What Comcast's Crunched HD Looks Like
In the black art of video compression, the trick is to fool the human visual system into seeing things that aren’t there.
All digital video is compressed. The technology that does this removes a lot of data, stripping out visual information in clever ways so it can be packed down, sent over a wire or satellite, then unpacked on the viewing end to a TV set.
Without squeezing HD signals down, a distributor couldn’t feasibly distribute them — uncompressed 1080i video simply takes up too much room (around 1.5 Gbps). Even the HD DVD formats use compression.
The question is: How tightly do you twist the screws? The more you squeeze, the more video impairments you get. A rule of thumb for MPEG-2 broadcasts has been about 19 Mbps is needed to ensure good quality. That means two HDs will comfortably fit in a 6-MHz carrier on 256-QAM cable networks.
Cable needs to add more HD channels (i.e., to keep up with marketing from DirecTV and Dish). But the challenge is doing that in capacity-constrained cable systems.
There’s switched digital video, which sends down a channel only when someone tunes to it. Cablevision, for instance, will be offering the 15 Voom HD channels this way.
There is also "3-in-1" compression: Comcast is distributing some of its HD channels using a variable bit-rate encoding technique that fits three HDs into one QAM. One of Comcast’s technology suppliers for this project, startup Imagine Communications, has touted the ability of its system to deliver lower bit rates without harming quality.
But as Ken Fowler, an A/V buff in Virginia, claims to have found in an analysis he posted to AVSForum.com, the differences between some of Comcast’s more highly compressed channels and Verizon’s FiOS TV are indeed noticeable (see "Test Shows Comcast’s HD Squeeze In Virginia").
Dramatic, you might even say.
Below are cropped sections of images Fowler grabbed of MTV Networks’ MHD high-definition music channel, airing a Red Hot Chili Peppers concert, which he extracted using two TiVos with CableCards connected to both providers.
I cut out 480 x 270 sections showing the band’s bassist, Flea, because the full images won’t fit on this page. I used Microsoft’s Digital Image 2006 editor to crop the images and convert them to JPG from PNG format.
In addition to his screen captures, Fowler uploaded 11-second MPEG clips of MHD’s Chili Peppers program (click to download his FiOS and Comcast clips).
Now, it’s important to note that — to my eye, anyway — the differences in picture quality in the actual video aren’t as striking as in the still images. Furthermore, the images I’ve posted here are one-sixteenth screensize cutouts to show detail.
For example, the blockiness and blurriness evident in the Comcast detail below don’t jump out as much in the full-screen video. Then again, I was watching this on a PC screen rather than a 50-inch flat-panel display.
Click on the links below to see the full-screen (1920 x 1080) versions Fowler originally posted.
FiOS TV: MHD Red Hot Chili Peppers
Average bit rate = 17.73 Mbps
Comcast: MHD Red Hot Chili Peppers
Average bit rate = 13.21 Mbps
Jerry commented:
This is quite a shame and sham and the cable providers and satellite providers should be pissed, like the rest of the professional video/film community is. I work in the TV industry and most of the HD professional equipment that we use (Sony HDCAM SR and Panasonic HD D5 decks) output @ 440 MBps and 270 ~ 323 MBps. What viewers are seeing at home is 5% of the original quality (440 Mbps -> 20 MBps). This is a disgrace. Come to the TV studio and look on the monitors to see what you're missing. By the way, Blue Ray is only 40 MBps.
In summary:
Professional Standards (the cream of the crop and the master standard)
----------------------
HDCAM SR 440 MBps (video bitrate)
Panasonic D5 HD 270 ~ 323 MBps
High end consumer crapo
--------------
Blue Ray 40 Mbps
Crapo / what a disaster
----------
Satellite/Cable providers
13 ~ 17 MBps
Me commented:
Can't we all just get along?
Lisa in PA commented:
When will they broaden the band width so that motion blur can be eliminated? A person turning his face or moving from a chair should not go out of focus. Very disturbed by this. Never happened with an analog signal.
Engineer commented:
Of course...most of the Comcast haters have never seen the inside of a headend...I forgot. So before you talk s-...know what you are talking about before you speak. I've seen most of you haters try to spit technical crap like you know what you're talking about. Try again...and do me and a few a favor the next time you try to write. Re-adjust the settings on that high def T.V. mom just bought you as you live in her basement. You're 30 or 40 years old. You spit out terms copied from the internet. Come up with something better. If you don't like the s- go to Satellite...you're still paying Comcast! Though you wouldn't know it...cause you're too busy masturbating waiting for you're mom to open up the basement door and call down..."
Engineer commented:
Comcast doesn't hire idiots. Wait...i take that back. People in our customer service are idiots. But we, including me...unfortunately...work the best equipment we're given with. Pictures like the above can be a number of things, but its most likely a line issue. Once analog is done though...I swear things will be better. We test on a daily basis...though I'm not supposed to say that. Motorola is mostly the issue...if we had S.A. in all markets...things would be better. Don't believe me? Go out of town to an S.A. market and look at the difference...
F*** That commented:
Meh,I don't really care.As long as I get the channels that I want,it doesen't matter.
Joeynach commented:
I think the underlying principle in all this is has cable in general, as a technology, seemingly reached the limit of transmitting quality HD programming. I dont really know the capacity of satellite systems, if they compress or not, and how much room for HD growth they have, but I think its safe to say cable has no room for HD growth. It seems less an issue of Comcast vs Verizon, as both will use whatever they feel they can get away with to keep up in the HD TV world, and more an issue of cable being at the end of its usefulness. I dont know what the future will hold, perhaps Fiberoptic systems (to the home) are where landline communication needs to go. For the meantime if I was comcast, verizon, or whoever I would do exactly what they are doing. Keep trying to inject HD programming onto their networks for as long as possible, until a higher performance and more affordable landline tecnology can be implemented. If i didn't think Directv was the most brittle technology on the planet, constant outages from weather, I would say at this point Satellite TV seems to have a technological advantage in supplying growing HD programming, its just not limited by comprehensive networks buried in the ground across the country.
hwertz commented:
"We've all missed the real story here.... That means Verizon is using the same compression techniques and is lying when it says it doesn't compress beyond what is received in the original signal (19Mbps)."
Not so. I'm not claiming what Verizon is doing or not, but the article just says 19Mbps is recommended for good quality. ATSC for instance is 19.38mbps *maximum*. This is a raw data rate of the MPEG-TS (MPEG transport stream). If the main video isn't using 19.38mbps, the remainder could go to other channels (weather etc), or even just be filled with null packets... it wouldn't make sense for cable or FiOS to transmit these nulls and mixed streams down the line.
Baff commented:
I copied the mpegs onto my Playstation 3 and played them on my 42" TV and the difference is quite noticable.
madBob commented:
And why the hell when I order NHL Center Ice package from Comcast, and Madison Square Garden broadcasts are in HD, the games are all shown in standard def? (channels 461-470 in San Francisco). What a scam!
Moose commented:
We've all missed the real story here. I was looking at the print version of the story and noticed that the bitrates for Verizon and Comcast were remarkably similar (on a chart), although slightly higher for Verizon. That means Verizon is using the same compression techniques and is lying when it says it doesn't compress beyond what is received in the original signal (19Mbps).
dd commented:
Not all compression is equal, and not all HD channels require the same amount of compression to maintain PQ. I would hope Comcast keeps Sports and Action channels at 2:1, and put things like Hallmark HD, etc. in 3:1 QAM's. They are trying to squeeze in as much HD as they can, and want to do so on the cheap.Some of their HD PQ is therefore in decline. SDV is the way to go, as other MSO's have already learned.
BTW- let's keep the FCC as far away as possible! And if Verizon's PQ is better, than Comcast will adapt to keep the customer base.
WPKaplan commented:
Hmm, do I dare suggest the FCC start regulating bandwidth per channel?? That's something that truly affects the viewing public instead of all the corporate politics Mr. Martin seems more interested in!!
Kriston commented:
One QAM channel holds approximately 25% more data than one ATSC channel. This is why the cable companies lobbied so hard to prevent ATSC from being required on their systems.
The comparison doesn't take in to account the smaller ATSC data bandwidth as compared to QAM. With slightly more compression in the transcoding step the cable company can easily fit two full-HD ATSC channels into one QAM channel with no real noticeable degradation.
What Comcast is doing is transcoding to an even higher level of compression as well as increasing the block size (no doubt using a trade-secret process from Imagine Communications).
Kordos commented:
Comcast PQ has gone downhill. I was an early adopter in 2004, back when there were just 5 channels. The picture quality was _astounding_.
Now, it's garbage - take a look at the white yardage lines in a football game, or the outline of a player - you'll see the tell-tale tiling of cranked-up MPEG compression.
Thank god the word is finally getting out there. Personally, I can't wait to switch to Verizon (ugh, did I just say that?)
I can see clearly now commented:
I was wondering why I thought the HD images were starting to look a bit choppy... Thanks! I'll be calling Verizon the moment FIOS is available.
knowitall commented:
alan sorry its the other way around. comcast for the most part DOES NOT compress local HD feeds. however some local HD stations reduce HD channel bandwith by adding local weather or news channels.comcast do however try to squeeze 3 HD channels into 1 QAM reducing bandwith by 50%. traditionally its 2 HD channels into 1 QAM. this of course is in response to satellite and phone company taking away their customers by theboat load.
Comcast User commented:
Actually I have been noticing HD broadcasts quality reducing. It amazed me watching national sports broadcast game versus the local HD sports even that was on (both recorded in 1080i) and see so many more artifact on the local feed.
I thought it was just me, but knowing bandwidth limitations and competition it does not surprise me that more and more aggressive compression has and will take place.
In the end, you may just have to move to the best bandwidth service available at the time.
I always laugh at Comcast when they say they have more HD programming, but of course less channels than say DirectTV. It's a twisted game, that consumers will figure out in the end.
-Alan
Jafraldo commented:
Your point seems accurate but academic. Like the difference between the actual definition of "broadband" and what telcos provide, for all practical purposes "High Definition" is what is given to us, not the real definition. Maybe grounds for a truth in advertising suit though.
Moose commented:
Interesting. Of course, the whole idea behind Variable Bit Rate encoding is that the human eye can't detect the lower resolution until the image is in motion, at which point the encoding kicks in to a higher bit rate. I've had Comcast HD for 3 years, and at first there were occasionally some problems with high-motion "artifacts", but I've got to say that over the past 18 months, the HD picture quality has been consistently impressive.
