Sorenson Compression of QuickTime Movies

Sorenson Video is a QuickTime software codec -- a method of compressing and decompressing QuickTime movies which does not require special hardware. The Sorenson compression process takes a QuickTime file in DV format and reduces the file size frame-by-frame. This is a slow process, but it produces a file that can be played back efficiently in real time by any PowerPC or Pentium computer that has QuickTime 3+ installed. The Sorenson3 version works with QuickTime 5+.

The Sorenson codec was developed by Sorenson Media and is available free with QuickTime. It can potentially produce a compressed movie with better resolution, larger window size, and higher frame rate (and all this at a smaller file size) than older codecs such as Cinepak. It is suitable for movies intended for web download or streaming.

There are a number of applications (including QuickTime Player which comes with QuickTime) that can access the Sorenson codec, to compress a movie for use in a multimedia presentation or on the World Wide Web. See Exporting Still Frames or QuickTime from Video.

This page shows still frames from files compressed in slightly different ways from the same source material, using the free version of the Sorenson codec. You can download some of the actual movie files to compare them in motion.

How These Examples Were Created

In order to produce the best-looking results, compression demos are often produced using the best possible source material. The resolution, steadiness and lighting of the original footage have a significant effect on the quality of the compressed movies at a given data rate (the number of kilobytes per second of video). Sorenson Media suggests this formula as a starting point for setting the data rate: (Height x Width x FPS) / 35,000.

The sequence used here, on the other hand, is a "torture test" for compression -- an example of the kind of material that may be available and of interest for "re-purposing" for the web or multimedia. The original footage was shot in 1969 at the Woodstock Pop Festival, at 18fps on Regular 8mm movie film. There is some unavoidable blur, graininess and camera movement, which make compression of the frames difficult for any codec. (See the original frames used for compression.)

The files were re-compressed using the free version of the Sorenson codec. The key frame setting (a frame fully compresed from the source, not dependent on previous frames) was every ten seconds. They are in Millions of Colors. There is no audio. The stills below are JPEG files exported from the movie files.

Click on any still below, to download the movie it is taken from. (Each 9 sec. movie is 250K, except the 36K/sec. version which is 350K.)

160x120 Movies

Cinepak26K/sec, 10fps

Cinepak's frequent key frames and less effective compression mean poorer resolution than in a Sorenson version at the same data rate. Cinepak's simpler decompression does require less CPU power, so scrolling and looping is smoother.
Sorenson 26K/sec, 10fps

This difficult clip compresses with slightly better resolution using Sorenson Video than it does using Cinepak. A simpler clip such as a talking head could be compressed to a much lower data rate and still achieve this resolution.
Sorenson 26K/sec, 15fps

With the frame rate increased by 50%, resolution is only slightly lower, partly because there are very few key frames. However, Sorenson Video is particularly sensitive to fast motion such as the clapping hands, which begin to produce blockiness in the image.

This is why Variable Bit Rate encoding, which could allocate a higher data rate to these frames, is essential to the best Sorenson compression.

240x180 Movies

Sorenson 36K/sec, 15fps

Just as with frame rate, an increase in frame size makes less difference to Sorenson than might be expected. Here, even though the frame area is now more than double that of 160x120, increasing the data rate by just 40% has maintained similar resolution.
Sorenson 26K/sec, 15 fps

Without that increased data rate, the blockiness in the frames where the hands are moving becomes more evident. The blocks are even more obvious in playback than in a still frame.
Sorenson 26K/sec, 15 fps
(120x90 Doubled in Playback)

If the resolution of individual frames is less important than smooth playback, a lower data rate can be achieved without blockiness by compressing at a smaller frame size, and saving with "double-size" playback selected.

Note: this clip shows the Jefferson Airplane onstage at Woodstock. See also the original frames used for compression in these tests.

For more stills from the video, see Woodstock 1969: How It Looked To Us.

Return to the FILM 410* Course Materials page.