There are many varieties of hard disk video recorders on the market. How to choose the most suitable hard disk video recorder is a difficult problem for many users, especially the novice, and it is impossible to start. When choosing a hard disk video recorder, we should focus on examining the image compression method of the hard disk video recorder, the picture Image quality, real-time, network performance, stability, after-sales service and price.
The industrial computer type hard disk video recorder (PC-BASEDDVR) takes the industrial computer as the basic hardware, the WINDOWS operating system as the basic software, and is equipped with video and audio compression boards. With the help of professional compression algorithms and PC software and hardware capabilities, the monitoring system video and audio signals are realized. A device for recording and playback. The core device of the industrial control DVR is the video and audio compression card. Compression method and image quality Compression technology is the core of the hard disk video recorder, and it is the most critical to choose which compression method. Here, not only the image quality, but also the storage capacity and transmission speed of the image should be considered. At present, the compression methods of hard disk video cards on the market mainly include M-JPE, wavelet algorithm, MPEG-1, H263, MPEG-4, H.264 and so on. 1) M-JPEG (we can think of it as the standard for monitoring compression) (a considerable number of domestic and foreign network cameras and video servers have adopted MJPEG compression technology since 1995, such as the SNC-Z2OP network camera launched by SONY, AXIS2130PTZ network camera, etc. .MJPEG refers to MotionJPEG, that is, dynamic JPEG. It uses JPEG algorithm to compress video signals at a speed of 25 frames per second (PAL system) to complete dynamic video compression. It is formulated by the JPEG expert group, and its image format is for each frame. Compression, usually a compression ratio of 6:1. Just like each frame is an independent image. The unit of the MJPEG image stream is a JPEG picture of one frame and one frame. Because each frame can be accessed arbitrarily, MJPEG Often used in video editing systems. Motion JPEG can produce high-quality, full-screen, full-motion video, but it requires additional hardware. Moreover, since MJPEG is not a standardized format, each manufacturer has its own version of MJPEG. The files on both sides cannot recognize each other. So MJPEG has begun to fade out of the video processing field.
The advantage of industrial computer MJPEG is that the picture quality is relatively clear, but the disadvantage is that the compression rate is low and the bandwidth is huge. Generally, a single channel occupies a bandwidth of about 2M to 3M. And when the processing volume is huge, the video workstation will not be able to complete too much video data, which will result in huge network data traffic and lead to network congestion. Since the compression is relatively small, the data storage capacity of the storage server will be an issue. On a 100MB network, 16 channels cannot be transmitted in real time at the same time, and the amount of video data per hour is more than 500MB. This problem is an even bigger obstacle for network equipment. Moreover, the Motion-JPEG compression technology does not support audio and video synchronization, so even if some companies have developed the audio part, the problem that audio and video cannot be synchronized frequently occurs. Many early products in Taiwan and Korea also used this compression method, but it has been gradually phased out of the market. 2) WaveletTransform (promoted since 1997) Wavelet transform is also an intra-frame compression technology. Since this compression method removes the high-frequency components of the image, only a single-frame image signal is retained, which is especially suitable for occasions where the screen changes frequently , and the compression ratio has also been improved to a certain extent, so it is also used by some network cameras, such as the NetCam-4 series of digital network cameras launched by BOSCH. However, the overall compressed data volume has not changed much, and the image quality has decreased significantly. 3) H.263 (second generation) (promoted since 1998) H.263 is a relatively mature standard, and the H.263 video coding standard is a low-bit rate image compression standard designed for medium and high-quality moving image compression. H.263 adopts a common coding method in motion video coding, and divides the coding process into two parts: intra-frame coding and inter-frame coding. The improved DCT transform and quantization are used in the frame, and the 1/2 pixel motion vector prediction compensation technology is used in the frame to make the motion compensation more effective. to get the final coding coefficients.
