Volume 10
2000 torque
Second phase
Second phase
Journal of College of Education of Corps
Journal of College of Education of Corps
vo 1。 10 No.2
June 2000
Comparison of the Application of Digital Technology and Analog Technology in Video Field
Han Lijun Yin Baoxiang
Audio-visual technology has undergone earth-shaking changes in the past twenty years, from the earliest Nissan home representative analog technology.
VHS video tape represents the digital technology of VCD and SVCD in China, as well as the increasingly developed DVD and audio-visual effects in the late 1990s.
Bring a pleasing feeling to people. This paper compares and analyzes the technical standards and advanced characteristics of video.
Analog signal; Digital signal; Coding; Compression; resolution ratio
one
, analog signals and digital signals, and comparison of their products.
Analog signal refers to the signal whose voltage or current changes continuously at any time. In the field of video, analog signals have been used in the past.
It has been dominant for more than twenty years. In the process of VHS video recording, the camera first converts the different optical signals of the scene into
For different electrical signals, the video recorder converts the electrical signals into magnetic signals and records them. During playback, the rubber recorder converts the magnetic signal into a magnetic signal.
It is converted into an electrical signal, and then converted into an optical signal on the screen by the television to display moving images. And reflect the bright process of the scene.
The color brightness signal and color tone signal are converted into magnetic signals by the video recorder and recorded on the magnetic tape, and the video recording is completed.
Cheng. The video tape is coated with hard magnetic material. When playing, the working gap of the magnetic head is fused with the magnetic tape, and the residual magnetic field lines on the magnetic tape
Through the closed coil on the iron core, according to the law of electromagnetic induction, the relationship between the induced electromotive force e at both ends of the coil and the magnetic flux
Yes:
E= an N dO.
When the tape is taken, it changes with time, so the voltage generated at both ends of the coil corresponding to the magnetic signal changes accordingly.
Magnetic signals become electrical signals. Complete the process of restoring electrical signals.
The above signals are all analog signals, and all kinds of peripherals needed for camera shooting and recording must adopt analog technology.
The dominant equipment can be seen from the technical parameters and playback process of VHS video tape. 1. TV image
The frequency band of is 6.54MHz. To produce such a broadband signal on video tape, two conditions must be met: the head office.
The gap is narrow, and the relative speed between the magnetic head and the magnetic tape is high. 2. The brightness signal in the video signal should be modulated and recorded. Set to record signal
When the gap between magnetic heads is g = k/2, the average value of magnetic flux between magnetic heads is equal to zero, and the output voltage of magnetic heads is always equal to.
At the zero point, when g = k/2, the output voltage is the maximum, g.
The minimum wavelength, and this wavelength is the wavelength of the high frequency end, so it seems that it is limited to reduce the head gap to increase the upper limit frequency.
Yes Another method is to increase the relative speed between the magnetic tape and the magnetic head. If the signal is a sine wave, the recording signal is set to one cycle.
The distance traveled is the recording wavelength, that is, recording wavelength = tape speed/signal frequency. When the recording wavelength (head gap) is the same,
The higher the tape speed, the higher the signal frequency that can be recorded and played. However, if the tape speed is too high, the required tape is too long, despite the high head.
Speed reversal method, but based on the speed of VHS video tape of 23.4 mm/s, an hour's program needs a magnetic field of 84 meters.
Belt, not only wastes materials, but also is bulky and inconvenient to carry. 2. because the induced electromotive force of the magnetic head e= N = NA~Jcos.
(+0), the frequency is reduced by half, and the output is reduced by 6dB, so the signal-to-noise ratio is too low, so it is necessary to record TV signals at 30-6.5 MHz.
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When the high-frequency amplification is just right, the low-frequency signal-to-noise ratio is too low, and the noise is amplified step by step after the signal is amplified at all levels. 3. Because of that video.
Hard magnetic materials coated on the surface are easily magnetized by electromagnetic waves such as the earth's magnetic field and light. Although it can be stored in a dark and magnetic environment,
If the time is too long, the magnetism will inevitably be affected. 4. From the tape format (attached table), its information recording density is small and its resolution is low.
There are 240 lines. Although the resolution is not proportional to the definition, the higher the resolution, the clearer the image.
. The tape format is as follows.
Project formula
Scanning mode 2: the width of the magnetic head scanning band is 12.5ram.
Brightness signal recording mode FM video head azimuth is 6 degrees.
Chromaticity signal recording mode conversion direct recording track number 2
Tape speed 23.4 RTMA/ s resolution (lines) 2.4I)
Video track width 49/tin
A digital signal refers to a signal that only takes some separated values, which is characterized in that the amplitude is limited to a limited number of values, and it
Not continuous, but discrete. In the binary number system, there are only two possible values "0" and "1", and the others are like four points.
The basic unit of flat code is also binary code, and continuous analog signals become digital signals after sampling, quantization and coding.
Because the correlation in the unit pulse sequence is expressed as:
∞
St<= σ 6 (t-nt) (t is the sampling period) is expressed by Fourier series:
N = one
∞ .,' 2
X
SVT)=σn .(co .=I .TC:2d
. ) answer. = "st (t) e-dt
n = 1∞ 1
2
Quantization and sampling are converted into digital signals by pulse code modulation (PCM).
So, what are the advantages of digital signals over analog signals? 1. Strong anti-interference ability and no noise accumulation. exist
In the process of digital signal transmission, the coded digital signal will produce noise because of the error-correcting code detection method.
The error code is found at the receiving end but not received, which can avoid the disadvantage of low signal-to-noise ratio in analog signal transmission. 2.
Easy to encrypt. In encryption, a binary number is added after each information unit, while in decryption, only the same information is generated at the detection end.
The binary number of can pass smoothly. : 3. Multiplexing communication is realized by time division multiplexing, which is used.
These channels occupy different time gaps, and the transmissions on the same channel do not interfere with each other. 4. The equipment is easy to be integrated, miniaturized and modularized.
Analog signal is different, because its equipment adopts digital circuit, which can be realized by LSI and VLSI.
Now 5. It occupies the channel frequency bandwidth, and its channel frequency band is 16 times that of the analog channel.
After a series of transformations, video signals need to occupy a lot of storage space. Traditional magnetic storage media
Unable to meet this requirement, the development of optical storage technology provides a guarantee for the storage of a large number of data. On the VCD disc
When storing information, the semiconductor laser writing head in the optical disk drive emits a micron-sized laser beam and projects it onto the high-speed rotating optical disk.
On the surface of the optical disc, information pits related to stored information are ablated, and the reflected laser beam contains specific.
Information, through photoelectric conversion circuit and D/A circuit, analog full TV signal can be obtained. You can store CDs with a diameter of 12cm.
650MB audio and video signal, its central aperture is 15ram, and the signal starting point is 45ram. VCD disc and VHS video tape
Compared with magnetic tape, its signal quality has the advantage of digital communication, and its compact disc is portable, small and easy to store, so it is made into compact disc.
The polycarbonate material of the sheet has low cost. Therefore, it is a qualitative change that VCD representing digital technology replaces VHS video tape.
Audio-visual effect and production cost are better than video tape.
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Second, data compression technology.
The amount of data of digitized image signals is very large. If the data volume of a 640×480 256-color image is 300KB,
The rate of digitized standard signal exceeds 100MB/s/s, and such a large amount of data is beyond the capacity of the current channel, so
The data must be compressed. After compression, a VCD with a capacity of 650M can store 74 minutes of audio-visual materials or moving images.
At present, before adopting compression technology, there are: JPEG static compression MPEG dynamic image compression standard, which has been defined at present.
MPEG 1, MPEG2 and MPEG4 are becoming more and more mature. MPEG 1 uses the comparison of the front and back pictures to cut out the repeated pictures. For a movie,
For complete image data, there are usually many repeated pictures in each frame, and these repeated pictures can be filtered out or only stored.
Second, when the second picture is needed, reading from the previous picture can save space and realize data compression. Yugao
With the emergence of HDTV, digital TV and DVD, MPEG-2 is becoming more and more perfect. The technical parameters of MPEG-2 and MPEG- 1 are compared.
As follows:
The comparison between MPEG 1 and MPEG2 is as follows:
Performance MPEG 1 MPEG2
The video specification is 352×288 720×576×25.
The data encoding speed is1.5mei/s1.5-2.5mb/s.
Picture unit frame frame and field
Dynamic images compare each field, frame and field.
Linear graphics in quantization mode do not need linear graphics.
Comparison of performance and technical parameters among VCD, CVD, SVCD and DVD
In China market, from the appearance of VCD to CVD, SVCD (Super VCD) to the final DVD, all kinds of business are constantly promoted.
When the new product was produced, they did not hesitate to spend a lot of money on advertising. At present, the supply of products exceeds demand, leading to price wars, and consumers are even more ignorant.
What are the characteristics and differences of VCD, CVD, SVCD and DVD? From a technical point of view, numbers are used.
Image compression technology, but there are some changes in technical parameters. Now compare its coding technology, compression standard and other technical parameters.
As follows:
The performance parameters of VCD and CVD are compared as follows (PAL):
Sexual energy
Video compression standard international standard MPEG 1 international standard MPEG2
Resolution (pixel) 352× 288.352× 576
Data rate 1. 15 MB/s fixed bit rate1.15-2.5mb/s variable bit rate.
Compatibility VCD CD VCD CD CVD
Disc specifications are the same as VCD and VCD.
Bandwidth 3.375 MHz 3.375 MHz
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Comparison of performance parameters among VCD, SVCD and DVD (PAL);
Project VCD SVCD research and development center
Pixel 352×288 480×576 720×576
Frame rate 25 25 25
MPEG 1 audio layer 2 unit MPEG 1 layer stereo is extensible.
Human voice MPEG2-2 multi-channel AC3
Meg 25. 1 channel
Video coding MPEG2 MPEG2
Bandwidth 3.375 MHz 4.5 MHz 6.75 MHz
Resolution 268 lines, 357 lines and 530 lines
The data rate is 1. 15mb/s (fixed), the lowest is 1. 15mi-iz/s, and the highest is 2.1.15-9.80mb/s.
Code rate 60 MHz/s variable code rate
Compatibility CD VCD CD SVCD VCD CD SVCD DVD
PAL analog TV can display digital images with a resolution of 360×450, so it is used to watch digital images.
The definition of DVD, CVD and SVCD is similar, much better than VCD.
To sum up, with the advent of the digital age, the development of audio-visual technology is changing with each passing day. Digital technology has replaced analog technology.
An inter-generational progress marks a major progress in the information revolution, and now analog TV sets can no longer adapt to DVD.
Need, can't reflect the characteristics of high definition, so digital TV is very popular now, and it will be one in the future.
A brand-new digital age.
refer to
1, Hao Qingwen: (for color cameras and video recorders), Electronic Industry Press.
2. Li Kedong: Application of Multimedia Technology in Teaching, Electronic Industry Press.
3. Wang Qinsheng: The Principle of First Word Communication, Beijing University of Posts and Telecommunications Press.
(The first author: Teachers of Education College of Bingtuan/Teachers College of Normal University)
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