Originally Posted by
bfdtv 
Last updated: March 16, 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 into ~25Mbps 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.
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:
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%
National Geographic HD 11.39 Mbps 10.32 Mbps -10.3%
Universal HD 12.72 Mbps 11.01 Mbps -15.5%
Starz HD 11.93 Mbps 9.76 Mbps -22.2%
Comparisons for more channels will be added next week.
As of Tuesday, March 11, a few channels like ESPN-HD and Comcast Sportsnet HD still remain at full quality, identical on both Verizon FiOS and Comcast.
Comparison screenshots of more channels coming next week.
Initial Thoughts
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 excess noise and detail loss. In contrast, higher-quality sources like Discovery Theater still look quite good, with the only obvious differences being added noise and loss of fine detail during motion.
Generally, 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 object transitions. 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 Discovery Theater images (above) are a good example of that. 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 excess motion blur seen with some video sequences on Comcast.
SourcesRound #1: Monday, March 9-11A&ETV HD - The Sopranos: All Due Respect
Discovery HD - Mythbusters: James Bond Special
Discovery HD - The Human Body Pushing the Limits: Sensation
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 HappinessRound #2 coming next week...
