Today, let’s introduce the concept of bit rate in video playback.
Data Rate refers to the Data flow Rate used by video files in unit time, also known as bit Rate, sampling Rate and bit Rate. Generally speaking, it is the most important part of picture quality control in video coding. Generally, the unit we use is KB /s or Mb/s. Generally speaking, with the same resolution, the larger the video stream, the smaller the compression ratio and the higher the picture quality. The larger the code flow, the larger the sampling rate per unit time, the higher the accuracy of the data flow, the closer the processed file is to the original file, the better the image quality, the clearer the picture quality, and the higher the decoding ability of the playback equipment is required.
Of course, the larger the bit stream, the larger the file size, which is calculated by the file size = time X bit rate /8. For example, a 720P RMVB file with a 90-minute 1Mbps bit stream is 5400 seconds ×1Mb/8=675MB.
Generally speaking, a video file includes pictures and sounds, such as an RMVB video file, which contains video information and audio information. Both audio and video have different sampling methods and bit rates, that is, the bit rates of audio and video in a video file are not the same. The bitrate size of a video file generally refers to the sum of the bitrate of audio and video information in the video file.
Take the most popular and familiar RMVB video files in China as an example. VB in RMVB refers to VBR, which is the abbreviation of Variable Bit Rate. The Chinese meaning is Variable Bit Rate, which means RMVB adopts dynamic encoding. The higher sampling rate is used for complex dynamic pictures (dancing, racing, war, action, etc.), while the lower sampling rate is used for static pictures, so as to achieve the effect of both quality and volume by rational use of resources.
The fundamental difference between bitrate and sample rate is that bitrate is specific to the source file.
There are many kinds of coding formats, in the case of continuous technological progress, for different purposes, produced a variety of coding formats. Different coding formats have different compression rates, and have their own characteristics, some in the case of low bit rate can maintain high picture quality, but in the case of high bit rate, but the picture quality hint is not good, some suitable for high bit rate to maintain high definition picture, but may not be good in the case of low bit rate. This section describes the common ones.
When RMVB/RM is formulated, the main consideration is network communication, aiming at using the unpleasant network speed to spread the visual acceptable picture quality. Therefore, RMVB/RM encoding format is characterized by better video quality at lower bit rate. But the case of high bit rate is not as good as other encoding formats. The same RM/RMVB suffix of the file, its internal coding format subdivision and R8/R9/R10, but in general, the above characteristics are still exist. It’s just that the compression rate is higher, so RMVB is not used in HD coding. RMVB pursues high compression rates and acceptable picture quality, so it often compresses out details that are not easy to notice. At first glance, the picture is good, but when I look closely, I find that the picture is not sharp and the hierarchy is not clear, and it always gives people a vague feeling. The same situation exists in the audio coding part of RMVB/RM suffix files. The sound compression rate is very high, but you can only listen to it, don’t expect to achieve the realm of sound.
The popularity of RMVB is due to the fact that the REAL format is suitable for transmission over low-speed networks and can achieve acceptable picture quality with small file sizes. Second, with the use of RMVB, there began to be transcoding software and solutions with relatively perfect functions, which facilitated video lovers and expanded video sources. But with faster Internet speeds, better encoders such as H264, and the fact that RMVB is not suitable for HD video production, and that other hardware vendors who want to support RMVB/RM have to pay REAL expensive royalties, many hardware vendors have dropped support for RMVB files. RMVB files are not the overwhelming advantage they used to be.
Now the network transmission of video files have been a lot of AVI, MKV, MP4, 3GP and other suffixes. Among them, MKV and other files are mostly used for HD video files, MP4 and 3GP are mostly used for mobile phones and portable devices, etc. AVI is more widely used, not only in high-definition video files, AVI is also used in portable devices.
The video encoding format used by VCD is MPEG1, and that of DVD is MPEG2. Both VCD and DVD are mainly used for home video and audio playback, and generally speaking, VCD uses MPEG1 encoding for fixed bit rate encoding. DVD can support MPEG2 encoding of dynamic bit rate. In order to ensure sharp changes in the picture of the definition, its default coding rate is higher, VCD standard coding rate 1152Kbps, DVD open, according to the film playback time, often set to 5000 Kbps -8000 Kbps between, Use as high a bit rate as possible to achieve higher resolution without wasting DVD capacity. MPEG1 and MPEG2 are not good at ultra-low bit rate, and when the bit rate is excessively increased, the picture effect is not obvious. MKV is encoded in H.264 for high-definition video, with a 30Mbps bit rate at 1080p.
Note: VBR: “variable bit rate”. VBR encoding refers to the fact that the output bit rate of the encoder (or the input bit rate of the decoder) can be adjusted adaptively according to the complexity of the input source signal of the encoder. The purpose is to keep the output quality constant rather than keep the output bit rate constant. VBR is suitable for storage (less suitable for streaming) and can make more efficient use of limited storage space: encode high complexity segments with more code words and low complexity segments with fewer. CBR: “fixed bit rate”, CBR encoding refers to the fact that the output bit rate of the encoder (or the input bit rate of the decoder) should be a fixed value (constant). CBR is useful for multimedia communication in a bandwidth-constrained channel where the maximum bit rate is limited and the CBR can more easily use such a channel. CBR is not suitable for storage, however, because CBR will result in insufficient bit rate to encode complex parts of content (resulting in quality degradation), while some bit rate will be wasted on simple parts of content.
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