How much better is 12 GOP?
 

While at first glance a 12 GOP may seem much better the way I understand the way GOP's work is that 12 GOP's isn't really all that much better than 6 GOP's. It is better but not no 2x better. Am I wrong on this one?

If you think about it the only difference between IBBPBBIBBPBB and IBBPBBPBBPBB is that the 12 GOP version has one less I frame and in it's place a P frame instead. That one frame reduction from I to P doesn't seem like it would free up a whole lot of bits.

The lower number of GOP the better the image. 12 is not better than 6. It is this number of frames it looks at to throw data away. This is why 6 GOP is better than the Sony HDV format for example.

The smaller the GOP the more I-Frames. 12 ain't better than 6.

I mean beter in terms of having to deal with twice the data. I realize a smaller GOP is better. Well until you start to get too small of a GOP. For example if it was I frame only then that 19.7 mb/s wouldn't leave very many bits for each frame.

I guess my whole point here is how many bits are we really saving by using a 12 GOP instead of a 6 GOP? Is it really enough to handle 2x the data? I personally do not think so but then again I haven't seen any of the 60p video so I cannot say.

Is 12 GOP's the rate for 720p 60p HDTV broadcasts?

You can see 60p in the JVC booth at the stand where they are showing the tiny, $8000 encoder/decoder.

They are running a D5 tape of ABC 720p60 to one screen and also sending the data through the new encoder which uses the new 60p chip. This is then decoded by JVC's $30K box and shown on a second screen.

Result -- no difference even with fast moving video.

Multiburst very clean to 30GHz through the new chip.

So the new chip easily handles 60p with its 12 frame GOP.

By the way, ATSC is 15 frames so by the logic that longer is "worser" -- the 12 frame is "better" than any 720p broadcasts you've ever seen. In fact ATSC 720p60 has a video encoding data rate of 15Mbps verses 18Mbps for JVC's ProHD -- so it's actually a higher data rate than what you've likely ever seen.

With MPEG -- the encoder is where the magic is done. The encoder can alter the "efficiency" so that you really can't compare data rates.

Also think about how MPEG-2 encodes. Why might 60fps be better to encode than 30?

Well 60 fps has less motion between frames compared to 30 fps. So for the most part the 12 GOP is covering about the same exact amount of time that a 30p 6 GOP would. Both cover 1/5 of a second in time but the 6 GOP version has bigger motion jumps between frames which are harder to predict.

Thanks Steve for the info. I was sure it looked good but I didn't know how good it was looking. I trust your eyes.

Just to clarify, ATSC doesn't impose any requirement on the length of a GOP. Some 720p60 stations use a 15-frame GOP (like WFXT, the Fox affiliate in Boston) and some use a 30-frame GOP (like WCVB, the ABC affiliate). You could use a 1-frame GOP, a 6-frame GOP, or even a 300-frame GOP if you really wanted to. Of course the last one would really be unpleasant for channel-surfers... :-) So ATSC does suggest an I-frame at least once every half-second.

There's also no 15 Mbps limit on the data rate. ATSC-conformant "terrestrial" decoders need to deal with video streams up to 19.4 Mbps. WCVB (the ABC affiliate with the 30-frame GOP) puts about 16.6 Mbps into the video.

Exactly Keith, Broadcasters have a 19.4Mb pipe to fit everything into. I've heard of some HD broadcast as low as 8Mb @ PBS. It's how the broadcaster wants to divi up the bandwidth between SD and HD

Smet,

You nailed it. 12GOP is the same fraction of time for 60fps as 6GOP would be for 30fps therefore our prediction was correct. JVC had to either A) Speed up the tape in the tape drive or B) make the encoding twice as efficient at the same datarate. I'l bet the encoder get's HOT.

You are exactly right.

Since compression works on the delta (difference) between frames -- motion between frames can be the biggest cause of differences -- the smaller the interval between frames, the smaller the delta, hence the less that needs to be compressed. The less that needs to be compressed, the less data to be recorded.

Steve I just want to thank you for all of the great information you have put out there over the last few months on how mpeg2 works and what the GOP structure does. You have really taught me a lot.

And boy does it show. PBS used to have some of the best looking broadcasts....now they're the worst. Your lucky to not see any compression artifacts even for a second or two.

I wish the stations would not be killing their content with multiple SD embedded streams with their HD streams. 19.2 is barely enough as it is for fast movement at 1080i and then reduce it further by embedding a virtually unwatched weather channel (NBC affiliate in my town does that) but the ABC signal has no other embedded stuff thank god and their programming always looks great.

I look forward to saturday or sunday night movies shown on ABC 720P.

There are two additional Kawality Killers.

1) USDTV buys what they call "unused" OTA bits from as many stations in an area as they can. Which means in the cities they serve (Las Vegas is one) can have ALL their OTA stations look like crap. Maybe not as bad as PBS, but not good.

2) While TWC does use "rate re-shaping" it appears Cox does. Which screws-up even Discovery and INHD. Moreover, because Mark Cuban supposedly will not sign with cable companies that lower his quality--we Cox company subscribers don't get HDNET. Doubly screwed!

The FCC needs to regulate cable transmission instead of worrying about Janet's t*ts.

Let's not gang up on PBS (the best broadcast content in America IMO). I see the same "breakup" @ ABC, NBC, FOX and CBS and many other "digital" channels. I've even seen full breakup of movies as if you were playing a DVD and it had some smudges on the disk while "on air" and broadcasting.

Local PBS broadcasters (WYIN, WTTW, WYCC) have invested millions in HDTV infrastructure. I think the 8Mb initially was just to get up and running with a signal. Now the backbone is in place I think they will comply with the 2006 mandate. I believe that was the government/FCC goal to begin with, getting stations they completely control (PBS) inline.

I'm not sure what "mandate" you are talking about, but if it's the DTV Mandate, then it has nothing to do with HD at all. Especially HDTV quality.

The plain fact is that 1080i requires a full 19.4Mbps. Any PBS station who chose 1080i should have realized that by doing so it ruled-out any option to multi-cast WHILE transmitting HD. But no! PBS ripe with taxpayer money wanted 1080i cause it had the biggest numbers. Then, to get contributions, they promised multiple-casting so they could say they had "kids" programming and "school" programming, etc.

At first they talked of multi-casting SD by day and HD at night. That would be fine, but that's not what they do. Instead, they pipe the same stuff night and day. So both the SD and HD are bit-starved and thus unwatchable.

Moreover, why should taxpayers pay for HD crap like Boohbah, Jakers!, Flip Flop Shop, and the dreadful Mustard Pancakes. Are kids really watching HD at 3AM! Do kids really need HD! Should they be watching TV at all?

The fact is PBS at al is stuck in the "voice of god" doc style from `50 TV.

Moreover, IMHO, Discovery, National Geographic, INHD, HDNET, TCM, Sundance, IFC, and AMC do everything PBS does--better. Unless, you think the oh so Politically Correct POV counts as doing something others don't do. These VHS/DV docs keep repeating the same 3 or 4 PC themes because they sound daring but in the 21st Century are really, very, very safe.