Wednesday, July 8, 2015

23.976 Judder


I remember spending countless hours trying to get my graphics card to hit exactly 23.976Hz when I first got involved in the HTPC scene years ago but only to find out you need to compensate for audio clock deviations as well. So the whole experiment starts all over again. These are the countless hours I'd never get back and I hope by writing this little post I'll save you the same trouble (and potentially lots of hours of your life).

The Problem

When you play 23.976 fps materials on a 60Hz display, you get what's called a "2:3 pulldown". Frames have to be played unevenly and we all know that. MPDN (Media Player .NET) has a feature called Fluid Motion that will get rid of the judder at the expense of clarity so it's generally advisable not to enable it if your display supports 23.976Hz refresh rate. In fact, MPDN's Fluid Motion is disabled by default if the display refresh rate is the same as (or a multiple of - more on that later) your video frame rate.

This is where the problem lies. It is terribly hard to get your graphics card to output the refresh rate you want and you often have to spend a lot of time tinkering with the custom refresh rates to get it right. You also have to take into account of the fact that video is played back at the speed of the audio (you get lip-sync problem otherwise) and not your system clock. MPDN calls this the reference clock.

What this means is you can't simply target 23.976Hz - you have to target 23.976Hz +/- ref clock. So even if your graphics card does 23.976Hz exactly out of the box, you'll still have the same rate mismatch (unless your reference clock is exactly 0%). Once you've corrected the mismatch for 23.976Hz, you have 24Hz, 30Hz, 50Hz, and 60Hz all requiring the same tedious corrections. Ref clock also changes with time and only gets more accurate the longer you let it play continuously - so you usually have to spend 5-10 minutes to find out if you've got the new settings right.

All of that assuming you know how to calculate your target refresh rate (video frame rate + ref clock deviation) in the first place and from what I've seen in the forum, most people don't.

The Solution

ReClock was the solution for a lot of people back in the days (myself included) but as we moved to 64-bit, ReClock is still firmly stuck in the 32-bit land. Since SlySoft (the creator of ReClock) have no plans to work on a 64-bit version, an alternative was clearly required. In fact, it is one of the most requested feature for MPDN for quite some time now (mostly via PMs).

Rather than trying to get the display to match our video, why not get the video to match the display instead? We do it for resolution (i.e. scaling) so why not do the same for refresh rate? Rate Tuner was thus created and comes with a calculator that automatically matches your video to your display refresh rate. Rate Tuner serves as a ReClock replacement and has the added benefit of it being fully integrated into MPDN.

Visit MPDN Extensions Wiki for more information about Rate Tuner.

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