In this article I attempt to explain the basic relationships between bit rates and file size and video and audio quality to try and explain what you should and shouldn’t do at the time of encoding your videos.
 
For the most part this article assumes that the target of your encoded video is ExposureRoom or some other video website. Please also note that in order to keep things simple and explainable, I’m assuming CBR (constant bit rate) mode of encoding even though VBR (variable bit rate) is what you should use when you encode your video.

 

8 bits = 1 Byte
b = bit
B = Byte

Notice the lower case "b" indicates bits, while the uppercase "B" indicates Bytes. This is a very important distinction to keep in mind. Also keep in mind when we talk about Internet bandwidth, download/upload speeds, video and audio bit rate we always use bits, while when we talk about file size or hard disk capacity or computer memory, we always talk in Bytes. For example your computer has 4GB of memory and not 4Gb.

1kb = 1 kilo bit

 

When it comes to the Internet and bandwidths or video and audio bit rates 1kb = 1000 bits and not 1024 bits as is common when we talk about file size, or disk space or computer memory.

The higher the video bit rate you encode your videos at, the better the video quality will be and larger the resulting file size. In other words, file size is directly proportional to video bit rate (and video quality).

 

Encoding your video at a bit rate higher than the bit rate of the source material adds no value to the quality of your video, but will increase the file size. That means there is no point in encoding your videos at a bit rate higher than that of your source video.

 

Some common video sources and their bit rates

All HDV cameras	25Mbps
Canon XH-A1 (this is an HDV camera)	25Mbps
Canon HV20/HV30/HV40 (these are HDV cameras)	25Mbps
XDCAM HD (HQ mode)	35Mbps
XDCAM HD (SP mode)	25Mbps
Canon 5D Mark II	40Mbps
Canon 7D	40Mbps

 

The higher the audio bit rate you encode your videos at, the better the audio quality will be and larger the resulting file size. So once again, file size is directly proportional to audio bit rate (and audio quality). And again, encoding your video at an audio bit rate higher than that of the source material adds no value to the quality of audio but will grow the file size.

 

There is no correlation between dimensions of your video and file size. As surprising as that may seem, it is true. The dimensions of your video have no impact on the resulting file size.

However, a video encoded at 1920x1080 will require a much higher video bit rate (for the same perceived visual quality) than a video encoded at 1280x720. How much higher? Well, let's first understand that we’re talking about the same perceived visual quality. That’s pretty hard to judge but math makes it really simple to determine. Let’s say you have a video that’s been encoded at a certain bit rate and dimensions of 1280x720 and it looks absolutely stunning. If you were to encode the same video at 1920x1080 to get the same stunning look, you’d have to encode your video at more than double the video bit rate. Of course, we know now, that that would also double the resulting file size!

Here is the math
1920x1080 = 2,073,600 pixels
1280x720 = 921,600 pixels

The difference in the number of pixels is 1,152,000 and that is 55.55% more pixels.

 

So what should you do? Well, that depends on your target. Again, if the target is online video and the largest dimensions the website transcodes your videos at is 1280x720, then stick to that, because you’ll get the best resulting video quality (quality of the video you see online) for  half the file size. Half the file size means it would take a lot less time for you to encode and half the time to upload the video to the website in question.

 

If your video source material’s video bit rate is higher than 5,000kbps then encode at 5000kbps. If not, then match the source material’s video bit rate (look at the table above for video bit rates of various sources).

If the audio source material’s audio bit rate is higher than 320kbps then encode at 320kbps. If not, then match the source material’s audio bit rate.

 

Keep in mind that if your source material is from mixed sources, then you should probably encode your audio at the bit rate of the source that has the lowest audio bit rate. But let’s say you’ve got one source material of background music at 128kbps and voice that’s been recorded at 1,411kbps (Let’s say you used the Canon 5D Mark II or 7D to record video and audio), what do you do? Since you want the “best” audio quality possible I suggest you encode your audio at 320kbps. This way, the voice will be as good as possible*. The background audio won’t get any better and the resulting file size will be unnecessarily larger than it needs to be, but it may well be worth it.

Why 320kbps? Note that ExposureRoom is the only website that encodes the audio of your videos at the same or best audio quality possible. All other websites encode your audio at 128kbps.

 

*Since most online videos are transcoded to Flash Video and Flash Video uses MP3 for audio, we limit the audio bit rate to 320kbs.

 

Find out (from the website you intend to upload your video to) what they encode your audio at and use that as your guide.