How to choose the applicable DVR
The DVR brands and types appearing on the market today can be said to be varied and colorful. Products that advertise fast recording speed, large storage capacity, large compression ratio, and high image clarity are everywhere. In the face of dazzling functions, we should How to distinguish its advantages and disadvantages, how to choose the product that suits you?
To distinguish the advantages and disadvantages of a digital video recorder (DVR), you can make judgments from the important performances such as the recording rate, storage capacity, picture clarity, and operation and usage methods. However, in the comparison of these points, there are many misunderstandings in the market. Many users are blindly pursuing fast speed, large capacity, and high resolution, but they do not know that these three indicators are actually mutually restrictive, and it is impossible to require all three to achieve the best; whether the operation is easy to use is also often used by most projects. The company's equipment selection staff ignored it and found out that it wasted a lot of time in vain during the installation process.
Indicators to distinguish the advantages and disadvantages of DVR
Video recording speed
As far as the recording rate is concerned, in fact all DVRs are 30 pictures / second or 60 picture fields / second (NTSC format) in real time, and any advertising that exceeds this indicator is unrealistic. If 16 channels of images are recorded at the same time, the rate of each channel is only 30/16 frames per second or 60/16 fields per second. During multi-channel recording, some DVRs use advanced image displacement detection to greatly increase the recording rate and activity. The recording speed of the image is almost real-time, which is many times faster than the video recorder without image displacement detection function. However, it should be noted that the image displacement detection recording refers to full-frame compression recording of the full frame of the camera with image displacement, rather than only compressing and recording the part of each camera image with image displacement activity. Although the latter can also increase the recording rate, it is In the law, such images will not be used as evidence in court.
2. Storage capacity and backup
The larger the recording capacity, the better, but the most important thing is to have an interface connected to an external digital storage device to back up image data. Only frequent backups can ensure that valuable images can be safely saved and facilitated. transmission. It is unwise to store several months of images on the built-in hard disk of the machine. Once the machine is damaged, all data will be completely lost.
3. Image clarity
The level of image clarity directly reflects the quality of the DVR, but from a technical point of view, the higher the resolution, the greater the storage capacity occupied, so it is best for users to adjust the level of resolution according to the actual situation. Design, this has been done by almost all brands.
4. Simple operation
Whether the operation is simple and convenient ultimately determines the adaptability of the product in practical applications. More and more users hope that the equipment they use is best to press a button to complete all functions. Many manufacturers ignore the overall quality of users ’electrical equipment. The product is designed with many functions and the operation is too complicated, so the result is difficult to be promoted. In order to improve this deficiency, many manufacturers are constantly improving their products, such as using a humanized keyboard, simplifying daily operations, or adding computer network interface functions, and conforming to the trend of network development, etc., to expand their market share in the international market. rate.
Choose the DVR that suits you best
When choosing a DVR, users should avoid falling into some of the above misunderstandings while ignoring the real advantages and disadvantages of the product; and should choose the most suitable product according to the actual application needs of their own projects, and not blindly pursue various high-tech indicators. The more functions and the higher the technicality, the more suitable for every user. The following three options are presented for reference by all engineering companies and end users.
1. Performance meets project requirements
When choosing a DVR, first determine where the device will be used? Is it a security monitoring project (such as a building, airport, museum, etc.), or a bank teller project that requires high recording speed? The products used for security monitoring projects are characterized by complete functions, good network transmission function, and slow recording speed, but through the linkage with sensors and motion detection, the recording resources can also be allocated to the places where they are most needed. The products used in the bank teller monitoring project are characterized by high recording speed, each channel can reach more than 20 frames, and must have real-time recording, multi-channel video and audio and audio and video synchronization functions, but due to the workload in image processing Very large, so its network and other functions are relatively weak. Different types of DVRs should be used in different types of monitoring projects to avoid wasting costs and ineffective operation. For example, if the equipment suitable for security monitoring projects is improperly planned in the bank teller project, the recording speed will not meet the needs of users. And because the bank teller project requires higher computer configuration and hard disk capacity, the cost is relatively higher. To put it another way, if a product suitable for a bank teller project is used when planning a security monitoring project, costs will be wasted, and security monitoring projects often have transmission and sub-control requirements, which is exactly what the bank teller project system Weaknesses. At the same time, it is necessary to remind everyone that DVR is a "one machine with multiple functions" instead of "one machine with all functions". There are often users who hope that one device can solve all the problems, but in fact, the current level of computer hardware and software technology is still basically unable to fully meet the requirements of users. Because the bus transmission speed of the computer, the processing speed of the CPU and the burning speed of the hard disk are limited, unless a supercomputer is used, it is impossible to transfer, process and burn so much data at the same time, but as a result, the cost is quite high. The user is unacceptable.
2. Stability verified
If the stability of the DVR is not enough, failure to record the video at an important moment will cause many serious problems. The stability of DVR can be mainly considered from the two aspects of hardware structure and software program. In view of the fact that there are many types of DVRs on the market currently based on PC-based, the following is only an example of PC-based models:
1. Hardware structure:
DVRs are all multi-input, and the hardware can be multi-card or single-card. Sometimes in order to improve product performance, such as increasing the recording speed, the multi-card method will be used, but in this way it is easy to cause conflicts between hardware, such as the aforementioned bank teller project equipment often have such problems. Generally speaking, the fewer capture cards you use, the better, because the single-card mode has high integration and stability is better than the multi-card mode. In addition, the heat dissipation of the equipment is also a factor to be considered.
2. Software program:
The software program is well written and must be verified through long-term operation and operation to be known. In particular, special attention should be paid to the issue of whether the hard disk is recycled and stable. One of the key advantages of DVR is that there is no need to continuously replace the storage medium. It will constantly refresh the hard disk, clear out the expired data, and save the latest data. However, some DVRs often have problems in this process, which is often The most easily overlooked by contractors and users. Therefore, it may be the easiest way to choose a product that has been widely put into use and has a good response from users.
3. Feasibility and cost of additional hard drives
Generally speaking, the capacity of DVR's built-in hard disk is insufficient. For example, the DVR used for bank teller monitoring requires 30 days of images to be saved, and the recording length is at least 200 hours. If an eight-channel real-time DVR is used, even if the amount of image data per frame is compressed to 2K, the required hard disk capacity is at least 300GB. And, the amount of image data is compressed to 2K, it is difficult to say whether the image quality is acceptable. Therefore, whether it is possible to add a hard disk to expand the storage capacity becomes a very important question. In addition, since the cost of hard disks is still high, it is also necessary to consider whether the additional cost that can be added later can be loaded.
in conclusion
To sum up, when choosing a DVR, engineering companies and users must fully consider whether the performance of the equipment meets the needs, and cannot force all functions to be powerful; the stability of the equipment should be investigated in many ways to avoid problems during use. ; And in order to ensure that the image retention time meets the requirements, the feasibility and convenience of adding a hard disk and the additional cost should be considered in advance; finally, it is necessary to know the limitations of the development level of computer hardware and software technology on the performance of the hard disk recorder, in order to be clear and correct In order to judge whether the function of the product is exaggerated, in addition, based on the principle of "seeing is believing", it is recommended to buy mainly to look at the performance of the product on the image and transmission, in order to choose the most suitable product among many products.
Myth 1: The operating system of Windows * is unstable and affects the stability of the hard disk recorder. The stability of the system depends on two aspects: hardware and software. The software has two parts: operating system and recording software, whether it is Windows98, Windows NT, Windows2000, It is Windows XP, which is a very good software that is universally used all over the world. As a hard disk recorder, only a small part of the function of the operating system is used. There is no reason to doubt the stability of any operating system.
But what causes problems such as unstable work and crashes during use? It can be said that 100% is caused by unreasonable hardware configuration or programming of recording software (application software), which caused the crash.
Myth 2: The multi-card in one machine causes hardware conflicts and affects system stability.
This problem is often caused by the use of immature video compression cards or pirated cards. Another cause is a problem with the hardware system settings. One machine with multiple cards is a widely used industry standard, and there is no reason to doubt its stability.
Myth 3: The hard disk recorder will always "freeze"
The new generation of digital hard disk recorders generally have a software or hardware watchdog protection system. When a hardware failure occurs, it will issue an alarm prompt; when a software failure occurs, it will automatically restart and automatically recover. Therefore, it can be used with confidence in an unmanned monitoring room.
Myth 4: Embedded DVR won't "crash"
Although the embedded hard disk video recorder deviates from the PC software environment and ostensibly solves the crash problem, if the hard disk fails, it will still be dead. In fact, the hard disk failure is one of the main failures of the hard disk recorder.
Myth 5: PC-based hard disk recorders have fallen behind, and embedded hard disk recorders are the development direction
PC-type hard disk recorders will always develop in sync with the IT industry. They are far superior to embedded systems in network transmission, picture quality, and video processing speed. In addition, they are intuitive, flexible, and fast in use. Yes, but its biggest weakness is that it is vulnerable to man-made malicious damage, and this is precisely the advantage of embedded systems. Therefore, these two types of DVRs have their own advantages, and there will be a period of coexistence.
Myth 6: The embedded all-in-one integrated system is not a PC system!
The all-in-one multimedia computer motherboard is just a highly integrated multimedia single board computer. Why did it become a product other than PC architecture?
Image quality Image quality is the core issue in evaluating the quality of digital video recorder products. After the analog video image signal is input to the DVR and becomes a digital signal, it is compressed, stored, and then decompressed. The quality of the conversion and the speed of the conversion can be from the clarity, grayscale, color reproduction, and real-time performance of the image. It is evaluated in many aspects, and clarity is the most important indicator for evaluating image quality. DVR0 definition can be divided into display definition, recording definition and playback definition. General display definition> recording definition> playback definition. The playback definition can express the overall effect of DVR recording and playback, and the definition evaluation measure The standard has two main aspects:
1. Image resolution The resolution of an image refers to how many pixels an image can be decomposed into. International standards are characterized by the product of its horizontal and vertical pixels. The most common image formats are 176 * 144 (QCIF), 352 * / 288 (CIF), and 704 * 576 (FCIF).
2. For the same image resolution, due to the difference in compression method and / or compression data rate applied, the contrast of the large area contrast and the small area contrast of the image is different, so that the image effect that people can observe with the naked eye It is also different, this is what we call the image quality.
The factors that affect the clarity of DVR playback are:
â‘ Clock (CP) speed and bandwidth of the video A / D converter;
â‘¡ RAM access speed;
â‘¢ Image compression method;
â‘£ Hard disk access speed. The selection of image compression method is very important. In addition, the hard disk is an electromechanical device, and its access speed may also be a bottleneck.
Compression method Compression method is the core technology of DVR. Compression method largely determines the image quality, compression ratio, transmission efficiency, transmission speed and other performance. It is an important part of evaluating the performance of DVR. With the development of multimedia technology, many compression coding standards have been introduced one after another. At present, there are mainly standards such as JPEG / M-JPEG, H.261 / H.263 and MPEG.
1. JPEG / M-JPEG
â‘ JPEG is a compression standard for still images, it is a standard intra-frame compression coding method. When the hardware processing speed is fast enough, JPEG can be used for video compression of real-time moving images. It can provide quite good image quality under the condition that the picture change is small, the transmission speed is fast, and the use is quite safe. The disadvantage is that the data volume is large.
â‘¡, M-JPEG is derived from JPEG compression technology, it is a simple intra-frame JPEG compression, the compressed image quality is better, no mosaic under the situation of picture changes, but due to the technical limitations of this compression itself, large-scale compression cannot be achieved , About 1-2GB of space per hour when recording, and 2M bandwidth is required for network transmission, so no matter recording or network transmission, it will consume a lot of hard disk capacity and bandwidth, not suitable for long-term continuous recording needs, not practical for video Network transmission of images.
2. H.261 / H.263
â‘ , H.261 standard is usually called P * 64, H.261 can achieve high compression ratio for full-color, real-time transmission of moving images, the algorithm is composed of intra-frame compression plus inter-frame compression encoding to provide video Fast processing of compression and decompression. Because only the last frame is predicted in the inter-frame compression algorithm, it has an advantage in duration, but the image quality is difficult to achieve high definition, and it is impossible to achieve large compression ratio and variable rate recording.
â‘¡ The basic coding method of H.263 is the same as that of H.261, but both are mixed coding methods. However, H.263 has been improved in all aspects of coding to adapt to the transmission of extremely low bit rates, such as saving code Words to improve the quality of the encoded image. In addition, H.263 also absorbs MPEG two-way motion prediction and other measures to further improve the prediction accuracy of inter-frame encoding. Generally speaking, at low bit rates, H.263 is used at only half the rate Image quality comparable to H.261 can be obtained.
3. MPEG
MPEG is a video and audio coding standard for compressing moving images and accompanying audio. It uses inter-frame compression and only stores the differences between consecutive frames to achieve a large compression ratio. MPEG currently has MPEG-1, MPEG-2 and MPEG-4 three versions to adapt to different bandwidth and image quality requirements.
①The MPEG-1 video compression algorithm relies on two basic technologies, one is motion compensation based on 16 * 16 (pixel * line) blocks, and the other is compression technology based on transform domain to reduce the spatial redundancy and the compression ratio. It is higher than M-JPEG, and can obtain better image quality for video signals that are not intensely moving, but when the movement is intense, the image will produce a mosaic phenomenon. MPEG-1 transmits video and audio signals at a data rate of 1.5Mbps. MPEG-1 is equivalent to the image quality of VHS recorders in terms of video image quality. The color mode of the clarity of video recording is ≥ 240TVL, and the quality of the two stereo sounds is close to that of CD. Sound quality. MPEG-1 is a compression algorithm for multi-frame prediction of front and back frames. It has great compression flexibility. It can compress video at a variable rate. Depending on the recording environment, different compression quality can be set, ranging from 80MB to 400MB per hour, but The amount of data and bandwidth are still relatively large.
â‘¡, MPEG-2 It is to obtain higher resolution (720 * 572) to provide broadcast-level video and audio coding standards. MPEG-2 is a compatible extension of MPEG-1. It supports interlaced video formats and many advanced features, including support for multi-level adjustable video encoding, and is suitable for occasions with multiple qualities such as multiple rates and multiple resolutions. It is suitable for real-time images with large motion changes and high image quality. It compresses 30 frames per second and 720 * 572 resolution video signal, and the data rate can reach 3-10Mbps. Due to the large amount of data, it is not suitable for long-term continuous recording needs.
â‘¢ MPEG-4 is a low-rate, high compression ratio video and audio coding standard formulated for mobile communication equipment to transmit video and audio signals in real time on the Internet. The MPEG-4 standard is an object-oriented compression method. Instead of simply dividing the image into blocks like MPEG-1 and MPEG-2, it separates the objects (objects, people, background) according to the content of the image , Respectively, intra-frame and inter-frame coding, and allows flexible allocation of code rates between different objects, more bytes are allocated to important objects, less bytes are allocated to secondary objects, which greatly improves Compression ratio, get better results at a lower bit rate, MPEG-4 supports most functions in MPEG-1, MPEG-2, provides rectangular graphics images with different video standard source formats, bit rates, and frame rates Effective coding. In short, MPEG-4 has three advantages â‘ , has a good compatibility;
â‘¡, MPEG-4 provides better compression ratio than other algorithms, up to 200: 1;
â‘¢ While MPEG-4 provides high compression ratio, the loss of data is very small. Therefore, the application of MPEG-4 can greatly reduce the video storage capacity and obtain higher video resolution, which is especially suitable for long-term real-time video recording needs, and has excellent network transmission capabilities at low bandwidth.
The total number of frames processed per second
The total number of frames processed by the DVR per second depends on the DVR image processing speed. In the application of early digital video recorders, the main consideration was the recording function, with a small number of channels and a large storage capacity. When more video surveillance points are required, it is inevitable It will reduce the time resolution of each channel or / and reduce the number of real-time recording channels. When the multi-machine cascade application is used to increase the number of input channels, it will make the control * very inconvenient. At the same time, in the image network transmission, it will Due to multi-machine transmission, remote clients need to switch logins one by one, resulting in a waste of network resources. Therefore, the total number of frames processed by DVR per second is also an aspect of evaluating the performance of DVR. The total number of frames processed by the DVR per second depends on the image processing speed of the DVR. In a PC-based DVR, the image processing speed is determined by the processing speed of the CPU. The transmission speed of the bus, the access speed of the hard disk, and the memory capacity. At present, in the DVR based on PC, the common methods to improve the image processing speed are:
â‘ Choose a PC with high CPU processing speed and large memory capacity;
â‘¡ Adopt pure hardware decompression method. Pure hardware decompression is used to save the CPU and memory resources of the PC as much as possible. In a DVR that uses a combination of software, hardware, and software decompression technology, the software decompression takes up more CPU and memory resources, so the ability to process and record images per second is affected. For example, the number of image frames processed by pure software decompressed digital video recorders generally cannot exceed 200 frames per second, and some require good image quality. Compression algorithms with large compression ratios can process fewer image frames. In addition, the use of pure hardware decompression will also reduce the uncertain factors of software operation and improve the working stability of DVR.
â‘¢ Video and audio acquisition, coding compression, decompression and display use an integrated chip, use the PCI bus structure of the PC to directly distribute and transmit multiple channels of video and audio streams to the graphics card, and the IDE bus performs image display and storage, so that the CPU only plays the role of control and Distribution role. However, the bus transmission speed, CPU processing speed and hard disk access speed are limited, so increasing the image processing speed of the DVR will be limited by the PC.
The monitoring function has the monitoring function is a characteristic of DVR's superior performance, the monitoring function of DVR should have:
â‘ , the alarm input and output and the control of the cloudy platform and the lens support various external alarms and linkage alarm equipment, and support the integrated dome of the pan / tilt and the lens, so that when an accident occurs, the camera is controlled to a predetermined position and the recorder is automatically started Make a video.
â‘¡, with matrix switching function and multi-screen processing display function, so that the DVR's recording function can be more fully exerted. â‘¢, remote monitoring can be real-time remote monitoring through the Internet network, local area network, telephone network, control the remote PTZ, lens, alarm, support remote setting alarm area, retrieval, playback and other functions * remote recording.
Stability Stability work is very important to DVR, and it is also the main problem of DVR at present.
For PC-based DVR, the main factors affecting its stable operation are:
â‘ The performance of the PC compatible PC for 24 hours of uninterrupted work is not very stable. The stability of the industrial control computer is relatively improved compared to the PC. It is suitable for a more complex working environment. Magnetic field interference has a certain suppression function.
② 〠The stability of the operating system Windows98 * is not high, and it is easy to crash. Windows NT, Linux and other operating systems have more stable performance than Windows 98.
â‘¢ The design of application software must also be tested for stability, and its capabilities should support multi-tasking concurrent processing, such as multi-tasking capabilities of monitoring, recording, playback, backup, alarm, control, remote access, etc.
â‘£ Hardware compression and software compression Hardware compression is superior to software compression in multiplexing mode and system stability. The main reason is that software compression occupies a lot of CPU resources, so that when other functions are running, the CPU has no time to deal with it and DVR crashes. .
⑤ Structure of video acquisition Video acquisition can adopt multi-card mode or single-card mode. The single-card mode has high integration and stability is better than multi-card mode. Sometimes in order to improve performance, such as increasing the image processing speed, Multi-card mode, even one-way one-card mode, so it is easy to form software and hardware conflicts, which have a greater impact on the stability of DVR. The fundamental solution to the stability of DVR lies in the selection of an integrated embedded DVR, but at present, this type of embedded DVR with more than 4 channels is relatively rare, and its functions in network functions, video and audio synchronization, etc. are also unsatisfactory. At present, some DVR products cannot guarantee the system's continuous normal operation for a long time, and the hardware watchdog function has an anti-crash restart circuit, which can maintain the stable operation of the DVR. Although it is impossible to record during startup, it is a safety issue. Loopholes, but as a stopgap emergency measure is still desirable.
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