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Multimedia videoconferencing audio video matrix Design Essentials
1. audio/video matrix effects in modern multimedia conference room, in order to meet the needs of different shows occasions, usually with a variety of different audio and video signal source and display terminals, although these audio and video signal source and display terminals or with composite video (Composite-Video), super video (S-Video), component video (Component-Video) or digital video (DVI, SDI) interface, but now in multimedia videoconferencing are commonly used or composite video matrix, the main reason in the following areas: composite video with good stability, compatibility and interoperability, transmission bandwidth small distances. But the chromaticity and luminance shared 4.2MHz (NTSC) or 5.0 ~ 5.5MHz (PAL) of a frequency bandwidth, each with relatively large crosstalk, equipment and transport requirements for standard cables, signal source, embedded cable investments low.
Super Video (S-Video) while reducing the luminance loss, brightness/color crosstalk in clear advantages over composite video, but for the present common LCD projectors, DLP projector does not reach the very obvious difference and embedded cable investment is twice as composite video, so the project is not universally by long distance transmission.
Component video signal format level has been significantly higher than composite video and super video, but for now most of the rooms is for computer display (VGA or RGBHV signal formats) services, equipment and transmission cables require a high level (depending on the intended design standards and investment budget), embedded cable investment is high.
Similar R-Y, Y, Y, B-Y, Cr, Cb component video signals are now mainly used in radio, film and television industry, and will gradually to SDI or HD-SDI transition of digital signal format, because the signal source and capital budget constraints, the Conference room use. DVI signal due to the effective transmission distance (5 meters), there is no widespread use.
The service that IMIS habits on audio and video matrix no special indication that the default is the composite video format. To the composite video format output of the main equipment: camera, physical stand, cable TV modem, remote video conferencing, video tape recorder, DVD player, audio and video matrix system in the video source and display or multiplexing Terminal, will among different audio/video signal source according to user's preference for centralized control. In accordance with the input and output channel, common video matrix generally have 8 × 8 × 2, 4, 8 x 8, 16 × 16 × 4, 8, 16 × 16, 32 × 32 × 8, 16, 32 × 32, 64 × 64 × 16, 32, 64 × 64, 128 × 128, etc. General understanding that multiply the preceding figures represent the number of input channel, take the number behind the figures represent the number of output channels. Whether matrix input the number of output channels, their control methods are more or less the same: the front panel pushbutton control, isolation keyboard control, third-party control (RS-232/422/485, etc.), and can achieve the following functions: 1) can use needs in different display Terminal at the same time show the same or a different video source content
2) you can set the camera, DVD player, VCR, cable TV, video conferencing, and other various video signal for convenient handling and call
3) administrators can independently monitor any one channel video signal, but does not affect other terminal display of content and results
4) administrators can on any video signal recording, but does not affect other terminal display of content and results
5) administrators can designate any one channel video signal sent to the meeting of the terminal or other tracks, but does not affect other port displays the content and effect, like the matrix with BGBHV design a basic principle of video matrix is based on the signal source and display the number of Terminal number to determine the rank of a matrix, the matrix of difference (the number of channels) in price reflects the very obvious, in certain cases, the budget, so select a matrix of channels will become more sensitive, for future expansion is also a challenge. In addition, the following describes some problems as equipment selection factors to be considered. 2. video signal bandwidth composite video signal according to the standard, the signal bandwidth also has certain differences, see the following table. Because composite video transmission bandwidth is relatively narrow, present in the system design is not a major consideration. PAL B/G/H PAL I PAL D PAL N PAL M NTSC SECAM
The number of lines per frame 625 line 625 line 625 line 625 line 525 line 525 line 625 lines
Row frequency range KHz 15.625 15.625 15.625 15.625 15.75 15.734 15.625
Field frequency range, Hz 50 50 50 50 50 60 60
Colour subcarrier MHz 4.43 4.43 4.43 3.58 3.58 3.58 4.4
Signal bandwidth MHz 5.5 6 6.5 4.5 4.5 4.5 6.5 3. -3dB decay point video signal on the desired result in the transmission bandwidth range or the transmission distance is called-3dB, also known as 1/2 power attenuation. At-3dB points within the signal will not be processed or transport of heavy losses arising in the course of, any signal input and output devices will have limited bandwidth, the video signal flat frequency response at-3dB points before, at the same time, we need to pay attention to the choice of equipment is in the "bandwidth full load (Fully Loaded)" State testing. Because there are many brands of nominal parameters is in the "ad hoc" state test, when the system is operating at full load, transfer bandwidth will be greatly reduced, the nominal parameters on the displayIt has no meaning. 4. video signal linear distortion due to system properties and distortion, has nothing to do with the signal itself, the output signal amplitude and the input signal between linear relations, formula, U2 = KU1 U2 on behalf of the output signal, K represents transmission function (frequency or time functions), U1 represents the input signal. System of amplitude-frequency characteristics and phase frequency characteristics are not uniform, is due to the circuit in the reactance of components and various distribution parameter.
5. the video signal of nonlinear distortion in signal transmission caused distortion and transferred the amplitude of the signal itself, this kind of distortion that nonlinear distortion between the input signal, the output is not a simple linear relationship. Formula: U2 = K (U1) U1, transfer function K (U1) is not only a function of frequency or time, and is a function of the input signal. Nonlinear distortion caused by nonlinear components, their argument with effect from their changes the signal level (by size affects the signal level). Transmission system for non-linear and dynamic range of the signal, the distortion in the range from signal components roughly divided into the following: · Brightness signal by nonlinear distortion
· Chroma gain of nonlinear distortion
· Chroma phase of nonlinear distortion
· Chroma, brightness of inter-modulation distortion
· Differential gain distortion
· Differential phase distortion
· Sync signal static nonlinear distortion
· Sync signal dynamic nonlinear distortion 6. Line time waveform distortion reflect MF 500KHz row frequency to distortion, represents the larger size image in the horizontal direction of brightness change, intermediate frequency distortion will cause the image horizontally ill-defined, serious cause horizontal tail, RGBHV signal is somewhat similar to low-frequency response of "bad". 7. color/brightness gain differential output signal of the brightness component and color component amplitude ratio and the ratio of the input signal amplitude change known as chroma/luma gain differential, because the channel on the chroma components and the brightness component of amplification is not consistent. Color saturation distortion, similar color saturation adjustment, gain difference is negative, the image color fading, people look bad; gain difference is negative, the color flood, profile not distinct, similar strong fill picture, lack of realism. 8. the brightness of nonlinear
When the average image level for a certain value, the starting level from blanking level gradually increased to the white level of small step, signal to channel input, the output of the corresponding step amplitude ratio between maximum difference. Brightness of nonlinear grey-scale image loss will result, reduction, resolution, lower level (as the chroma signal superposition in luminance signal), the resulting color saturation distortion. Brightness of nonlinear caused by nonlinear components: work point no, enter the margin is too large.
9. the differential gain distortion
Because the image brightness signal amplitude changes cause distortion of the chroma signal amplitude is called the differential gain distortion, different brightness background color saturation distortion effects, color effects (such as wearing red clothes from dark to light, bright clothes become dense or dimmed).
GY/T107-92 (TV broadcast system video running index) category a criteria require differential gain distortion ± 0.2 °, the good performance of video matrix will control at ± 0.1 °.
10. differential phase distortion
Because the image brightness signal amplitude changes cause distortion of the chroma signal phase is called the differential phase distortion, different brightness background color distortion that would create a color into a different color (such as wearing red clothes from dark to light, bright red dress is yellowish or purple).
In the NTSC system, the colour signal vector angle change represents the tonal variation, so the differential phase to signal effect is very serious. The PAL system because the line phase technology, so their compensation makes use of color saturation changes instead of tonal changes. Overall, the differential phase is used to describe the brightness signal amplitude variation on color tone of a parameter. GY/T107-92 (TV broadcast system video running index) category a criteria require differential phase distortion ± 0.2%, now have good performance video matrix will control at ± 0.1%.
11. differential causes distortion
Chrominance signal 4.43MHz ± 1.3M in video high-end, susceptible to the influence of distribution parameters in the channel: the working point of improper influence on junction capacitance, which influence transmission channel of impedance parameters. Control differential phase distortion and differential gain distortion, you need to ensure the transmission channel video amplifiers/video processing unit, must be sufficiently large dynamic range, to improve the transmission channel of amplitude-frequency characteristics and phase frequency characteristics, strictly control components distribution parameter.
12. color/brightness inter-modulation distortion of the provisions of the chrominance signal superposition in constant increase of the brightness signal and to channel inputs, and the average image level at a specific value, the output due to the superposition of the chrominance signal caused by the change of the brightness signal amplitude is called color/brightness inter-modulation distortion. In the color signal, the chroma signal is the superposition of the brightness signal, as the system on Nonlinear exist, will make the chrominance signal of positive and negative half week lose symmetry, equivalent to a DC component (axes), it appears the brightness signal, nonlinear distortion-distortion sizes with subcarrier amplitude variation, is the gain of the inverse differential phase. Image color subtitles, distortion more obvious, color subtitles that corresponds to the background brightness contrast on distortion. 13. random SNR random signal to noise ratio meaning the whole spectrum of clutter, but high-frequency noise interference in the image of tiny particles, the human eye unnoticeable, plus a weighted network interference cases in line with the human eye watching the actual condition, called weighted random signal to noise ratio. 14. cycle-to-noise ratio cycle-to-noise ratio from power disturbances: 1KHz harmonic sound inside the Exchange and its interference, because the voltage ripple coefficient, space AC Electromagnetic induction, ground or ground layout unreasonable (grounding ground potential differences of common-mode noise), clamp circuit bad result. Periodic noise can cause static or scroll black bars, black belt, seriousVertical image twist, destruction of synchronization.