Digital Video Compression Methods Part 2
For video storage and transmission, joint (composite) coding of video signals is also used, which can be characterized as a method of strong analog compression, as it allows the transmission of the three-component color images described above in the allocated bandwidth. for one-component black and white images.
After digitizing the analog signal, you can apply digital compression to it based on one or another digital encoding algorithm. In principle, it is possible to compress any signal presented in digital form, however, for composite video, only serial double compression (first strong analog and then digital) will be possible, which cannot be considered satisfactory, and the digital compression ratio will be it will inevitably be limited due to the high noise analog signal. So, for example, the most widespread digital compression standard today, MPEG, was developed exclusively for separate (component) video signals and is essentially a method of replacing full-color analog signals with digital signals, if possible. no preliminary analog compression.
Interlaced
Ideosystems traditionally use so-called interlaced scanning, when the lines of each frame are subdivided into even and odd lines, and the aggregates of even and odd lines are called fields. The alternating transmission of odd and even fields instead of a full frame was necessary at one point, not for a good life, so doubling the image refresh rate was guaranteed without doubling the required bandwidth.
Therefore, interlaced scanning can be thought of as a kind of analog compression. The defects of such “compression” are the well-known flickering between lines, poor resolution when transmitting fast-moving objects, and so on.
For best results, digital compression should ideally be done using material from a non-interlaced source, as otherwise it will revert to being a consecutive double compression (as is the case with a composite signal), from which you cannot expect nothing good.
The current prevalence of interlaced scanning in existing video systems means that, in practice, digital compression encoders must also accept interlaced source materials. Interlacing creates a particular difficulty for compression using motion compensation algorithms (such as MPEG), since it is quite difficult to track changes in a situation where successive fields do not describe the same picture points. Furthermore, interlacing complicates the formation of a differential image signal between fields.
It follows from all this that if your goal is to make DVD-video (MPEG-2), even if your player does not support progressive (progressive) scanning and your camcorder has such a mode, it is better to use it.
At the beginning At the beginning
Quantification
The term “quantization” appeared at the junction of analog and digital technologies and does not mean more than a method to represent the value of the signal at a reference point as a binary number with some precision. Therefore, a hardware quantizer divides the range (or voltage scale of an analog signal) into several equal intervals, each of which is represented by a separate number. Consequently, the quantizer generates the number of the interval in which the analog voltage falls (the value itself, in general terms, can be determined from the table).
If information about the position of the analog voltage within the range is lost, then we speak of a quantization error (or error), but since it cannot be greater than the quantization range, then this amount of error can be minimized using a number large enough of intervals.
Therefore, an 8-bit converter has 256 quantization ranges (providing a signal-to-noise ratio of about 50 dB), in a 10-bit converter, respectively, 1024 code words (a signal-to-noise ratio is 12 dB more high) etc.
By the way, the signal is restored in the same way: only the number of the quantization interval can be stored, and the restored value is taken from the corresponding table.
At first
Digital compression
Digital compression, also generally known as data compression or bit stream reduction, is a more sophisticated way of obtaining the most audiovisual results at the lowest cost than analog. And therefore, with digital encoding, sound and video can be brought to the viewer, significantly reducing stream or bandwidth.
