Comcast HD Quality Reduction: Details, Screenshots

[bfdtv]

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

                            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*

[keenan]

I can't believe Comcast will leave the quality that way. In that second shot it looks like she has a skin condition via the Comcast feed.

A comment: I know you've posted about this issue in other threads, but some supporting links to statements like "By early April, most Comcast systems will recompress and degrade their HD" would be nice.

[JimboG]

Wow, I can't see how degrading quality like that would make someone less likely to switch to DirecTV or Fios.

Why doesn't Comcast just kill off analog channels?

[Lodef]

Yes, this is already noticeable on the Boston-comcast system as well. There has also been an increase in stuttering when an upconverted broadcast is being shown on some of these channels. My guess is if the other two channels on the QAM are showing full HD, then the third one is being bitstarved cuasing this effect. Fios has been wiring my neighborhood so Comcast better be careful how far they go with this or they will end up paying the price!

[josephmckinney]

This is an absolute shame. I used to not mind paying Comcast's high prices because their HD quality was so good. I have also noticed a significant degradation in HD quality. I will imediately jump ship when FIOS comes to metro Detroit.

[Rakesh.S]

outside of sports on ESPN, I have no use for cable television -- tv shows on OTA 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.

[snatta]

I noticed this weekend that a lot of my HD channels didn't seem to be as clear. The Pistons/Spurs game on MoJo didn't look good. The screen looked like it had dust on it up close, I guess screen door effect. I have a 720p Sony 46 Bravia LCD, and didn't seem to notice this that much since I got the HDTV last year. I actually thought my Sony was going bad. The golf on NBC was horrible, but NBC usually is bad anyway. The NCAA hoops on CBS wasn't perfect either, and CBS usually is pretty good. ESPN seems to be okay. My area in South NJ is due to get 10 new HD channels starting April 12th. None are available right now, but I wonder if my Comcast already started to degrade and recompress some HD channels. Maybe it was just bad HD cameras.

[Cyrl]

Originally Posted by JimboG

Wow, I can't see how degrading quality like that would make someone less likely to switch to DirecTV or Fios.

Why doesn't Comcast just kill off analog channels?

That is one you have to ask the FCC about, they(comcast) would really really love nothing more than to do that considering just how much bandwidth those channels suck down.

(bold by me to emphasize what i was answering)

[DaveFi]

I'm all wired for FIOS, they just have to wire my condo grounds and I'm good to go. I think it's time to switch.

[richiephx]

I don't care who the provider is but, if anyone thinks for one minute that any of them, including D*, won't sacrifice picture quality to increase channel count or use bandwidth for other purposes that would improve their profit margin, is naive. Historically, they all did it to SD and I wouldn't be surprised if they did it again somewhere down the road.

[hcady]

Just can hope this temporary until more bandwidth is available for HD. They should start
dumping some expanded basic analog fairly soon, I would think.

[JimboG]

Originally Posted by Cyrl

That is one you have to ask the FCC about, they(comcast) would really really love nothing more than to do that considering just how much bandwidth those channels suck down.

(bold by me to emphasize what i was answering)

The FCC allows cable companies to go all-digital. It seems like Chairman Martin was pushing this at one time.

[bendog2784]

The FCC allows cable companies to go all-digital. It seems like Chairman Martin was pushing this at one time.

They are required to keep the analog until 2/2012, or at least make people with cable ready tvs able to view broadcast television.

http://news.digitaltrends.com/news/s..._tv_until_2012

[skriefal]

Well, there goes any interest I might have had in dropping Dish Network and moving back to Comcast...

[Ken H]

Originally Posted by Cyrl

That is one you have to ask the FCC about, they(comcast) would really really love nothing more than to do that considering just how much bandwidth those channels suck down.

(bold by me to emphasize what i was answering)

?

The FCC has absolutely nothing to do with Comcast eliminating analog channels. Zero, zip, zilch.

[CNiles3806]

Wow, if that is an accurate depiction I must say that its a shame. Is there nothing that can be done other than changing providers, if that option is even available in the local market. I was one who said eh, a little compression, just add a bunch more HD channels but after seeing that I must say I was dead wrong.

We all know that this will only spread as other cable companies will do the same in their markets. I don't know what the answer is to this, short of leaving the service.

I know full well about the compression however on my CV system with the Voom Channels, wish we never got that junk and the VS network, might not be as bad but its pretty obvious what they're up to.

[CNiles3806]

Originally Posted by richiephx

I don't care who the provider is but, if anyone thinks for one minute that any of them, including D*, won't sacrifice picture quality to increase channel count or use bandwidth for other purposes that would improve their profit margin, is naive. Historically, they all did it to SD and I wouldn't be surprised if they did it again somewhere down the road.

They'll keep pushing the envelope as far as they can soon we will be watching video quality no better than 16X9 widescreeen SD. They will do it again indeed.

Comcast does have the most HD though, right, according to their own logic anyway, sure better love those On Demand Channels

[keenan]

Apparently this 3-pak practice has popped up in the SF bay area as well. I haven't seen the consequences myself yet, still, being on a 550MHz system, but a very knowledgeable member has on his 860MHz system. I expect to see some reviews and maybe screenshots in our local thread soon. Channels involved are AETV, HGTV and SHOHD.

[maxman]

I keep hearing about 550 & 860MHz systems, etc. HOW THE H*LL DO I TELL WHAT MINE IS???

[keenan]

Originally Posted by maxman

I keep hearing about 550 & 860MHz systems, etc. HOW THE H*LL DO I TELL WHAT MINE IS???

How many HD channels do you have?

[bfdtv]

Originally Posted by keenan

A comment: I know you've posted about this issue in other threads, but some supporting links to statements like "By early April, most Comcast systems will recompress and degrade their HD" would be nice.

You are not going to find a public statement from Comcast that says "we will degrade our HD to make room for more channels." But this is how all Comcast divisions are being told to add new channels.

The current "three per QAM" groupings -- as distributed by Comcast Media Center in Denver:

Discovery Channel
SciFi
USA

Food
NatGeo
UHD

A&E
HGTV
Starz

Cinemax
HBO
TLC

Animal Planet
Discovery HD Theater
History HD

I don't have the ability to record three channels at once from Comcast, but recording from Food Network and National Geographic, followed by National Geographic and UHD, it looks like the total average bitrate for three channels on that QAM adds up to 35-36Mbps, not 38.8Mbps. Comcast probably has to reserve some bits for peaks.

[keenan]

No, I was referring to the article that talks about the system they plan to use(already?). I think you were the one that posted it here somewhere. I just thought it might provide some context to back up what you've been posting about the 3-per issue.

[sansri88]

For us it looks like they're still not grouped like that. UHD is on 243Mhz and NGC is on 543 MHz (or vice versa, I just took a quick glimpse at it).

Now, the way they're grouped in 3 per QAM, is that how CMC sends it out from Denver? Or is that done at the headend itself?

[bfdtv]

Originally Posted by sansri88

For us it looks like they're still not grouped like that. UHD is on 243Mhz and NGC is on 543 MHz (or vice versa, I just took a quick glimpse at it).

Now, the way they're grouped in 3 per QAM, is that how CMC sends it out from Denver? Or is that done at the headend itself?

I'm fairly certain they are sent that way out of Denver.

See transponders 8, 16, 20, and 24 on AMC18.

[mr. wally]

let's see...

most expensive service - comcast

fewest hd channels - comcast

hd pq equivalent to satcos - comcast


now why would i ever want comcast to be my provider. if the quality is no better than the satcos and d* & e* are cheaper, who in their right mind would ever subscribe to comcast.

i'd take fios in a flash if it were available here in the high tech capital of the world

[maxman]

Originally Posted by keenan

How many HD channels do you have?

25 total.

[aindik]

Any comparisons with the OTA channels?

[keenan]

Originally Posted by maxman

25 total.

Well, I would almost guarantee you that you ARE NOT on a 550MHz system, there just isn't enough room for that many. We have 11, total, HD channels on my 550MHz system.

To determine if you're 750, 860 or 1000MHz, you'll have to browse through the diagnostics screens to find the channel at the highest frequency.

[bfdtv]

Originally Posted by aindik

Any comparisons with the OTA channels?

I believe Comcast is still passing locals through as is, without any extra compression. But I'll double check that.

[sansri88]

Originally Posted by keenan

Well, I would almost guarantee you that you ARE NOT on a 550MHz system, there just isn't enough room for that many. We have 11, total, HD channels on my 550MHz system.

To determine if you're 750, 860 or 1000MHz, you'll have to browse through the diagnostics screens to find the channel at the highest frequency.

Well, there is a 600 or so MHz system in NWNJ that has more HD channels than most of Jersey.

Also, instead of diagnostic screens, you can search up your system in the FCC COALS database.