HD1080i De-mystify HDTV 1080i ::: know why before you buy

Saturday, August 19, 2006

HD & HDTV content delivery

... hope everyone is having a great summer, this has been a really excellent one - no hurricaines yet, Great private A-List parties http://dsmj.net/starroom/aug5.htm etc

So this time i will rant yet again, before getting to buiness and finally launching www.hd1080i.com this fall. ( oh and BTW i got another HD monitor - the westinghouse digital 1080p 37" LVM-37w3, and its also what i'm typing this on, demonstrating to myself that total digital experience convergence in my house is in full swing, since it's my TV, internet, video editing monitor and Game screen)

MACRO BLOCKING
bah. mpeg-ts.

The method used to get your digital video into your screen space is an MPEG transport stream.
why do you need to know this? ... because it is where the worst quality problems are happening.\ e.g. Why get a High Definition TV and digital Cable service/Dishwhatever and then end up with larger than life junky video frames. I see it everywhere, all services of all types, no one is immune.

Generally speaking a high quality well authored DVD or High Def DVD will be your best viewing experience, but quite frankly most of us wont be seeing the full real deal there in this calendar year, so i will speak to the broadcast quality digital HD and SD.

Ok so what is Mpeg and what is a transport stream?
Its your picture. Its Digital, and it can occasionally suck.
MPEG is a motion picture encoding scheme for compressing digital content, and transport stream is how it gets compressed at the source and decompressed on your end, that is the short of it. BOTH ends matter, but you basically have very little control over it, selecting the right provider company and having good equipment is about all you can do.

Basically the transport stream seeks to compress in a frame sequence optimization by only sending the content that is different in the next frame. So if a frame is mostly an unmoving background and the only things changing are the people talking ( headshots typical of news and talk shows ) then compression works really well and detail is preserved nicely because the bandwidth or size of the transport stream is composed of only the peoples heads in motion.
So in an HD video frame (1/30 sec) maybe only 10-20% of the 2 million pixels are different ,only 10-20% of the frame image data is needed to properly create the next frames. This is why Conan O'Brien in HD looks pixel perfect and really scary Conan's facial detail is always there, that guy can make his face move in such extreme ways and you do not miss a thing.

It all starts to go to hell when the entire frame is in motion and almost every pixel of the 2million in an HD frame are dramatically different, properly done, an Mpeg transport stream will then issue what is called a Key Frame whis is a much less compressed but complete image frame. That means 90-100% of the 2million pixels is needed to construct a new image for the next video frame. Uses More Bandwidth, Bigger data load.

"I See Big Squares"

Unfortunately, bandwidth numbers for the range of low data load to high data load is usually less than what is needed to send full frame transport stream keyframes every time, and the result is that you see the math in all that fail and the screen image is Blocky, There are small ones and Big ones, my biggest beef is the Big Blocks. Now your know what to call that problem. IT is a MACRO BLOCK. they are always there and the variation in its quality is what you notice, transport stream block failure, they come in various sizes 8x8 and 16x16, but bad Macro blocks are big and really botch an image past the point of tolerable pain.

In a worst case you may actually see completely missing blocks on your screen (black squares), or little squares of total garbage. These blocks are usually next to each other in a row when that happens, and it is a transport stream error correction problem , for cable that is a really lousy broadcast quality or system bandwidth overload at the provider/distribution end.

Macro Blocks are the BIG ONES = 16 x 16 pixels ...that is a data block of 256 computed pixels that is constructed at your end by your cable box / Dishthing and and then assembled to the screen memory of your TV by what is usually some very advanced firmware code. Welcome to the DIGITAL world, even if your equipment is totally high end your provider can and will send an MPEG Transport Stream that will make you feel like you are in a trailer park with a coathanger antenna...its the static crap of the digital age.

Ok lets get active. IF YOU HAVE GOOD EQUPMENT & SEE LOTS OF MACRO BLOCK ERRORS in your stuff, then i want you to IMMEDIATELY COMPLAIN in full detail about channel and time, by phone, to your provider.

... and conversely if you DO NOT COMPLAIN, then they will think thier stuff is delivering ok and continue to deliver junk possibly considering that you dont care or that such and issue is tolerable.

Most of these people behind the scenes of it all are trying hard but they cant always see everything at every endpoint in thier system and your feedback is in fact a component of thier quality control. There are a bazillion possible points of failure, including connection quality and signal corruption in the line between you and the Central Office, and if your dish or antenna is having issues, then system bandwidth overload is not the only problem, weather and sun-spots and whatall noise can corrupt the signal.

Ok i know i am way over-simplifying things, but if your video investment is important to you, you really need to know this at a minimum level. Geeklink = http://www.zenith.com/sub_hdtv/mpeg_tutorial/pixtypes.HTM

In Another Note:
Blah. dumb junk i get to read where people comment about the difference between 1080i and 1080p. IF YOU HAVE AN LCD type HD display, then you are seeing images presented full frame at a time. Progressive OUTPUT however is not a guarentee that the input is any good...Interlace errors you may see are almost always going to be the product of bad video editing at the source that was then made into a bad TS . Almost all HD source transport streams are HD1080i & therefore arrive to you interlaced, and your monitor assembles that into your image in a full frame (progressive), and scales the result to your screen.
This means that a nice 1366x768 is going to have 2 times the data incoming that it needs to create a Progressive HD image and basically it will look great. People with 1920 x 1080 displays need to be a bit pickier since all the data is used and not scaled, so all the transport stream data is seen. Fortunately most all 1080 screen manufacturers are very quality conscious and most if not all current 1080 screens are well equipped with internal computing engines and firmware.

2 Comments:

Blogger Doug said...

"I see big squares" I have macro blocks when watching ESPN News. Well that's the only station I remember it being on. But at the same time I won't have macro blocks on other stations. Can I be sure this isn't my equiptment?

9:53 AM  
Blogger Jeff said...

Hi Doug...
Probably your equipment is fine, but to really answer i would need to know if you are cable or dish or using an ATSC tuner.

There are several distrubution methods for HD content from the source channel that is then sent through a system to its subscribers..., some are a live stream replication, typically that can be a satellite feed. That said, i do not want dis DISH network or anyone, but satellite feed is prone to block errors, and that can depend on system loads, and weather, and rapid scene cuts/high action moments.

ESPN is a bit of an odd duck that way, they have a complex infrastructure, and in that particular case i would probably not complain about macroblock ESPN errors too much, we already know they are very attentive to quality.

10:32 AM  

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