ApsaraVideo Media Processing:Terms

An MPS queue is a queue for jobs. After jobs are added to an MPS queue, they are scheduled for processing by MPS. If an MPS queue contains a large number of jobs, the jobs are queued up. An MPS queue can be enabled or disabled. If an MPS queue is disabled, MPS does not schedule jobs in this MPS queue until the MPS queue is enabled again. However, ongoing jobs in this MPS queue are not affected. Different types of MPS queues support different concurrencies.

• Custom template: a transcoding template that is created in the MPS console or by using the API. No parameters are customized at the backend.

• Customized template: a transcoding template for which customized parameters are configured at the backend based on your personalized requirements. If you use a customized template, the parameters that you configure do not take effect. You cannot view or modify the customized parameters.

• Preset template: a transcoding template that is predefined in MPS based on the resolutions and network. Preset templates include static preset templates and intelligent preset templates. Static preset templates support regular transcoding, audio transcoding, container format conversion, Narrowband HD™ 1.0, and Narrowband HD™ 2.0. For more information, go to the MPS console or see Preset template details.

A Group of Pictures (GOP) refers to a group of continuous images in an MPEG-encoded video or video stream. It starts with an I-frame and ends with the next I-frame. A GOP contains the following image types:

• Intra coded picture (I-frame): the keyframe. An I-frame contains all information that is required to produce the picture for that frame. The I-frame is independently decoded and can be regarded as a static picture. The first frame in the video sequence is always an I-frame, and each GOP starts with an I-frame.

• Predictive coded picture (P-frame): A P-frame must be encoded based on the previous I-frame. A P-frame contains motion-compensated difference information relative to the previous I-frame or P-frame. During decoding, the difference defined by the current P-frame is superimposed with the previously cached image to generate the final image. P-frames occupy fewer data bits compared with I-frames. However, P-frames are sensitive to transmission errors due to the complex dependencies on the previous I-frame or P-frame.

• Bidirectionally predictive coded picture (B-frame): A B-frame contains motion-compensated difference information relative to the previous and subsequent frames. During decoding, the data of the current B-frame is superimposed with both the previously cached image and the decoded subsequent image to generate the final image. B-frames provide a high compression ratio and require high decoding performance.

The GOP value must be within an appropriate range to achieve balance among the video quality, file size that indicates bandwidth consumption, and seeking effect that indicates the speed of response to the drag and fast-forward operations.

• When the GOP value increases, the file size is reduced. However, if the GOP value is too large, the last frames of a GOP are distorted, and the video quality is reduced.

• The GOP value is also a key factor in determining the speed of response to seeking in a video. During seeking, the player locates the closest keyframe before a specific position. A larger GOP value indicates a longer distance between the specified position and the closest keyframe, which results in more predictive frames that need to be decoded. In this case, the loading time is extended and the seeking operation requires a long period of time to complete.

• Encoding P-frames and B-frames is more complex compared with encoding I-frames. A large GOP value results in many P-frames and B-frames. This decreases the encoding efficiency.

• However, if the GOP value is too small, the bitrate of the video must be increased to ensure that the image quality is not reduced. This process increases bandwidth consumption.

H.264 provides the following encoding profiles:

• Baseline profile: This profile provides basic image quality and is applicable to mobile devices. The baseline profile uses I-frames and P-frames, and supports only progressive videos and context-adaptive variable-length coding (CAVLC).

• Main profile: This profile provides mainstream image quality and is applicable to standard-definition devices, such as MP4 players that have relatively low decoding capabilities, portable video players, PSPs, and iPods. The main profile uses I-frames, P-frames, and B-frames, and supports progressive and interlaced videos. The main profile also supports CAVLC and context-adaptive binary arithmetic coding (CABAC).

• High profile: This profile provides high image quality and is applicable to high-definition devices that have big screens, such as broadcast and disc storage applications for Blu-ray Discs and high-definition television applications. The high profile supports 8 × 8 inter-prediction, custom quantization, lossless video coding, more YUV formats and the features of the main profile.

Advanced Audio Coding (AAC) provides the following encoding profiles:

• aac_low: Low Complexity AAC (LC)

• aac_he: High Efficiency AAC (HE-AAC)

• aac_he_v2: High Efficiency AAC version 2 (HE-AACv2)

• aac_ld: Low Delay AAC (LD-AAC)

• aac_eld: Enhanced Low Delay AAC (ELD-AAC)

Bitrate control methods refer to the methods that are used to control the bitrate of a coded stream. The following bitrate control methods are commonly used:

• Constant bitrate (CBR): This method is used to generate a file that has a fixed bitrate. If you use this method, the bitrate is fixed throughout the coded stream. CBR-compressed files are larger in size than VBR- and ABR-compressed files, and do not have much improvement in quality.

• Variable bitrate (VBR): This method is used to generate a file that has a variable bitrate. This method determines the bitrate of the output file based on the complexity of the input file during encoding. For a more complex input file, a file that has a higher bitrate is generated. For a simpler file, a file that has a lower bitrate is generated. This method is generally used with the Two-Pass encoding method. VBR is applicable to storage and allows you to use the limited storage space in a more reasonable manner. However, you cannot predict the size and bitrate fluctuation of the output file by using VBR.

• Average bitrate (ABR): This method is an average bitrate mode with interpolation parameters added. LAME has created this method to solve the size and quality mismatch of CBR-compressed files and the unpredictable file sizes of VBR. In a specific file range, ABR divides a stream into parts in the unit of 50 frames. 30 frames equal to about one second. ABR uses relatively low bitrates to code the less complex segments and high bitrates to code the more complex segments. ABR can be regarded as a compromise between VBR and CBR. The bitrate can reach the specified value within a specific time range, but the peak bitrate in some segments may exceed the specified bitrate. The average bitrate remains constant. ABR is a modified version of VBR. ABR ensures that the average output bitrate is within an appropriate range and codes videos within this range based on the complexity. By default, Alibaba Cloud uses ABR.

• Video Buffering Verifier (VBV): This method can ensure that the bitrate is lower than a specific value. This method is used based on the maxrate and bufsize parameters. The maxrate parameter specifies the maximum output bitrate, and the bufsize parameter specifies the buffer size. VBV can be used with the Two-Pass or Constant Rate Factor (CRF) encoding method. The CRF encoding method can also be replaced by Capped CRF.

• bufsize: the size of the video buffer. You can configure this parameter based on the expected bitrate fluctuation. In most cases, you can configure the bufsize parameter to twice the maxrate parameter. If the cache size of the client is small, set the bufsize parameter to the value of the maxrate parameter. If you want to limit the bitrate, set the bufsize parameter to half of the value of the maxrate parameter or less.

• CRF: This method controls the output bitrate by using the quality control factor. The video quality is quantified into different levels from 0 to 51. 0 specifies lossless image quality and 51 specifies the worst image quality that can be generated. You can ensure the general quality of the output video by using CRF. The bitrate varies based on the complexity of the input content. If you do not know which CRF level is suitable, we recommend that you use a level in the range of [23,29]. You can adjust the CRF level based on the complexity of the content. If you increase or decrease the CRF level by 6, the bitrate is reduced by half or doubled. To generate files with the same definition, you can set the CRF level for a computer-generated video to be larger than the CRF level for a live-action video. CRF can provide better video quality, but cannot be used to predict the size and bitrate fluctuation of the output file.

• Capped CRF: The bitrate of the output file by using CRF is not fixed. You can use CRF together with VBV to limit the range of the bitrate and prevent bitrate spikes.

• One-Pass: The encoding speed of this method is faster than that of Two-Pass. By default, Alibaba Cloud uses One-Pass.

• Two-Pass: The encoder is run twice to accurately assign the bitrate to obtain an output file that has a smaller size and higher quality. In the first pass, the encoder analyzes a video and generates log files. In the second pass, the encoder performs encoding based on the analysis results to obtain the best encoding quality. Two-Pass consumes more time than One-Pass. Therefore, Two-Pass cannot be used in scenarios that require high transcoding timeliness, such as live streaming and real-time communication. If the compression ratio of the input video is high, we recommend that you do not use Two-Pass. Otherwise, blocking artifact may occur.

OSS supports the following types of AccessKey pairs:

• AccessKey pairs applied for by the bucket owner.

• AccessKey pairs granted by the bucket owner by using Resource Access Management (RAM).

• AccessKey pairs granted by the bucket owner by using Security Token Service (STS).