Digital Video/Audio

1.4.3 Digital Video

As the cost of computer memory decreases and processor speeds increase, the capture, compression, storage and playback of digital video have become more prevalent. This has several important ramifications. First desktop animation has become inexpensive enough to the consumer. Second, compositing is no longer optical. Compositing each element in a film meant additional degradation of image quality. With the advent of digital compositing, the limit on the number of composited elements is removed. Third, these digital techniques can be used for wire removal and to apply special effects. These techniques have become the bread and butter of computer graphics.

 

There are several issues to determine the cost, speed, storage requirements, etc. Compression techniques conserve space or reduce transmission time, but some compression compromises the quality of the image and the speed of compression/decompression may restrict a particular technique’s suitability.

Formats used for storage and playback can be encoded off-line, but the decoding must support real-time playback. Video resolution require that 27MB/sec be supported for video playback. An hour of uncompressed video requires just under 100GB of storage. Most video compression is lossy(not all original signal is recoverable) because of the favorable quality/space trade-off. There are different digital video format. Better signal quality can be attained with the use of component.

 

1.4.4 Digital audio

Audio is just as important to computer animation. Audio technology has gone digital. Early audio recordings used an electromechanical stylus to etch representations of the signal into wax drums or plastic platters. Later, the signal was used to modulate the magnetization of some type of ferromagnetic material on plastic tape. Digital audio has the advantages for duplicating and editing. Digital audio can be copied, cut and pasted, transitioned, and looped over without any degradation in signal quality. The sound capability in personal computers has improved so that high-quality sound capability is standard. There are file formats and compression standards. When dealing with digital audio.

 

Digital musical device control

Musical instrument digital interface(MIDI) is developed in 1983 to control musical instrument without being tied to any one instrument. MIDI commands are key note commands to musical devices and are intended to represent a musical performance.

The commands take the form of “note x on/off” where x is any of the standard musical notes. There are also control commands to set pitch, loudness, etc. MIDI commands can be sent out as they played to record the performance when keys are depressed/released. MIDI supports up to 16 channels and devices can be daisy-chained(respond only to a particular track).

 

Digital audio sampling

Sounds are pressure waves of air. The pressure waves are converted to representation of the waveform. The wave is sampled at certain intervals-the sampling rate- with a certain number of bits per sample- the sample size- using a certain number of tracks. The sampling rate determines the highest frequency, called the Nyquist frequency, which can be reconstructed. A voice signal can be sampled at a much lower rate than CD-quality music because the highest frequency of a voice is lower than one in a piece of music. The number of bits per sample determines distortion in the signal. The number of tracks is how many independent signals comprise the music. A voice signal ‘s sampling rate is lower than CD-quality music.