12 December 2009

Sony BRAVIA problems with PAL playback

A couple of nights ago, I noticed my TV/DVD combination was acting up. PAL DVDs would play with a horrible irregular juddering motion. I checked broadcast TV and the PS3, and they were both fine; I checked the DVI/HDMI cable, that was fine also. I started to suspect the DVD player.

Friday, I bought a new DVD player. $44 at Fry’s Electronics. It had the same problem. Further investigation and experimentation eventually revealed that the issue was a setting on the TV.

The short summary: If you have an NTSC Sony BRAVIA TV but have no analog video sources, find the CineMotion setting in the TV setup, and make sure it is turned off, not set to auto or on. Even if you have some analog video sources, you should turn CineMotion off if you don’t use analog sources to watch movies. You don’t need it, you don’t want it, all it can ever do is mess up your signal. The long explanation follows…

Conventionally, movies are shot on film at 24 frames per second. When they are transferred to US NTSC video via a Telecine machine, the 24 frames per second must be converted to 29.97 frames per second, or 59.94 fields per second–which I’ll call 60 fields per second for the purposes of this brief discussion.

The process used is called 3:2 pulldown, because the first frame of film used to end up turning into 3 fields of video; the second frame of film ended up as 2 fields of video; the next was 3 fields again; and so on, alternating. Nowadays, frame buffers allow the 2 and 3 field allocation to be varied, so you tend to get 2:3:3:2, which results in fewer video frames whose contents are taken from two different film frames. But all pulldown options share the same fundamental defect, which is that the frames of the movie are no longer all of the same duration. This tends to make tracking shots and motion look somewhat odd.

Modern digital HDTVs don’t need pulldown. The HDTV standard, ATSC, mandates that TVs support 24 frames per second–and also 23.976 fps, which is the speed 24fps movies used to be slowed to before performing 3:2 pulldown, so as to end up with 59.94 fields per second. So an HDTV can display a 24fps movie at 24fps. If it’s a 120Hz set, it can even display movies with no split frames at all, as 24 goes evenly into 120.

Sony BRAVIA HDTVs therefore have a feature called CineMotion buried away in the setup menus. This detects incoming 3:2 pulldown video, and dynamically works out the pulldown pattern, reverses the pulldown and recombines the fields into 23.976 frames per second, buffers them, and then shows the result at exactly 23.976fps. Your movie motion looks smoother and more natural as a result.

My DVD player only pumps out a progressive signal (480p or 720p) via HDMI, so the TV never needs to do reverse pulldown. If the DVD is a movie, the MPEG-2 video file is 24fps, and the DVD player turns that into 24fps digital stream to the TV. So I have no use for this advanced CineMotion feature. It’s only applicable to analog interlaced video sources, and the only analog video source I have is the Wii–and that’s 480p via component cable and never shows 24fps movies, so it doesn’t need processing either.

Somehow, CineMotion got turned on in the setup menu–either as a default, or maybe I was playing with settings and turned it on without realizing what it was. And sadly, there’s a bug in Sony’s TV firmware: it doesn’t seem to check whether the source is interlaced before applying CineMotion post-processing. Instead, it just checks the frame rate of the decoded frames to decide whether to buffer them.

PAL video is 50 fields per second, or 25 frames per second. This seems to be close enough to 24 frames per second that it triggers the CineMotion buffering. The TV tries to take the incoming 25fps video, and show it at 23.976fps. This results in disaster; every now and again, the TV realizes it has fallen too far behind the incoming data stream, and drops an entire frame to catch up.

So that is why my UK DVDs were looking like crap. I turned the CineMotion feature off, and now everything looks good again.

So I have a second DVD player which is, strictly speaking, unnecessary. However, there’s an upside. My original player was state of the art 4 years ago, but technology has marched on. The new $44 DVD player upscales to full 1080p, the native resolution of the TV. This seems to give a better picture than upscaling to 720p and then having the TV upscale again. The fancy noise reduction and motion smoothing of the old DVD player are also unnecessary, as the new TV has even better implementations. As a final benefit, the new player has true HDMI out rather than DVI, meaning I get audio and video through the same cable, with no need to adjust timing between the two. I also notice that the new player is very light and runs cool, whereas the old one had a lot of circuitry packed in and would get hot. So, I’m keeping the new DVD player and retiring the old one.

Ah, technology, where the state of the art from 4 years ago is today’s doorstop.

Oh, and if you want a region-free DVD player, pick up a Coby DVD288 at Fry’s.

© mathew 2017