Interlacing, 601, 24P

This is a post complaining about video technology and the attempts at trying to make it work the best for everyone.

Interlacing

The first of my favourites to complain about is “Interlacing”. As a lot of people know (or at least those that have played around with television and game consoles, etc) there is a thing that exists in television land displays that doesn’t (for the most part) exist in the land of computer displays, this is called “fields”. In interlaced footage each “frame” is made up of two “fields”, one upper (even) and one lower (odd). Each of these fields is separated by half the frame duration, meaning the frame is separated into two temporal snapshots of the footage. The antonym of interlaced is progressive, which is where the frame is not split up in any temporal fashion. Basically this is where the “i” (for interlaced) and “p” (for progressive) come from in the various HD standards.

Comparison of Progressive and Interlaced footage.

So what’s to complain about!?… Well it all depends on what people want the project to “feel” like. Progressive footage “feels” expensive, but the motion can be jumpier or (if using motion-blur) too blurry, and interlaced footage feels smooth and fluid, but at the same time it feels “cheap”. Try to explain that to a client that wants it to be smooth AND expensive looking.  :-?

The solution is basically to combine both (interlacing and a bit of motion-blur), sure it takes a bit longer to render, but it gets most of the best parts of both alternatives. Simple!

601 Levels

Ok this one is simple too … if only! Everyone knows the image luminance (think of it as brightness… sort of) goes from black to white, right? Wrong, well at least in video land it goes from “almost black” to “almost white”, or more precisely in the 8 bit colour value system from 16 to 235. The reason for this goes back many decades to the original television specification and the transition from black and white to colour televisions and the accommodation of “overshot” and “undershot”, all a bit technical and complicated (although interesting for us video freaks). Although this reduced luminance range shouldn’t strictly be referred to 601, the term “601 levels” is generally used to refer to imagery at this reduced level. 601 (rec601 or BT601) actually refers to a CCIR standard to the encoding of video signals with chrominance sub-sampling.

Comparison Colour Range (Gamut).

So what does this mean? Well basically, the consequence is that you can’t reliably exploit the full colour gamut of your imagery without risking the image being TOO contrasty on the typically configured home (and cinema) theatre system. AHhhhhhh… “give me my full colour gamut, or give me death!”

DVD-Video can accommodate the full luminance spectrum, but most don’t use it because it makes the picture look too high contrast (because the default TV is set to high contrast to make typical 601 video levels normal). Most HDTV standards support full spectrum (with some even supporting over-bright or high dynamic range (HDR) imagery), but once again they aren’t being used because a majority of material have legacy levels, so the factory presets still tend to be set to 601 level.  Will we ever learn?… probably, because as more and more televisions are used for games and home media-centres the video levels coming out of this device are usually full spectrum and the end user may adjust their displays to match.

24P

This is one of the very sticky, and annoying standards that we often have to “explain”, particularly as a DCC to the creator/self-publishers. 24P is not video shot like film, what it really is is US television behaving like film. If you are authoring to 24P on Blu-Ray, you are actually creating content at 23.976 progressive frames per second. You are stilling publishing progressive frames, but they are ticking over at a slightly lower frame rate (the formula for which is frame-rate divided by the carrier’s stream overhead, namely 24/100.1 = 23.976). How these frames are displayed depends on the “black box” configuration of the decoder and display. A lot of systems just convert it to 50i (the Australia standard - if such a concept can exist in modern television).

The real issue starts when you add to sound to the pictures. For a film speed production, typically the sound mix is done at 24fps. Sounds like 24P to me!? No no no! This is where some the experience can go really pear-shaped for the audience. Remember it is really 23.976P not 24P. Doesn’t sound like much, but in a 10 minute programme you are going to be 346 frames (14 seconds) out between the audio/vision, and as we can only really tolerate two frames difference between sound and pictures we would definitely see/hear a problem even after a couple of minutes.

Ok that’s enough whinging for now… maybe

List of the Resources:

• Nonlinear 4 - the bible for all things non-linear by Michael Rubin (maybe there is a 5 in the works??)
• SMTPE - The society of moving pixels and stuff.
• The Lurker’s Guide - The evil lurking guide to video for programmers.
• Adobe DV primer (and others) - great resource for a start (but very focused on Adobe’s own products… of course)