HDTV aficionados with Comcast service might be in for a rude
awakening: the nation’s largest cable provider seems to have ratcheted up the
compression on its cable HDTV signals.
A thread at AV Science Forum updated last Monday
details what appears to be compression of up to 38%, allowing Comcast to
deliver more HDTV channels per line while using the same amount of bandwidth. A
side effect of this, however, means that HDTV’s pristine video is now jagged
and muddy for Comcast customers, full of MPEG-style compression artifacts and
stuttered movement:
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 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 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.
Still images comparing Verizon’s FiOS HDTV service with Comcast’s
HDTV service, taken at the exact same time in the exact same broadcast, show
Comcast’s images losing much of the legendary detail that HDTV is so well known
for – in a screenshot of the Red Hot
Chili Peppers playing live in Milan, the Comcast image was almost completely
stripped of all fine-grained detail; lead singer Anthony Kiedis’ textured
wristband becomes flat and blocky, and the tattoo on his left arm made pixellated
and blurry.
A request for comment was received by Comcast, but not
replied to.
The purpose of Comcast’s increase in compression is unclear;
however it would appear that the company is attempting to fit three HDTV video
streams inside of one QAM (Quadrature Amplitude Modulation, Comcast’s DTV
broadcast format) signal, as opposed to the previous two. In a bitrate
comparison between each provider’s broadcast of the same show, the Verizon
signal was recorded at 17.73 Mbps, while the Comcast signal recorded at 13.21
Mbps, a 34% reduction in size.
According to Ken Fowler, the A/V buff known as “bfdtv” at AV Science Forum, Comcast’s compression increase currently affects most
customers that were not originally in Adelphia’s cable system, which Comcast
purchased in 2005. Further, the increased compression only affects national
networks like A&E or HBO; local TV signals are rebroadcast at whatever
bitrate they were originally sent in.
