August 20, 2015
by Larry Jordan
I’ve written a lot about compression. So, in this article, I want to provide a Quick Start Guide containing key points you need to know about video compression.
Key Point 1
Compressed file size is totally dependent upon bit rate.
The smaller (slower) the bit rate, the smaller the resulting compressed file. Bit rate is measured in bps (“bits per second”).
Key Point 2
However, image quality is dependent upon five factors:
- Frame size of the compressed file
- Frame rate of the compressed file
- Compression codec
- Bit rate
- The amount of movement between frames
However, it is also true that a higher quality source file will yield better results than a source file that looks awful.
Key Point 3
The file size of the source file is irrelevant to how small it will be when compressed. The compression settings, not the source file, determine final file size and image quality.
As a corollary, the frame size of the source (master) file should be equal to, or larger than, the compressed file size. Enlarging an image always makes it soft.
Key Point 4
Most compression software today works by only compressing that portion of the image that changes from one frame to the next.
For portions of the image that are the same from one frame to the next, the compression algorithm basically says: “Repeat from the last frame.”
Key Point 5
It is harder to compress something that is moving than something which is still. Moving images generate more artifacts.
Therefore, a good way to test your compression settings is to compress a very short – 3-5 second – section containing typical movement. If you can get movement to look good when compressed, you can get the rest of your movie to look good as well.
Key Point 6
When judging image quality, always compare the compressed file to the master (source) file. Never compare one compressed version to another.
Key Point 7
Because of how compression software works, it is impossible to accurately predict the file size of a compressed file before compression starts when using a variable bit rate (VBR).
A series of still images will compress to virtually nothing, because there is so little movement between frames. However, a dance recital shot with a handheld camera requires a huge file because every pixel is changing from frame to frame.
Key Point 8
VBR compression yields smaller file sizes and generally higher image quality than CBR compression, but it takes longer.
1-pass VBR is much faster than 2-pass because, in general, 1-pass compression harnesses hardware acceleration to speed compression. However, 1-pass image quality may suffer and file sizes will be larger, when compared to 2-pass.
Key Point 9
When judging image quality, if it looks OK to your eye during playback, it probably is OK. If you see small, flickering rectangles dancing in the image, you are seeing compression artifacts. Increase the bit rate and recompress.
Key Point 10
Different codecs yield different results. Not all codecs will look good at the same bit rate.
For example, MPEG-2 files used on DVDs need to be twice as big as files compressed using MPEG-4 to achieve the same image quality; However, MPEG-4 files can’t be played on a DVD, though they can be played on a Blu-ray Disc.
H.265 files, when that codec becomes widely available, will be about 30% smaller than the same files compressed using H.264, yet offer similar image quality.
Key Point 11
MPEG-4 audio requires 64 kbps per channel for best quality. A mono clip should be set to 64 kbps. A stereo clip should be set to 128 kbps.
If you are compressing human speech, set the sample rate to either 22,050 samples or 32,000. If you are compressing music, set the sample rate to 44,100 samples. In all cases, set the bit depth to 16-bit.
Key Point 12
Transcoding from a smaller format (AVCHD) to a larger format (ProRes) does not generally damage image quality.
Transcoding from a larger format (ProRes) to a smaller format (H.264) does reduce image quality. Compress files as little as possible and never recompress a compressed file.
Key Point 13
YouTube always recompresses your files. Always. Therefore always compress files for YouTube using very high bit rates so that YouTube has data it can safely throw away during compression without damaging your image.
Key Point 14
At the risk of starting a bar fight, here are some suggested bit rate settings you can try when compressing your own files using the H.264 codec. (Keep in mind that there is no perfect setting that works for every movie. Please test your settings.)
|Frame Size||Bit Rate (in kbps)|
|480 x 270||500|
|640 x 360||1,000|
|960 x 540||1,750|
|1280 x 720||2,500|
|1920 x 1080||5,000|
Video compression can be very complex. However, the more you understand about how video compression works, the better your compressed images will look.
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