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Old 02-14-2008, 03:01 AM   #19
drmpeg drmpeg is offline
Compression Engineer
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Aug 2007

Originally Posted by damonous View Post
drmpeg (Ron), thanks for taking the time out for some great info on this Forum. I have a couple of long-burning questions you might be able to answer...

1) Since your background is broadcast application, what do you know of Cable; specifically TWC? I've read unsubstantiated info in the past that Cable providers sometimes re-encode MPEG streams to fit their bandwidth needs. I never used to think this, because compared to my friends' Dish/satellite HD, I've always felt Cable was superior.

But, of late, certain things on TWC in my area (Cleveland) has been looking VERY inconsistent, with some very unexpected 'smelly sights'. In particular, the Super Bowl looked atrocious with gobs of messy macroblocking. Ditto to the Grammys. Other times, I see some absolutely spectacular HD, like Prison Break on Fox (in 1080i even, according to my TV's indicator).

In total, though, all channels seem to be worse (more macroblocking) than in the past. Is it possible that TWC is re-encoding and selectively cranking-down the bits?
HD encoders are too expensive for head-ends to re-encode all of their HD channels. Instead, they use rate-shaping boxes called "cherry pickers" that reduce the bitrate of the incoming streams by re-quantizing the macroblocks in the MPEG-2 video elementary stream.

The only way to really tell what your cable company is up to is to capture some cable QAM bitstreams and compare them to an OTA capture made at the same time. If you can capture both versions, let me know, and I can analyze them for you.

Originally Posted by damonous View Post
2) You posted a page or two back about banding, and I'm curious about what it the primary cause. Is it the bit depth for color, or bit depth for encode, or the codec itself?

Case in point: I recently rented Sunshine on BD. I've always been VERY fond of the quality of Fox/MGM BD's, owning about 15.

For the most part (like 99.99999% ), this was another stunning transfer with spiderweb-fine lines clearly depicting the spaceship's communication arrays, and excellent compression and greyscale control (especially for a film advertised on the sleeve as AVC at only 14mbps!).

Yet, there is one absolutely atrocious moment at the 3/4 mark of the movie, where the ship is moving in space against an eclipse of Mercury against the Sun. Suddenly, a frightening blooming mush of muddy 256-color-looking orange halo around the ship (where a proper haze/mist cloud should have been).

It's a BD50, --so what gives? ...Was the compressionist asleep at the wheel? Is it a limitation of AVC? Is it a limitation of available colorspace?
It's mostly a problem with 8-bit video, but the encoder (of any codec, not just AVC) can make it worse. A look at the quantization levels for those frames would show if the encoder was making things worse.

Without seeing the 10-bit source, it's difficult to know what the compressionist was thinking. The compressionist may have taken a look at an 8-bit version of the source and decided the encode was faithful (enough) to that.

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