Saturday, December 10, 2011

QuickTime, 20 years of digital video - I

On December 2, 1991, Apple gave the world the first version of QuickTime, which triggered a technological revolution. At a time when digital video was completely replaced analogue, it is time to render unto Caesar what is his by right.

We imagine perhaps with difficulty, but without QuickTime, we would not have digital terrestrial TV or Blu-ray in every home, as we know it today.

The digital has many advantages over analog, it allows the identical copy of any signal without any degradation, and opens to the capabilities of mathematics, for its treatment using number of algorithms, the Fast Fourier Transform to the Wavelets. An analog signal, once digitized, can undergo a variety of effects, but it is also possible to use a bandwidth considerably reduced by digital compression. Thus, the transition of television from analog to digital microwave radio has multiplied the number of channels, previously limited to six in France. Another advantage is that digital has yielded stills perfectly static.


If the sound and digital images have long benefited from compression-decompression systems (co-December), with QuickTime came the advent of time compression in addition to the compression space.

Let's start by explaining the principle of spatial compression, which applies to still images. Taking the case of GIF for example, this format uses spatial compression algorithm (ie in a given area) of Lempel-Ziv-Welch (LZW), which summarizes the identical pixels in successive of the same image. The JPEG in turn operates the way the human eye perceives images, focusing on major changes in chrominance and luminance compression occurring on the less noticeable details (such as we see blue light least although the red and green, our retina is lined with fewer photoreceptors dedicated to blue, there is no need to store the blue layer definition equal to the other two. We perceive this variation particularly in highly compressed images at the edges of objects placed on a blue background).

MJPEG format, used by the first digital cameras, is actually a series of JPEG compressed images independently of each other. QuickTime add to this temporal compression Spatial compression: codecs reduce redundancy and graphic similarities not only in a single image, but also from one image to another, a technique particularly suitable for the video in the same plane -sequence, since the principle of the illusion of animation, based on the persistence of vision, based on the similarities and differences from one image to another. To maintain a baseline of quality and prevent excessive degradation over time, keyframes (ie independent of the images that precede them) are inserted regularly, and forced to change plans (the animated GIF operates also the temporal compression, but can sync with a soundtrack or a constraint real-time playback).

This compression was critical in 1991, since not only the storage media were much more limited today, but then the whole chain of data transmission was also much more restricted. In short, if the digital video was not compressed, it would have been impossible to not only store but also to read it. Digital compression allowed to transmit only the "recipe" for video, CPU load to reconstruct in real time.

But specifically, in the 90 processors are inherently more limited than today, he had to find a delicate balance between the flow of data that a processor can process in real time, and ability to reconstruct each image. The ancestor of QuickTime also relied on a dedicated hardware architecture: called QuickScan, it required the presence of a dedicated graphics chip to allow the first Mac to view the video medium in the late 80's. Created by engineer Steve Perlman at Apple, the project remained dormant until the Mac becomes powerful enough to allow the same performance entirely in software (Perlman has since founded the company that allows OnLive "streamer" the images of video games in real time via the Internet, which relies heavily on advances in digital video, play all games on Mac).

Early versions of QuickTime therefore incorporated a number of different codecs that were specialized for each type of image or animation given to offer based on a more efficient compression. If QuickTime is not the first system to display digital video on computer, the revolution he has introduced is to allow the display of digital video, any type, on the computer man in the world.

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