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Understanding encoding

From: Compressor 4 Essential Training

Video: Understanding encoding

When we talk about the concept of encoding, as differentiated from transcoding, encoding is pretty much taking big video and making it small. Now, I do want to mention that, again, this concept of transcoding and encoding technically they mean the same thing, but I tend to refer to transcoding as the idea that you're changing from one editorial format to another editorial format, or to an archival codec, where when I talk about encoding I'm really trying to make the file smaller and smaller for the use of distribution.

Understanding encoding

When we talk about the concept of encoding, as differentiated from transcoding, encoding is pretty much taking big video and making it small. Now, I do want to mention that, again, this concept of transcoding and encoding technically they mean the same thing, but I tend to refer to transcoding as the idea that you're changing from one editorial format to another editorial format, or to an archival codec, where when I talk about encoding I'm really trying to make the file smaller and smaller for the use of distribution.

You can see here I've got a half-gig file. This is my Mission Statement Starting Point. This is stuff I have encoded in earlier sections in this title. You can see that I've made it as small as 9 megabytes. I've made it really, like, significantly smaller, and that's what Compressor's major features is going to be for us. When I go inhere into Compressor, you can see that most of these presets, most of these settings--whether it's Apple devices for your iPhone, disc burning for DVDs, or video sharing sizes meant for Vimeo or, say, YouTube--all of these are taking your big original gorgeous video and compressing it down to small as it can get to be handled for distribution.

And there are three real ways that this happens, and let's take a look at these graphics here. We're adjusting the frame size, we're doing what's called spatial compression, and we're doing what's known as temporal compression-- the simplest of which is frame size, so we're going to start with that. And the way frame size works is this. We're going to take something and make it a smaller picture. The smaller the picture, the less compression that needs to be done--the less area we physically need. Very common to go from 1920 x 1080 down to 960 x 540.

That 960 x 540, that is still larger than standard-definition video. We could still call that a high-def file, but we've reduced the vertical, and we've reduced the horizontal by one half each, meaning we're only dealing with a quarter of the amount of area. The beauty here, for you as a practical user, literally speaking, we've made the file one quarter of its original size, merely by sizing it down. For those of you who are working at 720, we have the same ability. We're working here from 1280 x 720, and it becomes 640 x 360.

This idea that we cut the frame size really can make a difference in making a file small. It may not be ideal in every situation. For example, if I'm trying to project this and can carry it around on a thumb drive, I don't want to make the frame size any smaller. But certainly for distribution on the web, for giving a client a quick file, this becomes a great way to make the file smaller. The second method, what's known as spatial compression, you're already familiar with if you've ever used or made a JPEG file, say in Photoshop. We start with large frames.

All editing codecs have all the information on every frame; that's what an IFrame is. Since every frame has all the information, your editorial software doesn't have to work very hard. What's very common is that we sit back and we do some very basic JPEG-like compression to make every frame smaller, and as we make every frame smaller, we make the file smaller. Now this is no different than sitting back and feeding every frame into Photoshop and just turning up the JPEG slider; in fact, editorial codecs often already do this.

So if you're working with, say, DVCProHD, it's already has some spatial compression occurring, making the file smaller, meaning that it's more efficient for your editorial tool at the cost of a little bit of quality. Where the real magic in distribution compression occurs is what's known as temporal compression. In temporal compression we don't have all those beautiful green I-frames; what we have is we've got a full frame and then we're only sending the changes. So if it's a talking head, maybe you're seeing a couple pixels change around the mouth and the head, but the background isn't changing whatsoever.

The way this is done is what's called a group of pictures, where we've got a full frame, a full I-frame, with all the information every 15 or so frames, and the red frames are just the changes. Since we're only sending the changes, this is a lot less information to send. The line itself represents what the average of that is, and you'll see there is some points where the red frames are above the line and some where the red frames are below the line. It's the average we talked about. But with the sort of compression, we can often get ten times, twenty times, as much as fifty times compression, because we're only sending the changes, rather than having to show every single frame.

So this is the beauty of compression; this is the real magic in it. We change the frame sizes on occasion, we do spatial compression the way we'd work with JPEG, and we do a temporal compression where we only send the changes. And that's really what the goal of sending material through a Compressor is, is to take big files and make them small.

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Compressor 4 Essential Training

32 video lessons · 8856 viewers

Jeff I. Greenberg
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