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Introduction

The need to write this material arose after many novice digital video lovers began to send frames from their videos, on which image distortions were clearly visible and they asked a reasonable question - what am I doing wrong, the image is normal on the TV, but not on the computer?

Television was invented decades ago, long before the advent of electronic computers, then there was no question of any digital transformations, so everything was created with the aim of convenient and cheap signal transmission and subsequent display on the screen, which was simple for the level of the then technology. Since the video image on a computer cannot be formed in the same way as on a TV, and there are problems, questions about them, etc. As answers to most of these questions, this material is conceived.

Transmission and formation of a television image

Most people watching TV believe that they are seeing an image at a frequency of 25 frames per second (hereinafter, the information will refer to PAL / SECAM television systems). This is not entirely true. In fact, the image on the screen changes 50 times per second, but not the entire image, but only half of it. First, one half of the lines of the image frame is drawn, then the other. Each of the halves is called a field. Therefore, it is correct to assume that 50 fields per second are drawn on the TV screen. This technology is well illustrated by the figures below:

Source Frame

It is the same, but divided into two fields

A person does not notice the "half-heartedness" of each image, both because of the inertia of human vision, and because of the afterglow of the phosphor of the cathode-ray tube of the TV. Nevertheless, many viewers can easily distinguish movies from television films precisely by the greater discreteness of the movement of objects in a movie. Television films are shot on video cameras with the same 50 frames per second, and movies on a movie camera at 24 frames per second. When preparing a film for a television show, each frame is converted into two "half" fields, but since there is no movement within one film frame, these fields, superimposed on each other in the eyes of the viewer, simply restore the original frame from the film.

Some figures characterizing the transmission of a television image

The maximum number of vertical lines that can be displayed on a TV set horizontally is 768. Such a number of lines can even be seen by applying a rectangular signal with a frequency of 15625 Hz to the low-frequency input of the TV set. The lines will alternate - 0 signal white, 1 black. Thus, the full television frame will be 768x625. After discarding the service lines and reversing the vertical scan, the real resolution is 720x576. This resolution is specified for full-screen video on a computer in all video editing programs.

The bandwidth required to transmit a full television picture is simply calculated: 768 (horizontal lines) / 2 (one line is white, the other is black) = 384 * 625 (number of lines per frame) = 240000 * 25 (number of frames per second) = 6000000Hz = 6MHz.

The horizontal scanning frequency is 15625Hz, thus the duration of one line is 64 microseconds.

The vertical scanning frequency is 50Hz, the duration of one field is 20 milliseconds, respectively.

The number of lines drawn in 20 milliseconds is 312.5 (0.020/0.000064). In the whole frame, respectively, 312.5x2=625.

How to take into account the specifics of the television signal

It is precisely because of ignorance of the specifics of the television signal that many users often have perplexed questions after resetting the video to a computer. The same questions arise when compressing video into various MPEG formats. So, the most common questions and answers to them:

I watch a video captured from a digital camera on a computer and see that the image is much darker than when viewing the same fragment on a TV. Thus, I cannot carefully edit the film without having an idea of ​​​​the actual colors and brightness of the frame. Why is this happening and how to fix this situation?

This phenomenon is well known and is due to the nature of the DV codecs used to decompress DV and display it on the screen. Indeed, the image on the monitor screen looks very dark, despite the fact that the same image sent to the camera will look perfectly normal on the TV. There are no radical means of combating this phenomenon, but it is possible to significantly reduce the differences between television and computer images. Since both video editors and just Windows players use the overlay mode to display video, you can adjust the contrast (Contrast), brightness (Brightness), color tone (Hue), color saturation (Saturation) specifically for the overlay window "i. Video cards provide this opportunity on processors from NVidia.In the settings of all modern drivers from this manufacturer there is a corresponding tab Overlay Color Control.Unfortunately, another eminent manufacturer of processors for video cards, Matrox Graphics, does not provide settings in Overlay mode.Settings of parameters of the overlay window do not affect nothing but video output.

The view of the settings window is shown in the figure:

To adjust the video image, load any video clip into a regular Windows player, click the Stop button, then bring up the overlay settings panel shown above and adjust the settings to your liking (it's even smarter to watch the same image on TV).

When viewing the frames that I took and saved to disk from my video, I found an artifact that can be called a "comb" - teeth on moving objects or stationary, but when the camera moves. On the TV, everything is fine. How can such distortions be avoided and is it possible to somehow correct the shots already taken?

Let's consider a small example. Here is a snapshot of people caught in the frame of a fast-moving camera:

This picture shows that the edges of all objects in the picture are distorted by a "comb". This artifact is caused by the superposition of two fields, the images on which are shifted relative to each other. The offset corresponds to the distance that the camera has traveled in 1/50 of a second. It is impossible to avoid such distortions when shooting with conventional video cameras. Only cameras with progressive scan can significantly reduce, and at a low speed of the object / camera, remove completely similar distortions. If you would still like to save such a frame for placement on a WEB page or printing on a printer, then you can improve the image quality by applying the Video / De-Interlaced filter in Adobe Photoshop or Ulead Photoimpact 8. The result will be like this:

It should only be taken into account that the vertical resolution after such an operation falls by half, since in fact one field is deleted and the lines of the other field are doubled.

I captured video from a digital camera using Ulead MediaStudio Pro (Adobe Premiere, Vegas Video, etc.) and found a strange picture - the Windows player shows that the resolution of the video I recorded is only 360x288, although it should be 720x576. Why is this happening and how can I watch the video in full resolution?

It really is. By default, the Windows player displays DV Video in 360x288 resolution. To convert the display to full resolution, do the following:

Having switched the Windows player to full resolution display mode, one should not forget that the player will display each frame from 2 superimposed fields, which will lead to a "comb" at the edges of moving objects. This phenomenon has been described in detail above. In the 360x288 mode, only one shelf is shown and there are no such distortions.

What does "progressive scan camera", "normal scan camera" mean - in theory, a television image should always be interlaced and nothing else? And on the same topic - like any digital video camera makes a "photo" - there is no "comb" there, moreover, on the most ordinary camera without "progressive" abilities?

Indeed, a TV can only display an interlaced image, but there really is no contradiction with the capabilities of some camcorders to shoot with progressive scan, but to understand this, the technology of shooting a camcorder should be described in detail:

• Shooting with interlaced scanning - Consider the process of shooting with a conventional video camera that uses only interlaced scanning. With such shooting, the camera actually shoots 50 times per second, and the information from its CCD (CCD) is read precisely in even or odd lines - i.e. odd lines (1, 3,..., 623, 625) are read first, then, after 1/50 of a second, even lines (2, 4,..., 622, 624). Therefore, when the subject moves relative to the camera, the images in different fields will differ from each other, and the greater the speed of movement of the subject, the more noticeable the differences will be and, accordingly, the larger the "comb". This type of shooting has one clear advantage - a smooth display of moving objects, since no "comb" will be clearly visible on the TV. The disadvantages of shooting in interlaced mode are noticeable only when editing on a computer - it is almost impossible to make high-quality still frames of moving objects for printing photographs and / or creating albums of files with the most interesting shots.
• Shooting with progressive scan - The process of shooting with a progressive scan camera is different in that the camera takes a picture every 1/25 of a second and then records two fields from it, as expected by the television standard. It is clear that in this case there are no violations of the rules, but the image on one field will never be shifted relative to another field. A video film shot with such a camera, when shown on a TV, will very much resemble a movie, which, as everyone knows, is filmed at 24 frames per second. The ability to shoot progressive scan is available on a small number of not-so-cheap camcorders, so don't expect it to be available on inexpensive camcorders.
• Photo mode in digital camcorders - The photo mode in all modern digital camcorders works the same regardless of the other characteristics of the camcorder. This mode is a special case of progressive scan shooting. The only difference is that in this way only one picture is taken every 6-7 seconds and the recording of the actual picture is accompanied by sound recording during this time. But the picture itself is remembered in exactly the same way, the same frame is simply recorded during the entire time of the "photography". Such a picture will also not have a "comb" and does not need to be subjected to the procedure described above in Adobe Photoshop.

Resolution: lines or lines?

A sore point for many novice video lovers is how to evaluate the quality of shooting your own camera or the one you plan to buy. One of the most important parameters of a video camera is its resolution, which is usually measured in TVL (Television Lines). For most modern digital video cameras, the value of this parameter reaches 500 or more TVL according to their passport data. The most popular question on this topic is why does a camera have only 500 lines, but a TV picture should have 625 lines? In fact, these are completely different concepts - the lines into which the television image is decomposed (there are actually 625 of them) and TVL, which characterize the quality of the image. To understand what TVL is, the easiest way is to look at a fragment of a conventional television test chart:

The numbers next to the lines just characterize the resolution. If you can distinguish the lines next to the number 500, for example, then the resolution is written as "not worse than 500 TVL". For example, you can look at two real shots of a TV table: a shot with a SONY Digital 8 camera and a shot of the unofficial champion in resolution SONY DCR-trV900, taken from the John Beale website.

It should be taken into account that the maximum resolution in the transmission of terrestrial television is determined by the channel bandwidth and cannot be improved, since the bandwidths have been standardized for a long time.

An ordinary household VHS video recorder has a maximum resolution of only 240 TV lines, S-VHS video recorders and camcorders - 400 TV lines.

Not all users who read the documentation for a video camera or advertising for them pay attention to the note that often accompanies the TVL value for a digital video camera. Usually the note says - the value is specified only for recording / playback on the camera's VCR. In fact, in this case, the quality characteristic of the encoder / decoder of the camera that converts the analog video signal to digital and vice versa is indicated. Why is such a tricky remark given? The fact is that the resolution of a video camera depends on many factors, listed below:

• Quality CCD (CCD).
• Number of CCDs (CCD), more (3) is better.
• The number of photosensitive elements (pixels) in a CCD (CCD).
• The quality of the DV encoder/decoder of the camera.
• The quality of the camera optics.

Since the DV standard strictly stipulates the format for recording DV to tape, then for a tape recorder of a digital video camera, the resolution for recording video will be the same for any DV camera costing both $650 and $4000. As a matter of fact, the very concept of "resolution" for a digital tape recorder is absurd - there is no concept of "resolution of a hard disk", for example. The correct value, which determines the overall quality of the video camera, will be the resolution on the captured image, but this parameter is not often given - it can be significantly lower than the TVL value for the video camera tape recorder.

What does the computer show?

Many novice video lovers believe that in order to output video from a computer to a TV or VCR, it is enough to have an ordinary good video card with video output and that's it - the results of your work can be easily saved in this way on a regular VHS cassette. In fact, this output method is almost impossible to use due to the low quality of the video image on the TV output of any, even the most expensive and best video card. There are several reasons here:

• The video image is always shown only frame by frame, and the frame is formed by superimposing fields with all the ensuing consequences in the form of artifacts, one of which is the "comb" mentioned above.
• The resolution of 720x576 is not listed among the screen resolutions allowed for the video card, therefore, if the video window resolution is correct, it will be bordered to the video card resolution of 800x600 or 1024x768. If you select the full-screen view mode, the image will be scaled as the video card and its driver see fit.
• The video card will decompose by fields for TV output without any connection with the original video image.

Currently, only Matrox video cards are practically devoid of the above disadvantages, but these video cards are quite expensive, rarely found on sale and have low parameters in terms of working with three-dimensional graphics.

The digitized video film itself is stored on the computer disk in the original, i. sorted by fields. But its correct playback on a regular TV is possible only through:

1. A special board for video input/output (the video fragment itself, of course, must have a format supported by this board).
2. Digital video camera for video output in DV format. This is possible due to the fact that any digital video camera, when data is received at the digital input, converts them into an analog signal that can be used to watch DV on a regular TV and / or record on a home video recorder. Unfortunately, locked digital cameras sold in European Community countries cannot be used in this way due to their customs restrictions.

Author: Spline Company; Publication: pctuner.ru

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