Canon XL 1 User Manual - Page 20

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distribute 8-bit audio anymore. Modern 16-bit codecs provide better quality at a

smaller file size than 8-bit codecs were ever able to.

• 16-bit sampling This bit depth is the current standard for audio distribution. Most

modern codecs and all audio CDs use 16-bit sampling as well.

• 20-bit sampling This bit depth is used for professional audio recording to provide a

better dynamic range than 16 bits. Codecs that support 24-bit audio can effectively

take advantage of 20-bit sources.

• 24-bit sampling This bit depth is rapidly becoming the standard for professional

audio recording because it provides more sonic detail than 16-bit sampling, meaning

you can use many more effects and layers without introducing errors.

Channels

Most audio from computers is either mono with a single channel or stereo with two

channels. Low bit rates generally use mono because it requires fewer bits.

There are techniques you can use to produce video that compresses well:

• Camera motion The amount and type of camera motion has a huge effect on the

compressibility of content. Camera motion changes every pixel of every frame.

Lack of camera motion is the simplest, of course. But modern codecs can encode

camera motion, such as that created using dollies, quite well. Tilts and pans are

more difficult. Depending on the codec, rotating the camera (a Dutch) can be trivial

or extremely costly in compression efficiency. The most difficult motion for any

codec is hand-held camera motion with random jostling.

• Shutter speed This speed controls how long the aperture is open in the camera, and

hence, how long light hits the film or the CCD. Slower shutter speeds let through

more light, and hence less grain, which helps compression. Slower shutter speeds

also cause moving parts of the image to become blurry, which makes them easier to

compress.

• Backgrounds A difficult motion for codecs is foliage rustling in the breeze. Foliage

is green, which makes up the majority of luma and therefore gets allocated many

bits in the compression. Leaves have hard, irregularly shaped edges, and details that

require many bits to encode well. Additionally, the motion of foliage in the breeze is

largely random, with leaves constantly overlapping and revealing, all motions that

are very difficult for motion estimation to handle.

• Depth of field A useful and subtle technique for controlling the complexity of

images is depth of field. By managing the depth of field, you can keep unimportant

objects in the background or close foreground out of focus while clearly showing

the actual subject of the shot. Depth of field is very common in cinematography and

photography, so it looks natural to the audience.