4/19/2006 6:52 AM
Let’s start with an acceptable fact: MPEG-2 uses motion prediction on anything more than a GOP of one.
Even small two-frame GOP MPEG-2 IB codecs use motion prediction to code the B frames based on a forward prediction from a previous I frame, as well as a backward prediction from a succeeding I frame. What this means is that the B frame is not a representation of what really
occurred, but a computer estimate based on what happened just before and just after.
TiVo’s multiple quality levels use MPEG-2. It’s been reported that a TiVo GOP has more than 18 frames. One report states a TiVo GOP of 58 frames.
You know where this is going don’t you?
There’s a sports league, whose name I cannot divulge, that has decided to do instant replay on the cheap. I’m OK with that for arena playback, but this league is using it for officiating.
And what are they using? TiVo (or maybe it’s a TiVo-like device that they call TiVo, but that’s what I’ve been told). To be fair, I would have the same problem if they were using a DVD recorder with its 18 frame GOP.
The problem is, that unless the official is looking at an I frame, which is a complete frame not based on any motion estimation, what that official sees is not what happened, but what a complex algorithm thinks happened.
Why would a league do this? The answer could be as simple as wanting to have at least the same technology as their home viewers for reviewing a call. While all TiVos may be the same, how they operate isn’t.
What if the on-site TiVo is set to the highest quality and the home viewer’s TiVo is set to a lesser quality? The home viewer’s GOP is going to be larger than the on-site GOP. At the same moment in time, the same viewed frame on-site and in the viewer’s home could (and probably does) show slightly different pictures.
Even if the quality levels are the same, with a GOP of 18, the odds are 18:1 that the I frames of each TiVo have fallen in exactly the same spot in time. Add in some noise and some frame syncs due to transmission to the home, and those predicted frames could still look different. (Let’s not even talk about the fact that the signal might be delivered digitally and be compressed before the home TiVo even touches it.)
I love instant replay. To make the viewing experience better, it’s a great tool.
I even love officiating replay and I’m OK with an IB GOP of two. (And yes, you can do I frame only MPEG-2, but what would be the point?) But 18?
Here’s what I’m waiting for: some controversial call to be upheld or overturned based on a long GOP MPEG-2 officiating replay system. But lo and behold, a news crew was shooting the same play, from a similar angle with an intraframe system like DVCPRO or DVCAM. Each of their frames is a real representation of what happened.
Now the best part: the intraframe system shows something different than the officiating system’s best guess which predicted
the frames used to uphold or overturn the call.
Does anyone other than me think that MPEG-2 for officiating isn’t a good idea?
Michael Silbergleid is the editor and associate publisher of Television Broadcast. He can be reached at firstname.lastname@example.org.