Image Quality vs. Bit-Rate


If you've ever exported a video, bit-rate is a concept that you should probably know about.

This blog post is designed to help everyone handle their Premiere Pro/Avid/Resolve exports - and I'm going to shed some light on the advantages of working with a manual bit-rate workflow.

If you already well versed with bit-rate, scroll down to find a handy bit-rate guide for 25FPS exports. Otherwise, let's get into it!

Exporting a file can be overwhelming.

Bit-rate was one of those settings that I never felt qualified to fiddle or experiment with until I got a job as a data wrangler - now I've been through that process let me give you the gist.

WHAT IS BIT-RATE?

The definition of bit-rate is actually fairly self-explanatory.

Bit-rate is the amount of data per second that is used to encode video or audio. The higher the bit-rate, the higher the quality of the file and the higher the file size. A low bit-rate encode can cause a drastic loss in quality while creating manageable, small file sizes.

When you set your target or maximum bit-rate, you will either be presented with a Kilobits per second (Kbps) or a Megabits per second (Mbps) option.

Kilobits are 1000x smaller than a Megabit (36mbps = 36,000kbps)

Let's see that visually, shall we?

A nine-second HD clip exported at 30mbps = file size of 31.2 MB


A nine-second HD clip exported at 1mbps = file size of 1.9 MB

Higher frame rates and/or higher video resolutions require a higher bit-rate to maintain the same level of image quality. This is to accommodate the increased information per second that high resolutions/frame rates bring to the table.

So if you're putting a UHD video onto your website, keep in mind that your bit-rate is going to be substantially higher to accommodate that resolution. Not only that, but remember that a higher bit-rate will increase the bandwidth that the user will have to chew up in order to stream your video.


YouTube's recommendation for bit-rate encoding based on resolution and frame rate.


Vimeo's recommendation for bit-rate encoding based on resolution.

The above tables from YouTube and Vimeo outline ideal suggestions for bit-rate, balancing upload speed and quality. As you can see, UHD content is recommended to have a bit-rate 6x higher than HD! NB: If you're interested to check out the best compression settings for Vimeo exports - check out this fantastic blog post by Zena Hirsch (vimeo.com/blog/post/untangling-the-knotty-myths-of-video-compression).

CODECS & BIT-RATES

Wait a minute. So can you change the bit-rate setting for every video codec out there?

The short answer is no. The long answer is, it depends.

Codecs such as ProRes and DNxHD have set, defined bit-rates that are the user cannot change during exports. Wide-spread variations of these codecs are available to allow a broad selection of quality/file size options. Side-bar, this is great! Bit-rate isn't something you really have to to think about when you export these codecs. If you're a nerd like me though - it is interesting to know what bit-rates these codecs use internally. I have listed some below for reference, please be aware that these only apply to 25FPS footage.


ProRes 422 Options (1920x1080)


As you can see, Apple doesn't include the bit-rate in the naming convention, instead indicating the quality with an LT or HQ label. Classic Apple, keeping it super simple!


DNxHD Options (1920x1080)

The "x" indicates 10-bit footage

As you probably noticed, DNxHD handily places the bit-rate at the end of the codec name. Rather strangely, this can actually cause confusion for some exports. If you don't believe me, have a look below:


If you've ever stumbled upon this funky looking configuration above and been confused, I don't blame you. So, what does it mean?

Well, higher frame-rate footage requires higher bit-rates to maintain image quality, right? The naming convention above states the different bit-rates for three common- frame-rates 30FPS, 25FPS and 23.98FPS.

WHEN CAN I CHANGE MY BIT-RATE SETTINGS?

I'll give you the answer in five characters. H.264

Well, strictly speaking, there are a few more codecs that will allow you to manipulate bit-rates (the most relevant one among them being H.265) - but H.264 is the important one for now, and it is the delivery codec that I use the most. In order to understand how I choose my bit-rate settings, it's important to understand what types of H.264 exports I deliver every week. I can define them as follows:

1. Approval File

A low-quality/resolution file for client approval, most of the time uploaded over the internet via services such as WeTransfer or Vimeo.

2. Client Screener File

A mid-quality/resolution file for a client screening session, generally played on a television or projector as a final pass before signing off the project.

3. Deliverable/H264 Master File

A high resolution file designed to stream on the web and retain the highest quality possible. Uploaded via Vimeo or YouTube/archived on HDD or LTO tape.

So, with no further ado, here is a quick bit-rate guide for exporting 25FPS footage to the above specifications:

QUICK BIT-RATE GUIDE FOR 25FPS FOOTAGE

1920x1080

Mid-quality: 15MBPS use for approval/client screener files.

High-quality: 25MBPS use for higher quality screeners/deliverable files.

Extreme-quality: 35MBPS

use sparingly for 1080p deliverables from noisy or grainy content or if the client has requested the best possible quality.

1280x720 Mid-quality: 5MPBS use for approval/client screener files.

High-quality: 7.5MBPS

use for higher quality screeners/deliverable files.

Extreme-quality: 10MBPS

use sparingly for 720p deliverables from noisy or grainy content or if the client has requested the best possible quality.


You should be able to follow this guide to choose the minimum bit-rate required to maintain image quality, while reducing file size and decreasing the likelihood of bandwidth issues.

Wellington, New Zealand

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