loose a substantial number of high paying customers if they do not put a plan in place to have comparable picture quality as FiOS. Your low end customers might stay with Comcast because they won't know the difference, but your high end customers which Comcast makes the most profit from will desert Comcast! If they don't make the investment now they will loose more money from customers switching to FiOS!
 
#332 ·
I'm not sure if this is the right area to post my observation but let me just say that in view of all the chat about Comcast allegedly (gotta throw that in) giving us less than HD and assuming its true.........(getting "less" for more dollarwise) would having a 720P TV show less PQ degradation...........OR...would a 1080 set be more likely to show a lower quality picture?


I suspect that a 720P set would show a "better" (less affected) PQ loss. My "logic" (and I may well be wrong) is that the 1080 takes more "bits" of information to fill the screen than a 720P. In other words, it seems that if a company like Comcast is "cutting back" then there would be more info taken away from a 1080 picture than the 720? If someone could help clear this up in plain English without "technobabble", I/we would sure appreciate it!
 
#333 ·
Here's a screenshot comparison showing Kidnapped on UHD. This is not an intensive program, and the scene has relatively slow movement.


UHD on FiOS



UHD on Comcast



As always, click each for the full-resolution PNG.


From watching UHD on both providers, it is clear that the original feed is already overcompressed. Applying further compression on top of that -- as Comcast has done -- makes for horrific results on more intensive programs like Action Sports. Expect to see shots of that shortly.
 
#334 ·

Quote:
Originally Posted by kmil /forum/post/13490060


I suspect that a 720P set would show a "better" (less affected) PQ loss. My "logic" (and I may well be wrong) is that the 1080 takes more "bits" of information to fill the screen than a 720P. In other words, it seems that if a company like Comcast is "cutting back" then there would be more info taken away from a 1080 picture than the 720? If someone could help clear this up in plain English without "technobabble", I/we would sure appreciate it!

I agree, 720P at lower bitrates, still holds better PQ then 1080I.....
 
#335 ·

Quote:
Originally Posted by bfdtv /forum/post/13490350


Here's a screenshot comparison showing Kidnapped on UHD. This is not an intensive program, and the scene has relatively little movement.


As always, click each for the full-resolution PNG.


From watching UHD on both providers, it is clear that the original feed is already overcompressed. Applying further compression on top of that -- as Comcast has done -- makes for horrific results on more intensive programs like Action Sports. Expect to see shots of that shortly.

Wow, the difference is really astounding, and bfdtv, thanks for posting, and here is to hoping that this thread makes big news and forces cableco's to fix the compression issues!!
 
#336 ·
Ugh!! I just discovered this thread yesterday and it took me this long to read through it.


I'd begun to suspect that something was up with some of Comcast's HD channels and I did a Google search on "Comcast HD quality" yesterday. This thread was in the top seven results. Now I'm disgusted, but I can't say that I'm totally surprised.


It's a real bummer because I don't expect FiOS to be available here in Northern California anytime soon, and my wife isn't likely to agree to D* due to the equipment cost and the multiple receivers required. Seems Comcast has me right where they want me...


-Steve
 
#340 ·

Quote:
Originally Posted by Wizio /forum/post/13489412


I currently pay Comcast close to $200 a month for the Triple Play option with the movie channels. As soon as AT&T gets the rights for television in Tennessee, I'm switching, that is if Comcast doesn't get the message on both quality of service as well as price. Then I will urge my family and friends to do the same. I have been a long time customer who feels like I am getting the shaft. Hope you are reading this, Comcast!


we pay about $175 for cable/internet and I agree, I would switch in a heartbeat if FIOS TV was available here (gotta love Pittsburgh and the whole monopoly obsession) the cable HD quality over the last couple months has gone significantly downhill



would seriously consider satellite instead as well, except our apartment building disallows it*
 
#341 ·
We just got done watching August Rush on HD-VOD. I have a free VOD coupon so that's what I used it on.


GREAT MOVIE!!!


But, I noticed the same macroblocking MPEG noise during fast motion scenes. This blocking looks EXACTLY like the screen shots for UHD and MHD. So it must of been a 3:1 encoding, or a 12mbps or whatever VOD file.


There was a few audio drops near the end, but overall it was pretty decent. I'd still get an HD-VOD movie over an SD-VOD movie mainly because they are the same price. Heck in this case FREE, because I have a coupon!


So check your HD-VOD and see if you notice the same compression. Perhaps it's only on certain VOD files and not others.
 
#342 ·
#343 ·

Quote:
Originally Posted by bicker1 /forum/post/13484860


Yes, thanks for that clarification. The essential point I was making, however, stands: Bit rate is no part of the definition of HD, even if we would want it to be.

There is no legal definition of HD. The govenment including the FCC passed on that one. The CEA attempted their own and on this forum we also tend to accept 720p and 1080x or above as hidef.


But, as the name implies, generally it is nice to think of High Definition TV as having a higher definition picture. And that idea rapidly and disappointingly fails when the video is broadcast with an obviously insufficient bit rate.


As consumers we like to share information about vendors peddling low quality product, legally or not. And as AV enthusiasts the products of interest on this forum are obviously audio and video related.


- Tom
 
#345 ·
wow, 38% for discovery HD


i'm glad i still have alien insect praying mantis in my DVR from last year
 
#346 ·

Quote:
Originally Posted by PaulGo /forum/post/13487264


But if Comcast makes an infrastructure investment of about $100 per subscriber we can have both HD quality and and quantity.

Comcast investors will expect that $100 per subscriber to come from the subscribers, over a limited period of time, perhaps as little as two years. Furthermore, many subscribers are lifeline subscribers, so the rest of us will have to pick up their share. Many people feel cable is too expensive already; even assuming your $100 is accurate, it would like be too excessive for most customers.
 
#347 ·

Quote:
Originally Posted by davehancock /forum/post/13488962


In fact, the FCC is REQUIRING cable to provide analog signals to their customers for a minimum of 3 years past the cut-off.


But:
  • This only applies to qualified OTA stations that cable carries (does not mandate any approach for the typical cable channels like USA or CNN).
  • Cable can get around this by going 100% digital and providing digital boxes to the customer AT NO COST to the customer.
  • The FCC has made it clear that they may well extend this past 3 years.


That TerraPIX gateway, that I mentioned before would certainly help cable out here.

As far as I recall, the stuff you put in bold did not come from the regulation -- that there was nothing in the dual-must-carry regulation that said the digital boxes had to be provided "at no cost".
 
#348 ·

Quote:
Originally Posted by trbarry /forum/post/13491900


But, as the name implies, generally it is nice to think of High Definition TV as having a higher definition picture.

And for sure, even at lower bit rates, the HD provided by even the worst HD Lite implementation is higher definition than analog.

Quote:
Originally Posted by trbarry /forum/post/13491900


As consumers we like to share information about vendors peddling low quality product, legally or not. And as AV enthusiasts the products of interest on this forum are obviously audio and video related.

As bfdtv did, clearly stating the actuality. He didn't say that it wasn't HD (because it is). He just said (showed, really) that it wasn't as clear as some other implementation of HD.
 
#349 ·
Congratulations to bfdtv for this thread


There are a lot of unhappy Comcast customers in my area: they are being forced to get digital boxes because several analog channels are being moved to digital only: and they do charge extra for the boxes


the line at the Comcast counter is out the door and there is a limit of one digital box per customer : needless to say folks are very unhappy: and forget calling...you will spend a lot of time on hold and get nothing done


I turned in my cablecards ,sold my S3, and cancelled Comcast TV service (kept internet for now) :there are too few HD channels to make it worthwhile... happier with DirecTV HD


and FIOS is coming soon (already in my town but not down my street yet)
 
#350 ·

Quote:
Originally Posted by bicker1 /forum/post/13494045


And for sure, even at lower bit rates, the HD provided by even the worst HD Lite implementation is higher definition than analog.


As bfdtv did, clearly stating the actuality. He didn't say that it wasn't HD (because it is). He just said (showed, really) that it wasn't as clear as some other implementation of HD.

I didn't say it's not HD either. But it does appear to be becoming very low quality HD.


But again, the name is High Definition TV, not High Resolution TV. So I would like to see well defined images, not over quantized softness, blocks, and other artifacts. Or we can start calling it HRLQ TV, for High Resolution Low Quality.


- Tom
 
Status
Not open for further replies.
You have insufficient privileges to reply here.
Top