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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*
 
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10
#303 ·

Quote:
Originally Posted by VideoGrabber /forum/post/13478748


cypherstream commented:

> I did think Starz looked blurred and more like a good DVD transfer rather than HD when watching the Davinci Code on Sunday.
 
#304 ·

Quote:
Originally Posted by rallen /forum/post/13484815


I provided my feedback about all this to Comcast, and yesterday, found a message on my voicemail asking me to call back about it. Interesting.


Wonder how the conversation will go? Will it be a denial of any compression or.....

I too voiced my opinion to Comcast via an email regarding the HD compression. I received a response to the effect that they forwarded my comments to the appropriate management team for further review and action. I am not holding my breath but will update if I receive any further response.
 
#305 ·

Quote:
Originally Posted by jsb_hburg /forum/post/13484880


On the Comcast of Harrisburg system, I have seen the same softening and macroblocking due to the 3:1 loading. We are supposed to get FiOS in June or soon thereafter to the rescue.

I sure hope Fios makes it's way down here soon. I'll sign up the moment it's available.
 
#306 ·
Comcast Manchester NH.


I've been seeing the quality reduction from Comcast across all channels for the

past 3-4 months now. If Verizon Fios was available I'd switch in a heartbeat.

Quote:
Quote:

Originally Posted by PaulGo View Post

And that's why we need to let Comcast know how displeased we are with the video quality! I understand that Comcast still has very high bit rate on their HD VOD service. It would be interesting if someone who has the equipment to measure this could confirm this. Another solution as to how Comcast could regain HD picture quality is if they go to Mpeg-4 as the satellite services are doing, but that would require a large investment in new hardware. Also the Spectrum overlay technology could be yet another solution for gaining additional bandwidth.

http://www.avsforum.com/avs-vb/showt...9#post10338739

The VOD service is done on a local (node basis) and is similar to SDV. That allows the bitrate to be higher than signals from central head end QAMs.

Last night I watched John Adams pt.3 and there was macroblocking within, so HBO On Demand video is not precluded from the HD compression and all the other crap that Comcast is doing.
 
#308 ·
I talked to the guy at Comcast today and he was very nice. He acknowledged my concerns and admitted they had already gotten several complaints locally, for us that's the entire Delmarva Peninsula. He was advancing all his complaints up the corporate ladder, but wanted to let me know he got it and understood it. All I could really expect I guess. Still, if everyone starts complaining, it might just get them to do something.


Get those emails going. It might just have an effect if they get enough.
 
#309 ·

Quote:
Originally Posted by rallen /forum/post/13486963



Get those emails going. It might just have an effect if they get enough.

Well from enough pressure they decided to stop throttling Bit Torrent connections. So they listen eventually...
http://www.dslreports.com/shownews/C...rottling-93022


I just don't want to see my new HD channels all of a sudden disappear from my lineup. I'll take Discovery HD at 1920x1080i in a 3:1 mux over Discovery SD on a soupy looking 576x480i 12:1 mux ANY day of the week.


I'd rather have Medium to semi-HD over crap looking SD!
 
#310 ·

Quote:
Originally Posted by cypherstream /forum/post/13487146


Well from enough pressure they decided to stop throttling Bit Torrent connections. So they listen eventually...
http://www.dslreports.com/shownews/C...rottling-93022


I just don't want to see my new HD channels all of a sudden disappear from my lineup. I'll take Discovery HD at 1920x1080i in a 3:1 mux over Discovery SD on a soupy looking 576x480i 12:1 mux ANY day of the week.


I'd rather have Medium to semi-HD over crap looking SD!

But if Comcast makes an infrastructure investment of about $100 per subscriber we can have both HD quality and and quantity. I am willing to put up witha a short term reduction in quality. But if we don't complain it could become long term or permanent. Remember it's the squeaky wheel that gets oiled. No complaints mean Comcast will not make the investment and instead raise dividends to stockholders.
 
#311 ·
Well that explains it. Here in Colorado Springs I'd been noticing a drop in picture quality over the last month or so and now I know why. Very disappointing -- I'm gonna go send an email to Comcast!
 
#312 ·

Quote:
Originally Posted by MikeSM /forum/post/13477961


I think HBO, and possibly the other premium channels have elements in their contracts that specify what the distributer can do to the video they send them. People pay a lot for these channels, and HBO doesn't want the quality knocked down.

Contrary to the nope you were given, HBO does go out to cable headends to confirm that they are distributing the service at a quality HBO finds acceptable. I know this for a fact. You sir are on the right track with your thinking.
 
#313 ·

Quote:
Originally Posted by ja2bk /forum/post/13487343


Contrary to the nope you were given, HBO does go out to cable headends to confirm that they are distributing the service at a quality HBO finds acceptable. I know this for a fact. You sir are on the right track with your thinking.

I know this was the case a few years ago, and given the personalities at HBO, I could not see how that policy could have changed. Let's just say they are not the easiest people to deal with, on a number of topics. In this particular case, it happens to align with consumer's interests. :)
 
#314 ·

Quote:
Originally Posted by cypherstream /forum/post/13487146


Well from enough pressure they decided to stop throttling Bit Torrent connections. So they listen eventually...
http://www.dslreports.com/shownews/C...rottling-93022


I just don't want to see my new HD channels all of a sudden disappear from my lineup. I'll take Discovery HD at 1920x1080i in a 3:1 mux over Discovery SD on a soupy looking 576x480i 12:1 mux ANY day of the week.


I'd rather have Medium to semi-HD over crap looking SD!

I agree with your point about better to have semi-HD vs. crappy SD, but I fear that if HD subscribers accept that as a compromise without speaking up, Comcast will not be motivated to spend the money to address the issue.
 
#315 ·

Quote:
Originally Posted by LawrenceR /forum/post/13487546


I agree with your point about better to have semi-HD vs. crappy SD, but I fear that if HD subscribers accept that as a compromise without speaking up, Comcast will not be motivated to spend the money to address the issue.

I don't think it's that simple, but in any case, if Comcast marketed it as semi-HD channel, I would have a lot less issues with it. But they aren't doing that, which is why they have to change what they are doing.
 
#316 ·
Well I think the 3:1 mux has to be in effect now, just to get the channels on the system. If Comcast wants to spend the time and money taking 750 MHz systems out to 1 GHz, or they want to deploy SDV to better manage bandwidth, then I would hope they use the new found bandwidth to best use.


I do think 3:1 muxes 'could' work in theory with the talking heads channel, or channels with lots of solid colors (like cartoons, or weather graphics). Or even mix 720p with two 1080i channels, or two 720p's with one 1080i (although there's not many 720p channels to go around). Heck you could even put lesser watched HD's on 3:1 muxes, like RFD TV HD, or other niche networks. Some channels don't need 2:1 full blown HD, but semi-HD is good enough, stuff like Fox Business, CNBC, Nickelodeon, VH1, MTV, CNN HD, Weather HD, etc.. Leave the major top 50 in 2:1 muxes (Like discovery, usa, sci-fi, etc..)


I think a switched digital video approach is the most logical way to go here, because you can't simply do massive analog bandwidth reclamation overnight due to the horrible customer backlash about needing boxes for all of their TV's... and you can't just go out and do a 1 GHz upgrade overnight either. Theres millions of miles of cable plant to comb through, and millions of dollars of nodes, amplifiers/bridgers, line extenders, and even feeder that may need to be replaced. You can't really think about fiber to the home either, without a huge architectural change costing even more time and money. So in my humble opinion, bring on the SDV.


But who's to say they make a bandwidth upgrade, or deploy SDV... that the source is STILL coming from a 3:1 mux on Hits Quantum AMC18? What good does it do taking a 12 mbps 3:1 mux from AMC18 and putting it on a 2:1 SDV Carrier. What are you going to just pad the 12 mbps rate to fit a 19.2 mbps "slot"? That won't get you anywhere. The problem I think is at the encoding source. Either a newer firmware for this Imagine multiplexer which handles the compression better, or a different 3:1 mux, say CNN HD, Discovery HD, Weather HD on one transponder, and CNBC HD+, ABC Family HD, SciFi HD on another, etc...


A solution needs to be reached, one though that still allows for massive HD channel expansion, good quality audio video, and minimal cost (I know thats a tough one). By cost, not only dollars, but political costs, the cost of customer churn from analog spectrum reclamation or increasing rates to fund infrastructure upgrades, etc...


It's a really tough business indeed. I hope they are up to the challenge. I think cable TV is one of the most scrutinized industries in the world today. Maybe between that and the technology involved in the transmission networks is what attracts me to follow the industry so closely as I do.
 
#318 ·

Quote:
Originally Posted by briansemerick /forum/post/13487963


wouldn't the easiest fix at least temporarily be to get someone to design a cheap, tiny box that is like an external cablecard, and just give them out with cable subscriptions for those who don't have HD? they already have this tiny black ones, which i assume do mostly what i just said. they can't cost that much.

A CableCARD doesn't tune channels. The tuner for a CableCARD device is in the device. The CableCARD just tells the device's tuner where to tune for a particular channel number, and decrypts the signal.


The device you're talking about would have to have a tuner in it. It would essentially be an HD box. I can't imagine them being much cheaper than the boxes being rolled out now.


Also, you're telling every subscriber that they need to have one of those on every single TV. More than that if they still want to use a VCR to record one thing and watch something else. Even if you give them the box for free (which, if they wanted to do that, they'd be doing it now), that's still a PITA factor that some people just don't want to deal with.
 
#319 ·

Quote:
Originally Posted by briansemerick /forum/post/13487963


wouldn't the easiest fix at least temporarily be to get someone to design a cheap, tiny box that is like an external cablecard, and just give them out with cable subscriptions for those who don't have HD? they already have this tiny black ones, which i assume do mostly what i just said. they can't cost that much.

Well the problem with that is that there's three competing (not really competing directly) but 3 different encryption standards. There's Motorola MediaCipher, SA PowerKEY, and a NDS Nagravision variant that runs on a DirecTV like access card for SA boxes on Cablevision.


The FCC created a mandate that went into effect on 7/1/07 which states that the device that does the security decryption MUST BE SEPERATE from the actual cable box 'front end' so to speak. What box manufacturers came up with was to move the decryption to an industry standard "Cable Card" and incorporate cable card slots into reworked, and more expensive cable set top boxes. This was all because of the FCC requires that this is THE LAW. The only way out is to obtain a waiver, which only select small operators were granted. All new cable boxes purchased for service MUST be the more expensive "separable security" version to satisfy the nasty FCC's rule. Because of this, you may have noticed (or will notice) an increase in monthly equipment rentals from SD boxes, to HD boxes, to DVR service. The more expensive cable card boxes must be funded from somewhere, and sadly that's us, the consumer.


One solution might be to eliminate analog COMPLETELY from the 750 MHz pipe. Sure this would really piss off thousands of subscribers, so what may be possible is a residential gateway device containing a chip from BroadLogic(tm). BroadLogic is a company that has created a chip which takes a 70 Channel digital only input and remodulates it on a standard 70 analog channel NTSC output. This device would only need to be installed in analog only subs homes, as digital subs have a box or DVR or whatever. You could simply put this device in the basement (or outside simular to a FIOS ONT), and any analog TV's could be connected after this converter by the use of splitters. If you have a hybrid setup, for example a DVR, an SD box, and 4 analog TV's, you would simply split the all digital signal.... send one end to a splitter to your digital boxes, and the other leg goes to the digital/analog converter which will then have a splitter to all of the analog only TV's.

It sounds like a major task to be able to tune and remodulate 70 analog channels from a pure digital system, but somehow BroadLogic found a way to do it with their "Headend on a Chip". I know Comcast invested in BroadLogic, but I have no idea how the progress of this device is going. Since most Comcast area's are already fully digital simulcast, deploying this within a 6-8 month timeframe while both analog and digital signals co-exist wouldn't be a problem. Then when all required subscribers have this device, they could simply shut off all of the analog signals and reclaim that bandwidth for all of the glorious HD we all want and deserve.
 
#320 ·

Quote:
Originally Posted by MikeSM /forum/post/13487742


I don't think it's that simple, but in any case, if Comcast marketed it as semi-HD channel, I would have a lot less issues with it. But they aren't doing that, which is why they have to change what they are doing.

Mike,


I agree with you and I probably should have clarified this more. In theory I agree semi-HD is preferable to crappy SD but I do not find it as an acceptable substitute or compromise for HD service. I think if enough subscribers complain something will get done.
 
#323 ·
cypher, thanks for the clarification. i misspoke about the cablecards, i realize they are just a decoder. they could create a tiny box with two tuners that takes up no space and can be shoved behind the unit with an IR receiver out front, possibly. it would just need to downconvert. honestly it really can't be that expensive - and they're making a killing on service as it is. it would just be a stop-gap until everyone eventually purchased newer TV's. so maybe 5-10 years. better than the alternative of screwing all the people who want the new tech, which is where it's headed anyway. we're not going to wait that long. the general public just need to bite the bullet on this one.


really neat about the Broadlogic stuff. i'm not crazy right, everything is going digital on Feb 2009 anyway? so won't this be moot since everyone will need one of these boxes who doesn't have HD?
 
#324 ·

Quote:
Originally Posted by briansemerick /forum/post/13488715


i'm not crazy right, everything is going digital on Feb 2009 anyway? so won't this be moot since everyone will need one of these boxes who doesn't have HD?

Nope, the Feb 2009 date only applies to OTA (Over the Air) transmission. Some Cable operators may use this date to migrate a bunch of analogs to digital as a cover... but private CATV networks are not mandated to go digital.


In our system, despite the incredible rush in new box requests from the analog drops, people are VERY aware of the large negative publicity that the local media seemed to love highlighting from analog channels moving. (Based off what I've heard based off customer response, the people in the newspapers other local media, and Comcast employees). Because of this negative press I think i did hear that they are looking to avoid dropping anymore analogs for awhile, but that does not mean no more HD launches. Every local market varies. I'm speaking for my market, Central PA.
 
#325 ·

Quote:
Originally Posted by aindik /forum/post/13488082

Quote:
Originally Posted by briansemerick

wouldn't the easiest fix at least temporarily be to get someone to design a cheap, tiny box that is like an external cablecard, and just give them out with cable subscriptions for those who don't have HD? they already have this tiny black ones, which i assume do mostly what i just said. they can't cost that much.

A CableCARD doesn't tune channels. The tuner for a CableCARD device is in the device. The CableCARD just tells the device's tuner where to tune for a particular channel number, and decrypts the signal.

I think that you guys missed his point and dived into a CableCARD discussion. He was really just talking about the concept of a cheap low cost EXTERNAL box.


The fact is that these boxes do exist, but still cost cable money - so they can't just give them away. In some markets Comcrap has been switching to 100% digital and giving the first box away, but charging $5-6/month extra for each additional one. Most households have multiple analog TVs - so this is an issue. I, personally, have 8, in various locations in the house. Only one has a cable box. Now if that pricing philosophy applied to me my bill would go up by $30 or so/month.


Another approach, which I haven't heard much from lately, is the Broadlogic TerraPIX Residential Gateway.
This gateway converts the digital channels to analog at the entrance to the house. One box for the whole house. But, it's apparently still in development.


Any of the approaches takes time. It took TW here at least 6 months to install and get SDV running - and that was with a couple of years of planning.
 
#326 ·

Quote:
Originally Posted by briansemerick /forum/post/13488715


cypher, thanks for the clarification. i misspoke about the cablecards, i realize they are just a decoder. they could create a tiny box with two tuners that takes up no space and can be shoved behind the unit with an IR receiver out front, possibly. it would just need to downconvert. honestly it really can't be that expensive - and they're making a killing on service as it is. it would just be a stop-gap until everyone eventually purchased newer TV's. so maybe 5-10 years. better than the alternative of screwing all the people who want the new tech, which is where it's headed anyway. we're not going to wait that long. the general public just need to bite the bullet on this one.


really neat about the Broadlogic stuff. i'm not crazy right, everything is going digital on Feb 2009 anyway? so won't this be moot since everyone will need one of these boxes who doesn't have HD?

We're getting off topic, but maybe something like the below?


Comcast Pursuing $35 Digital Dongle
http://www.lightreading.com/document...46733&site=cdn
 
#327 ·

Quote:
Originally Posted by cypherstream /forum/post/13488839


Nope, the Feb 2009 date only applies to OTA (Over the Air) transmission. Some Cable operators may use this date to migrate a bunch of analogs to digital as a cover... but private CATV networks are not mandated to go digital.

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.
 
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