Overview of the Transcoder API

This page provides a technical overview of the Transcoder API, including features, terminology, and useful concepts. The Transcoder API implements a REST and RPC API that lets you submit, monitor, and manage transcoding jobs in Google Cloud. To submit jobs using the Transcoder API, you first upload media assets to Cloud Storage. After processing a job, the Transcoder API saves the resulting media back to Cloud Storage.

Features

The Transcoder API includes support for the following features:

  • Insert ad keyframes to allow video player clients to insert ads
  • Create thumbnails from a generated spritesheet of video frames
  • Create job templates to save and reuse custom or complex configurations for transcoding jobs
  • For more information about the MP4 and MPEG-DASH standards, see The Moving Picture Experts Group website. For more information about the HLS standard, see HTTP Live Streaming.

    Access control

    The Transcoder API access control model is based on Google's Identity and Access Management. IAM's fine-grained permissions give you control over what can be done with media data, and by whom.

    Limits

    The following table shows limits per transcoding job.

    Job limits
    Total input size400 GB
    Total output size400 GB
    Maximum EditList duration24 hours
    Maximum number of video streams30
    Maximum number of audio streams50
    Maximum number of text streams50
    Maximum number of mux streams100
    Maximum number of manifests100
    Maximum number of spritesheets10

    Concepts

    This section provides important concepts regarding video files and how they are used with the Transcoder API.

    Components of a video file

    Each video file has a container, which is the wrapper for the entire file. The Transcoder API uses MuxStreams to define the container. Each container includes a set of ElementaryStreams to define the encoding of the video, audio, and caption text tracks for the file. Video and audio are compressed using codecs.

    In the following example, the video is compressed using H.264 and the audio is compressed using AAC. Both are placed in an MP4 container.

    Components of a video file and an example
    Figure 1. Components of a video file (left) and an example MP4 file (right).

    Streaming protocol structure

    For streaming media, content providers encode the same content at multiple bitrates (measured in Kilobits per second). The provider then generates a manifest file that points to the different files with different bitrates. The streaming media player uses the manifest file to select the appropriate file, and then pulls video a few seconds at a time. Each bitrate may be a single file or multiple short files, depending upon what the player supports.

    Different resolutions of media are typically encoded to target different bitrates. For example, lower bitrates are encoded at standard definition (SD) instead of high definition (HD). The set of bitrates, resolutions, and codecs is called the Adaptive Bitrate (ABR) ladder. Streaming content providers may tune their own ladder based on CDN costs, user device types, bandwidth in-region, and other factors.

    Components of a streaming protocol structure and an example
    Figure 2. Components of a streaming protocol structure (top) and an example HTTP Live Streaming (HLS) implementation (bottom).

    Terminology

    This section provides a glossary of useful terms for working with the Transcoder API.

    Ad break

    An ad break is a short advertisement that appears before or during media playback. The Transcoder API supports an ad break keyframe in the job configuration. The Transcoder API does not insert or play ads, or stop media playback; video player clients are responsible for handling the keyframe.

    Atom

    An atom is a foundational data structure for defining a video's metadata and location. A video can comprise a complex nested hierarchy of different types of atoms, including audio, edit, and text atoms.

    Audio atom

    An audio atom maps the audio from an elementary stream to an edit list.

    Codec type and profile

    When selecting the codec for a video stream, you specify the codec type, such as H.264 and the profile, such as the default high profile.

    Container

    A container is a wrapper that describes the relationship between the various components in a multiplexed stream, including media files and metadata. The Transcoder API supports the MP4, MPEG-DASH, and HLS container formats.

    Edit atom

    An edit atom defines the start and end offsets for the individual segments of a stream that you want to combine in an edit list.

    Edit list

    An edit list defines a sequence of edits as a timeline for the resulting file or manifest from a transcoding job.

    Elementary stream

    An elementary stream is an encoding of an input file, such as an audio, video, or caption text track. You must package elementary streams before mapping and sharing the stream to different output formats.

    Entropy encoding

    Entropy encoding is a form of lossless compression that the Transcoder API supports. When configuring jobs, you can specify either the Context-Adaptive Variable-Length Coding (CAVLC) or Context-Adaptive Binary Arithmetic Coding (CABAC) entropy coders.

    Job

    A job is the basic unit for managing work with the Transcoder API. When you submit a job to the Transcoder API, it is processed asynchronously in a geographic location known as a region. You can list and manage all jobs for a region.

    The lifecycle of a job includes three steps: prepare, transcode, and package.

    • Prepare
      • Download inputs from Cloud Storage
      • Analyze inputs
      • Validate inputs
    • Transcode
      • Run transcoding operations on inputs
    • Package
      • Stitch inputs
      • Multiplex inputs
      • Upload outputs to Cloud Storage

    A job has an updated state. The service reports errors using the error field.

    Job configuration

    A job configuration represents many of the various settings you can customize when creating and submitting a job to the Transcoder API. You can specify configuration settings such as edit lists and where to insert ad break tags in an output manifest. You can create reusable job configurations as job templates for use in a Google Cloud region.

    Job template

    By default, the Transcoder API applies a preset template called preset/web-hd for populating a job configuration. This job configuration produces the following output files:

    • manifest.m3u8: The primary playlist for an HLS media stream. This file contains references to playlists for the high definition (HD) variant of the output and the standard definition (SD) variant of the output.
      • media-hd.m3u8: Playlist for the high definition variant
        • media-hd0000000000.ts: High definition video segment file
      • media-sd.m3u8: Playlist for the standard definition variant
        • media-sd0000000000.ts: Standard definition video segment file
    • manifest.mpd: The playlist for an MPEG-DASH media stream. This file contains references to video-only and audio-only segment files.
      • audio-only0000000000.m4s: Audio-only segment file
      • video-only-hd0000000000.m4s: High definition video-only segment file
      • video-only-sd0000000000.m4s: Standard definition video-only segment file
    • sd.mp4: Standalone standard definition video file
    • hd.mp4: Standalone high definition video file

    You can create and manage your own custom job templates and specify them when creating jobs.

    Preprocessing

    Preprocessing is the stage of a job that takes place prior to the main transcoding.

    Preprocessing configuration

    A preprocessing configuration represents the settings you can apply to a video before the transcoding stage of a job. You can apply cropping or padding as part of this configuration.

    Rate control mode

    The rate control mode indicates whether to process a job using either the constant rate factor (CRF) or variable bitrate (VBR) modes. CRF ensures constant quality throughout the processed media assets. VBR optimizes the encoding process to reduce the file size of the processed media assets. For streaming use cases, select the VBR rate control mode. For archival purposes, select the CRF rate control mode.

    Manifest

    A manifest is a description of the available contents and metadata of an adaptive media stream to a client. In the Transcoder API, you can configure a job to output a manifest with a file name, a list of multiplexed streams, as well as the type of the manifest, which must be either HLS or MPEG-DASH.

    Text atom

    A text atom maps the text from an elementary stream to an edit list.

    Text stream

    A text stream encodes text data associated with a video, such as closed captions or subtitles.

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