Video bit rate

Today, we are going to popularize the concept of bitrate in video playback.

When formulating RMVB/RM, the main consideration is network communication and the purpose is to use unpleasant network speed to deliver visually acceptable image quality. Therefore, the characteristic of RMVB/RM encoding format is that better video quality can be obtained at lower bit rate. However, in the case of high bitrate, it is not as good as other encoding formats. It is also an RM/RMVB suffixed file, and its internal encoding format is subdivided into R8/R9/R10, etc., but in general, the above-mentioned characteristics are maintained. It’s just that the compression rate is higher, so RMVB is not used in the field of HD encoding. RMVB looks for a high compression rate and acceptable image quality, so it often compresses some details that are not easy to notice. At first glance, the image is good, but if you look closely, you will find that the image is not sharp, the layers are not clear, and it always gives people a feeling of being blurry all the time. The audio encoding part of the RMVB/RM suffix file also has that situation. The sound compression rate is very high, but it can only be heard. Don’t expect beautiful sound.

The popularity of RMVB is due to the fact that the REAL format is suitable for low-speed network transmission and can achieve acceptable image quality with a smaller file size. Second, with the use of RMVB, relatively full-featured transcoding software and solutions have begun to appear, which is convenient for video lovers and expands the video source. However, as network speeds get faster, better encoders like H264 appear, and because RMVB is not suitable for high-definition video production, and if other hardware manufacturers want to support the format RMVB/RM, you have to pay REAL quite expensive The patent fee caused many hardware manufacturers to stop supporting RMVB files. RMVB documentation is not the completely overwhelming advantage it once was.

Many video files spread on the Internet now have suffixes like AVI, MKV, MP4 and 3GP. Among them, MKV is mainly used for HD video files, MP4, 3GP, etc. They are mainly used in mobile phones and portable devices, etc. AVI is more widely used, not only in high-definition video files, but also in portable devices.

The video encoding format used by VCD is MPEG1, and DVD is MPEG2. Both VCD and DVD are mainly used for home video and audio playback, and generally speaking, VCD is encoded with MPEG1 as a fixed bit rate encoding. DVD can support dynamic bit rate MPEG2 encoding.

In order to ensure the clarity of the image when the image changes drastically, the default encoding rate is high, the standard encoding rate of VCD is 1152Kbps, and the DVD is more open. According to the playing time of the movie, it is generally set between 5000Kbps -8000Kbps, so as not to waste DVD discs Under the premise of capacity, please use a higher bit rate as much as possible to get a higher definition. MPEG1 and MPEG2 are not very effective in the case of ultra-low bit rate, and if the bit rate increases too much, the indications generated by the image effect are not obvious. MKV is encoded in H.264 for high definition video and the 1080p image reaches a bit rate of 30 Mbps.

Note:

VBR: “variable bit rate”, VBR encoding means that the encoder’s output bit rate (or decoder’s input bit rate) can be adaptively adjusted based on the complexity of the encoder’s input source signal , to keep The output quality remains the same instead of keeping the output bitrate the same. VBR is suitable for storage (not very suitable for streaming) and can use limited storage space more efficiently: use more codewords to encode high-complexity segments and use fewer codewords to encode low-complexity segments .

CBR: “fixed bit rate”, CBR encoding means that the output bit rate of the encoder (or the input bit rate of the decoder) must be a fixed (constant) value. CBR is very useful when performing multimedia communications on a channel with limited bandwidth, because the higher bit rate is limited at this time, and CBR can use that channel more easily. But CBR is not suitable for storage,

Video bit rate

Today, we are going to popularize the concept of bitrate in video playback.

Data rate refers to the data flow used by a video file in a unit of time, also called bit rate, sampling rate, and bit stream rate. A popular understanding is that it is the most important part of image quality control in video encoding. Generally, the unit we use is kb/s or Mb/s. Generally speaking, at the same resolution, the higher the code stream of the video file, the lower the compression ratio and the higher the image quality. The higher the bit stream, the higher the sampling rate per unit time, the higher the data stream, the higher the accuracy, the closer the processed file is to the original file, the better the image quality, the clearer the image quality and the higher the decoding capability of the playback device is required.

Of course, the larger the code stream, the larger the file size. The calculation formula is file size = time X code rate/8. For example, a 720P RMVB file with a 1 Mbps stream of 90 minutes is common on the Internet and its volume is = 5400 seconds × 1 Mb/8 = 675 MB.

Generally speaking, a video file includes both image and sound, such as an RMVB video file, which contains both video information and audio information. Audio and video have their own sampling methods and different bit rates, that is, the same video Audio and video file bit rate is not the same. And what we’re talking about is the bitrate of a video file, which generally refers to the sum of the bitrate of the audio and video information in the video file.

Taking the most popular and familiar RMVB video file in China as an example, VB in RMVB refers to VBR, which is short for Variable Bit Rate. The Chinese meaning is variable bit rate, which means that RMVB adopts the dynamic encoding method. , use a higher sampling rate for complex dynamic images (singing and dancing, flying cars, wars, actions, etc.), and use a lower sampling rate for static images, make rational use of resources and achieve the image quality and volume effect

The most fundamental difference between code rate and sample rate is that the code rate is for the source file.

There are many types of encoding formats. With the continuous advancement of technology, various encoding formats have been produced for different purposes. The compression rates of different encoding formats are different and have their own characteristics. Some of them can maintain high image quality in the case of low bitrate, but in the case of high bitrate, the image quality indication is not great and some are suitable for high bit rates. High definition image is maintained, but may not work well at low bit rates. Present some common ones.

How does bitrate affect video quality?

To understand how bitrate affects video quality, we first need to understand how video compression works.

When you’re compressing video, the compression algorithm does two things:

Convert video from “pixel domain” to “frequency domain” by DCT (Discrete Cosine Transform). If you are not familiar with DCT, you can read our previous article: How to explain DCT to a 5-year-old?

Some of the information in the frequency domain is discarded through a technique called quantization, with the expectation that the human eye will not perceive this loss of information.

Essentially, when you compress a video, you’re throwing away some information while hoping there’s no loss in video quality. When you overcompress the video, you throw away a lot of information, and the human eye perceives the encoding loss. If you don’t compress the video too much, the file size will be huge, while the video quality will be excellent.

This is known as the “rate-distortion exchange (RDT)” in video compression, and the lower the bitrate, the worse the video quality (assuming the resolution is fixed).

What does RTD mean?

Let’s look at the example below. We use FFmpeg to compress the example video (CrowdRun) to 1.5 mbps and 5 mbps, respectively. With the video screenshots below, can you tell me which one is from the higher bitrate video and which one is from the lower bitrate video?

Obviously the bitrate in the image above is lower and the bitrate in the image below is higher. When using a bitrate of 5 mbps @ 1080p, the video quality is excellent. For the same video, when we use a bitrate of 1.5 mbps, the video quality of CrowdRun becomes very poor.

But is it always like this? Does low bitrate always produce poor quality video? We answer this question with a simple experiment.

Let’s test the “low bitrate = low quality” hypothesis using animated videos. For the following video, we used the popular Simpsons trailer and compressed the video to 2.5mbps and 1mbps using H.264/AVC (make sure all encoder settings are consistent).