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supercharging-compressor.pngYou can find more compression settings articles in our Supercharging Compressor series index.

You can be plenty good at video work if you only know that bigger bitrates are usually better -- but if you're going to be a guru when it comes to the quality of your video output, it's worth taking a closer look at the concept from the ground up.

Luckily, it's a fairly simple concept.  Let's take NTSC video as our example: 30 frames per second, each 720 pixels wide and about 480 high.  Each broadcast-safe pixel, in RGB space, could take up 235 values for each color (red green and blue), so it needs 24 bits of information to describe it.  If we were to try to store a second of NTSC-type video completely uncompressed using this bitmap scheme, we would need

30 * 720 * 480 * 24 = 248,832,000 bits, or about 30MB -- per second!

Try slapping THAT on YouTube -- or a DVD, for that matter.
Codecs (abbreviated from "compressor/decompressors") exist to try to strike a balance between the complexity of the original video, the limits on the bits that we can use store to reproduce that original video, and some model of what kinds of corners it can cut without the human eye noticing.  Think of it like running your personal budget: in your vision of a perfect life, there are a certain number of things you'd ideally love to buy, but you've only got a certain amount of money to spend -- so you have to decide which things are most important to spend money on in order to come close to that "perfect ideal."

Just like you'd throw away that luxury vacation before you gave up your basic food money, the compressor's job is to throw away less important things like color detail before, say, making your video flicker.

CBR (Constant Bitrate) encoding

A CBR encoding strategy uses the same amount of bits for every part of the video.  This has the upside of being the fastest encoding method, because it requires the least "thought" -- but it's generally the poorest in quality.  The codec spends bits blindly: to call on the budget metaphor, it's kind of like taking your date to McDonald's because "I spend $10 on food EVERY day!"  It would be wiser to settle for Ramen on a couple of weeknights, then splurge on date night, right?

VBR (Variable Bitrate) encoding
VBR does just that.  It generally provides the best quality available from a given codec, but it also takes a bit longer to encode.  When an encoder is using a VBR strategy, it tries to budget those precious bits so that it has more to "spend" on complicated scenes.  This means that, as the codec tries to decide which parts of the video to sacrifice, it looks across the whole video instead of just individual frames.

There's a downside, though: VBR strategies lead to spikes in bitrate during complicated scenes.  Depending on the specific codec and your specific arrangements, you may find that CBR videos are more suitable for streaming video over the internet while VBR video is more suitable for downloading or distributing on physical media.


One Pass or Two?

Let's extend that budget metaphor one more step: one-pass VBR is like getting up each morning and saying "Hmm, today is a pretty ordinary day.  I can eat Ramen." or "Hmm, I've got a date tonight.  Probably better to spend a little more."  It takes the video as it comes, and just tries to use its best judgment about how many bits to spend.  This is fine until the video equivalent of a wedding or something comes up -- if you'd known that there was something even more important coming up, you might have saved more money/bits to spend at that event.

Two-pass VBR, by contrast, uses its first pass over the video to sit down and map out its budget for the entire video, then returns to actually perform the encoding according to that map.  It's like sitting down with your calendar and your budget, and planning where every dollar will be best used.  It takes a long time, but the end result is the best possible budget you could come up with.

The Big Exception
There's one case where every single encoding method performs just about the same: when you're bumping up against a maximum bitrate.  Take DVDs for example: if your video is less than about 90 minutes long, then encoding at the very highest bitrate allowed by DVD standards (about 9Mbps) will never fill up the DVD.  In money terms, you're fabulously wealthy -- you can buy the finest dinner in the house every single night and not even come close to blowing your budget.  In cases like this, by all means, save time and use CBR!



In summary, then:

CBR - Poorest quality - fastest encode
VBR 1-Pass - Good quality - faster encode
VBR 2-Pass - Best quality - not-so-fast encode

comments  

Fedrick-video frames said:

Most of time videos seems to be compress but slightly, undone.So,nice suggestion is to divide the videos into frames and packet using a software....video frames

Alex said:

Fedrick, I'm not sure what you mean, but if you'd like to elaborate, I'm happy to respond to your comment!

steve said:

Thank you, Alex, for your thoughtful and succinct explanation.

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Using Final Cut's Color Correction Layout was the previous entry in this blog.

Create Chapter Markers in Final Cut - not DVD Studio! is the next entry in this blog.

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