What is sampling? Sampling frequency

What is discretization, knows today any professional in the field of digital photography. However, many people who are just beginning to get acquainted with this field do not know the basic features of it, so they can make mistakes.

What is it?

What is sampling? This is an undesirable effect, which leads to the fact that the quality of the photograph deteriorates noticeably. This phenomenon can be associated with any device or process in which information is divided into several separate samples. In this case, sampling can be considered as an interference pattern if there is a certain relationship between the frequency of a given phenomenon and a certain periodic structure in the data.

The person's eye constantly strives to perceive a certain ratio as an interference pattern that can obscure the real meaning of a particular image. Considering examples of what is discretization, it is worth highlighting moiré, which is not quite an exact effect of it, but it can also show how a person is misled when two patterns begin to interact with one another, forming a third.

What is moiré?

Moire is an incomprehensible undulating pattern that was not originally present on the subject. This effect often occurs on various images that are obtained with the help of digital devices. And the problem here is that the pattern of the object is superimposed on the pattern of placing the pixels on the matrix, resulting in the appearance of a third, which is called moiré.

In most cases, this effect occurs on detailed high-contrast images that do not correspond to the initially configured resolution of the sensors. In particular, it is often enough to be found in the case when objects such as hair or fabrics are taken, as well as plots that contain a large number of repeating details. Often, moiré can not be found in nature, since it occurs in images that are obtained with a digital camera or later with an incorrect scan.

Quite often in modern digital cameras, in order to reduce this effect, a specialized optical low-pass filter is used, so if you are really going to professionally engage in photography, then you should definitely think about a model with this function that will be able to provide the proper quality beyond Depending on secondary conditions.

Discretization in modern cameras

In modern digital devices, the effect of sampling is caused by the fact that information is divided into several samples at a regular interval. In particular, one of the patterns in this case is the location of the pixels on the matrix, the second pattern will be any elements in the picture that can be repeated over a large area or that change through a certain number of pixels in the transverse or longitudinal direction.

For those who do not understand what is discretization and when it arises, you can give a concrete example. When there is insufficient number of pixels in order to convey reliable information from the picture, then in this case it is done in not the best quality. In the standard version, it would be sufficient to simply select a higher resolution, thus ensuring the required number of pixels for transferring the details to a picture with a certain accuracy, and if the number of pixels were insufficient, we could simply see a small number of image elements.

However, in fact, this is not entirely true. The theory of discretization states that in reality the situation is much more negative, and if we do not have enough pixels to take a certain picture, then the image quality will constantly deteriorate.

How many pixels do you need?

Take, for example, a situation where there are simply 20 black and white lines in the image, each with a width of 5 pixels. In this case, if at least one pixel is provided on each line, the picture can be recorded. Naturally, if the pixels do not fall clearly into the center of each line, then each pixel will turn out to be gray, not white or black, and its hue will directly depend on how the pixel is positioned relative to the lines.

If the number of pixels in the image decreases, then some of them start to appear between the lines, as a result of which the above pattern appears in the image, which will change constantly depending on the ratio of the line spacing and the number of pixels. Of course, such an image will no longer be an accurate reproduction of the original, since the regular structure of the lines will be already noticeably broken. It is this phenomenon that is generally accepted in professional circles as "discretization of data".

What to do?

To solve this problem, you need to soften the image somewhat before reducing the number of pixels. Thus, you can completely get rid of the sharp boundaries on each line, and pixels can take intermediate values. In other words, the picture becomes softer, but the overall impression of the picture remains.

How does this affect the image?

Of course, repetitive and regular line structures are rarely seen on images of various natural objects - their presence is often limited to images of various artificial structures such as buildings and so on. However, in any case, the depth of sampling can be impressive, so this effect should always be avoided, while taking pictures of any objects.

It should be noted that the quality of images can be completely different even if they have the same number of pixels. After all, among other things, the difference between pictures can also be in the way they were obtained. For example, in one case, a snapshot can be somewhat relaxed by skipping it through a low-pass filter to obtain intermediate pixel values before decreasing the size, while another image may simply decrease in size without introducing any additional changes And not getting intermediate values at the boundaries of objects, where too sharp changes in brightness are observed.

How to check?

To understand how this works, it's enough to just take one picture, then make a copy of it. The original is filtered by resizing using the so-called bicubic filtering option, which is available in standard Adobe Photoshop. Thus, the picture will be relaxed. Despite the noticeable decrease in pixels, the transitions between the keys eventually turn out to be smooth and quite suitable for the number of pixels available.

After that, the previously made copy of the image is reduced, say, up to 30% of the original size, using the option "nearest nearest point" in the same program. This operation will ultimately give you the discretization effect, which will be evident.

Audio Sampling

Sound sampling is the filtration of a sound wave before it is stored in a sound file. In other words, in the final file there will not be an exact copy of the sound wave, but only an approximate one. On the one hand, sound sampling provides a certain reduction in the amount of stored file, but on the other hand there is a mass of audio vibrations that do not necessarily need to be stored on the hard disk.

Such a sound filtering is called a "sampling frequency". It should be noted that in fact only in nature there is a sound without this effect, although few people know about it. The sampling frequency is the imposition of a specific grid on the sound wave, as well as recording only certain key elements.

It would be quite difficult to record the entire sound wave completely. It is for this reason that it is much more common to find such a situation when a two-channel sound recording with a sampling frequency of 44.1 kHz was made. The latter is chosen most often, since this is the most optimal parameter.

In principle, considering sound processing, you need to pay special attention to parameters such as the depth of coding and the sampling frequency, because the higher these numbers are, the more the digital signal will correspond to the analog signal.