Video quality monitoring in nonlinear editing

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With the rapid development of computer software and hardware technology and digital recording technology, non-linear systems, with their flexible system configuration, high system compatibility and integration, powerful and convenient post production methods, have become an indispensable means in the field of film and television program production. It can be predicted that with the further development of computer software and hardware technology and the digitalization and networking reform of film and television program production and broadcasting methods, the application range of nonlinearity will be further expanded

in addition to providing convenient and fast editing functions, many non-linear software also integrate subtitle production, two-dimensional and three-dimensional stunt synthesis and other functions, so that the produced film and television programs have rich picture effects. Because the color signal processing range of the computer is not exactly the same as that of the ordinary TV broadcasting and receiving and restoring equipment, there is a phenomenon that the color caused by the computer can not be reproduced normally on the TV restoring equipment. In the field of TV production and broadcasting, the requirements for the indicators of broadcast level TV signals are very strict, so it is necessary to pay attention to whether the signal indicators meet the requirements for broadcasting due to small experimental power at any time, The ultimate goal is to restore clear and lifelike images on viewers' TV sets. The quality and safety of video signal technology in the process of nonlinear production are mainly reflected in two aspects: display range and signal safety

premiere is a general non-linear software that can run on a PC. It has comprehensive functions and the widest compatibility. It is compatible with almost all acquisition cards on the market. In addition, it has and is compatible with a large number of third-party audio and video plug-ins. It is highly praised by video professionals and video enthusiasts

in April, 2004, Adobe Corporation of the United States launched the latest professional non-linear video software Premiere Pro 1.5, which has been greatly improved compared with the previous version. Here we pay more attention to its professional color monitoring features. This paper will take this software as an example to introduce the monitoring methods for video signal quality and safety in the process of program production

1 security and control of display range and providing additional enhanced performance

the reason for the security problem of display range is that the TV receiving and restoring equipment has "overscan" phenomenon when displaying video signals. The so-called "overscanning" simply means that when the scanning system in the TV is scanning and reproducing images, the range of field scanning and line scanning is too large, which exceeds the range of the TV frame, so that a circle around the picture is cut off. Because the size and center position of the picture of different program sources (even each channel of the TV station program) are somewhat different, it is necessary to overscan appropriately. The overscan amplitude of professional monitors is strictly controlled. It is generally considered that the overscan amplitude of ordinary TV sets should be controlled below 5%

most non-linear software provides a safety frame to provide a reference for producers to design subtitles or stunt positions, so as to avoid incomplete TV pictures seen by viewers due to overscanning. The safety frame is generally in the shape of "back", which is composed of two inner and outer boxes with different distances from the picture edge. It will not be recorded or output

in the monitor window of Premiere Pro, press the "safe margins" button on the control panel to display the safe margins in the monitor window. The peripheral box is called "action  safe zone", which means that the picture movement and transfer beyond the area may not be completely displayed; The small box in the middle is called "title  safe zone", which means that the subtitles in this area can be normally displayed on the audience's screen

the size of the security border is not fixed and can generally be changed by setting. In premiere, the default range outside the action safety zone is 10% and the range outside the caption safety zone is 20%. You can set the size of the two types of safety zones in the horizontal and vertical directions in the property dialog box of project setting. It is worth noting that the values of 10% and 20% outside the safe area mentioned here refer to the proportion of the picture on each side. For example, 10% in the horizontal direction means that there are 10% non safe areas on the left and right sides of the picture

for Premiere Pro and most other non-linear software, the method of controlling the display range is basically the same and relatively simple. That is, pay attention to the position of subtitles or action display at any time during the design process. The ideal method is to use a professional monitor or a TV with over scanning standard to monitor the output video signal in real time, so as to avoid the loss of pictures caused by over scanning

when making a video program for network publishing, since the display mode is consistent with the equipment and time, it is unnecessary to consider the restriction of the security area. The picture range seen in the monitor window will also be the picture range seen by the end user

2 signal safety

the dynamic range of video equipment is certain. Maintaining the minimum and maximum amplitude of the signal within the limited range can ensure that the video signal level will not exceed the working range of the quantizer, maintain the correct picture color balance, contrast and brightness, and ensure that the details and colors of bright and dark parts in the picture can be restored normally. It is also important to keep the video signal amplitude normal and relatively uniform during the program, so as to ensure that the image will not cause serious changes in picture brightness or color before and after the point. Therefore, we must control the amplitude of video signal within the limit of transmission permission and effective conversion to other video formats, especially when the signal needs to be modulated before transmission, the modulation process will amplify the defects of the original signal, resulting in more serious image distortion

the quality of video image is the primary indicator of the technical quality of TV programs. At present, the widely used video recording equipment provides a considerable number of audio and video level automatic control circuits to ensure the signal quality. These automatic control circuits work according to the preset technical parameters and can adapt to most shooting environments. However, with these automatic functions or preset technical parameters, in some special recording environments, the captured video signals may be inconsistent with the expected results, so that the recorded signals may exceed the normal requirements. In addition, due to the difference between the signal channel of the video recording equipment and the final signal transmission channel, it is also necessary to carry out necessary detection or adjustment before sending the video signal to the transmission channel

in the traditional TV program production system, in order to produce high-quality video signals that meet the broadcasting requirements, TV producers generally use special instruments to monitor and control the quality of video recording in real time. The commonly used instrument is the waveform monitor, which measures and displays the brightness (brightness component) level of video in a graphical way; The other is a vector oscilloscope, which measures the chrominance (color) information of video in real time

these instruments are usually independent equipment. Even the relatively cheap ones cost tens of thousands of yuan, and the high-precision ones cost hundreds of thousands of yuan. In many cases, it is unrealistic to monitor the program at any time with the help of such equipment. Moreover, due to the high integration of nonlinear systems, the detection of video signal quality is often ignored

fortunately, most of the non-linear software currently running in the computer provides the window function of waveform oscilloscope and vector oscilloscope, which allows users to monitor whether the video signal is normal at any time without additional equipment investment. According to the comparison with the measurement results of medium and high-end oscilloscopes, the measurement accuracy of these software for video signal quality can be comparable to that of professional oscilloscopes worth more than 100000 yuan. Of course, there is a big gap with professional oscilloscopes in terms of measurement items and convenience of use, but for video signal monitoring in nonlinear process, it can generally meet the needs

in non-linear software, the monitoring information of waveform monitor and vector oscilloscope can be selectively displayed in the monitoring window, and some software have separate waveform and vector information monitoring windows. Mastering the basic methods of monitoring and controlling video signals is necessary for producing high-quality TV programs

in Premiere Pro 1.5, you can select the type to be monitored in the pop-up menu by clicking the "output" button at the bottom of the monitor window. The signal level monitoring types provided by Premiere Pro include vectorscope, YC waveform, YCbCr component oscilloscope and RGB component oscilloscope

combined with different output forms, it provides users with an accurate reference for adjusting the quality and effect of video signals. It will no longer "follow the feeling" according to the picture results as before, but a big step forward in the direction of specialization

(1) use the waveform monitor window to monitor the amplitude of the video signal

the waveform monitor is mainly an oscilloscope used to monitor the amplitude of the video signal. At the same time, it can detect all pulse scanning patterns of the signal in the unit time, and can be displayed in the form of a plan view, allowing the user to see the distribution of the brightness signal in the current picture. In the very demanding professional video shooting, professional waveform monitors are even used to measure the quality of scene video. During, they are often used to monitor and ensure the consistency of video quality and scene to scene video quality

the unit of measurement commonly used by Chinese TV stations and equipment manufacturers is volt (V), which is suitable for direct measurement with oscilloscope. The national standard stipulates that the standard value of pal/d full TV signal amplitude is 1.0 V (P-P value), and the blanking level is the zero reference level, in which the synchronous pulse amplitude is -0.3 V downward and the peak white level of image signal is 0.7 V upward. The YC waveform oscilloscope window provided by Premiere Pro can monitor the brightness signal of the current video signal and the full TV signal level after superimposing the chrominance signal. The vertical direction in the window indicates the level, and the unit of measurement is volt (V). The horizontal direction indicates the distribution of luminance information in the current picture. The user can decide whether to superimpose chrominance information by clicking the "chrominance" check box at the top of the window

the blanking level (i.e. black level) in the waveform monitor window of Premiere Pro PAL system is 0.3 v. according to national regulations, the normal white level of all TV signals should be less than 1.0 V. This method is not consistent with the customary measurement method that takes the blanking level as the reference zero level, but it is intuitive and easy to describe

the former Ministry of radio, film and television (now the State Administration of radio, film and television) stipulated in the measures for awarding the technical quality award for TV program recording of the Ministry of radio, film and television (Provisional) and the detailed rules for evaluating the technical quality award for TV program recording issued by the Ministry of radio, film and television in 1996 that the peak level of all TV signals shall not be greater than 800 MV, and the peak level of brightness signals shall not be greater than 770 MV. According to this regulation, the full TV signal peak limit can be increased to 1.1 V, and the brightness signal peak level limit can be increased to 1.07 v. That is, at pal

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