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Digital Video Compression Methods Part 3

Compression rules

You don’t need to know all the intricacies of the compression process if you follow these general rules of thumb:

If compression isn’t absolutely necessary, don’t use it! Until the editing and design of the material is completed, it is better to work with the original source to have a sufficient quality margin and compress it only at the last stage, that is, immediately before the output to the medium.
If compression is unavoidable, try to use the lowest possible compression ratio. In other words, always use the highest possible bit rate.
Do not turn on the compression systems one after the other and, if possible, avoid compression. This always leads to a loss of quality, and the lower the flow rate, the more pronounced this deterioration will be. The quality will be even worse if between applying the codecs you perform some editing actions or apply special effects.
Digital encoding systems always introduce a time delay and complicate video editing, and the use of compression can lead to desynchronization of audio and video streams.
Any compression system works best with the purest video footage. Signal noise, jitter, graininess and poor lighting when shooting, as well as poor decoding of the original video signal make compression difficult and produce poor results.
Compressed data is always much more susceptible to transmission errors than uncompressed data.
Do not believe the claims of manufacturers comparing codec quality to analog video (“VHS quality”, “quality comparable to Betacam SP”, etc.), because the defects of a compressed digital image are different from those of a Analog video, even if Analog video is played from a consumer VCR known for its defects.
The quality of the image obtained by this or that codec depends directly on the type and condition of the source material. Therefore, question even the most compelling demos, as you can use specially selected material to ensure that a particular codec works perfectly. Always use your own material for testing, selected based on a reasonable balance of “easy” and “hard” stories.
The quality of compression can only be assessed subjectively.

Many types of digital video compression are based on the fact that high-amplitude signals do not occur simultaneously at all frequencies, and when compressing audio signals, it is necessary to perform a frequency analysis of the input signal to develop a masking effect. . Spectral transformations are typically used for such purposes, which are also used in the phase correlation method to measure motion. A special case of spectral transformations are Fourier transforms, which are used in most of the known compression algorithms for audio and video signals. Fourier transforms are a processing technique used to analyze changes in signals over time and express them as a spectrum.

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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.

Digital video compression methods

Gradually, all the equipment for the production of television and video programs in the world is going digital. Why is this happening?

It is not at all due to the fact that everyone admires digital methods and that the image quality is radically improving, the reasons for such a transition are mainly economic, because now both the price and the cost of operating a digital complex are lower. than a traditional analog with the same functions. Now that digital technology has reached a certain maturity, analog equipment is suddenly less efficient, less reliable and much less profitable.

EN The very foundations of video production and delivery of images to the viewer are currently being revised, and the dynamics of change in the current age – digital – is much higher than in analog. Today, for example, the 4: 3 aspect ratio is gradually being replaced by the 16: 9 aspect ratio, interlaced-progressive (non-interlaced) scanning and changing encoding standards increase image quality. The economy also dictates the need to compress video information. Naturally, any compression degrades the quality of the rendering and is applied not for a good life, but out of necessity.

Do not think that video compression was not used before and that it only appeared with the advent of digital technologies. No, compression has always been used, but only before it was analog, and today it is necessary to make it completely digital, getting rid, as much as possible, of double compression, that is, both analog and digital.

Modern digital compression methods, which replaced analog, especially when combined with computer technology, can not only improve the quality of the video itself, but also expand the possibilities of video production and optimize the display of audiovisual products.

Basic concepts of digital transformation

Essentially, a digital representation, or digitization, is a partition of the domain of a continuous function in some intervals and the representation of this function as a set of values ​​at the end of these intervals. So, for a digital audio signal, the second interval on the time scale is divided into 32, 44 or 48 intervals, in each of these samples the sound is measured and its value is stored with a certain representation precision, generally 14 to 20 bits. These operations are called sampling processes, that is, the representation of any continuous quantity (in this case, sound) through periodic discrete measurements. After that, they say that the sound is digitized with a sample rate of 32 to 48 kHz, respectively, and a bit depth of 14 to 20 bits. Therefore, the digital current

In digital video, the sampling process for a value is simply generalized to a multivalued function that has a range of values, not a number, but an image (that is, it becomes three-dimensional) to break solid lines of a value. image (scanning beam) in so-called pixels, or image elements, our three-dimensional space will be because when the image is presented at each moment of time, not a value is involved, as in sound, but the whole image – a frame . As a result, the video signal is also represented as a sequence of integers, in other words, as a stream of data.

In the case of (separate) component analog video signals, each channel must simultaneously transmit three color components (red – red, green – green and blue – blue, i.e. RGB) or a luminance signal together with signals of color difference (YUV). The conversion of RGB color components to YUV components is one by one, but the latter method makes it possible to highlight the change in signal brightness, for which the human eye provides a separate perception mechanism, with the help of rod cells. in the retina, in contrast to the perception of color by cone cells. Rods are more sensitive than cones, so in systems with separate component coding (e.g.

Sampling theory, in turn, requires that the sampling frequency be at least twice the bandwidth of the signal to be digitized. In the case of a broadband signal (for sound, that is, for example, the simultaneous reproduction of several octaves of frequencies), the sampling frequency must be at least twice the highest frequency of the input signal analog (Fig. 1).

Figure: 1. An example of sampling an analog signal: a – correct; b – wrong. In the first case, the original signal is easily reconstructed in the form of a sample envelope, and in the second, when the signal frequency exceeds half the frequency