Codecs vs. Containers: Understanding the Essentials

In the world of digital media, the terms "codec" and "container" are frequently used interchangeably. However, understanding the distinction between these two concepts is essential for anyone working with audio and video files. In this blog post, we'll explore the differences between audio and video codecs and the containers they reside in, as well as the pros and cons of the most common options available today.

Defining Codecs and Containers

A codec (short for "coder-decoder") is an algorithm or software that compresses and decompresses digital audio and video data. Codecs are essential for reducing the file size of multimedia content while maintaining an acceptable level of quality. Audio codecs, like MP3, AAC, or FLAC, focus solely on compressing audio data, while video codecs, such as H.264, H.265, or VP9, compress video data.

A container, sometimes referred to as a "wrapper," is a file format that encapsulates and stores one or more compressed audio, video, and metadata streams. Containers do not compress data themselves but provide a structure for holding and organizing the various streams. Popular container formats include MP4, MKV, AVI, and MOV.

First, let's look at the most common codecs in use today and their comparative strengths and weaknesses.

  1. H.264 (Video)

    Widely supported by various devices and platforms
    Provides good video quality at relatively low bitrates
    Efficient compression, which results in smaller file sizes

    Limited support for 4K and 8K resolutions compared to newer codecs
    Not as efficient as newer codecs like H.265, which offer better compression
  2. H.265 (Video)

    Improved compression efficiency compared to H.264, resulting in smaller file sizes
    Supports resolutions up to 8K
    Better suited for high dynamic range (HDR) content

    Requires more processing power for encoding and decoding
    Not as widely supported as H.264
  3. MP3 (Audio)

    Nearly universal compatibility with devices and platforms
    Small file sizes due to lossy compression

    Lacks support for high-resolution audio
    Limited to two audio channels
  4. AAC (Audio)

    Better audio quality at similar bitrates compared to MP3
    Widely supported by devices and platforms, including Apple devices
    Efficient compression, resulting in smaller file sizes
    Supports up to 48 full audio channels and 16 low frequency channels

    Not as universally supported as MP3
  5. AC3 (Audio)

    Also known as Dolby Digital, widely used in DVD and Blu-ray audio tracks
    Provides 5.1 surround sound support, enhancing the immersive listening experience
    Good audio quality at moderate bitrates

    Not as efficient in terms of compression as newer codecs like AAC or Opus
  6. Opus (Audio)

    Highly versatile, designed for both low-latency real-time communication and high-quality music streaming
    Excellent audio quality at low bitrates compared to other lossy codecs
    Open-source and royalty-free, making it accessible for various applications and platforms

    Not as widely supported on older devices and platforms compared to more established codecs like MP3 or AAC

The process of combining those separate raw audio and video streams into a single file that can be shared or transmitted is known as multiplexing. The audio and video streams are synchronized and then combined into a single data stream which we call a container. In addition to audio and video, containers can also accept streams for subtitles, chapters, tags, production information, and other metadata. Containers are recognizable as file name extensions like .mp4, .mov, .mkv, .avi, .webm, and others. Some containers can hold a practically unlimited number of streams, including multiple video and audio tracks. An example would be Harmy's Star Wars Despecialized, which has over 20 separate audio tracks with different mixes.

Let's examine the different types of containers and their different characteristics. Novices often make the mistake of saying things like "MP4 is better quality than AVI" or things of that nature when, in reality, the container has nothing to do with quality and everything to do with compatibility and extra features. Quality is determined by the codec, and the container is only the vehicle for it.

  1. MP4 (Container)

    Wide compatibility with devices and platforms
    Supports multiple audio and video codecs
    Can store metadata, such as subtitles and chapter markers

    Limited support for certain codecs
    Limited support for features like chapters and subtitles compared to MKV
  2. MKV (Container)

    Highly flexible, supporting a wide range of audio and video codecs
    Can store multiple audio and subtitle tracks
    Supports advanced features, such as chapters and menus

    Not as widely supported as MP4, especially on mobile devices
  3. AVI (Container)

    Widely recognized and supported by various devices and platforms
    Can accommodate a range of older audio and video codecs


    Limited support for advanced features, such as multiple subtitle or audio tracks
    Restricted to a single audio and video track

    Does not support modern video codecs like H.265
  4. MOV (Container)

    Developed by Apple and optimized for QuickTime and MacOS
    Supports a wide range of audio and video codecs
    Can store multiple audio and subtitle tracks, as well as metadata

    Less widely supported on non-Apple devices and platforms compared to MP4

Understanding the differences between audio and video codecs and the containers they reside in is crucial for anyone working with digital media. When choosing a codec and container, it's essential to consider factors like compatibility, file size, and quality requirements. By weighing the pros and cons of the most common options, you can make informed decisions that best suit your needs and ensure a smooth playback experience across a range of devices and platforms.

Alan Burns March 22, 2023
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