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Codec and container, differences. Explanation

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Codec and container

Confusion between codec and container is very common. And even more if we add file extensions. This post aims to clarify these terms and show the relationship between them.

What is a codec?

The codec term comes from the concatenation of the initials of two words: encoder and decoder. Strictly speaking, a codec is a specification on how to encode a type of information and then be able to decode it. For example, a codec determines how video and audio information is compressed and decompressed. Remember that compression is a specific case of coding in which space is reduced, eliminating redundant information or low perceptual impact

What is a container?

Within the multimedia arena, a container is a specification of how different types of encrypted multimedia content are arranged within a file. These different contents are usually mainly video, audio and text.

Consider the case of a film in digital format. This movie is in an archive. When we open that file, a player runs and we can enjoy the movie. Notice that a movie has video, has audio, subtitles, information about its author, information about its duration, etc. For the convenience of the user, all these elements are grouped within a single file. The order in which they are saved in that file is the container specification.

Connection

The video information that is inside a container is usually compressed, that is, it usually follows the specification of a specific codec. The same goes for audio information. The confusion appears when there are containers and codecs with very similar or even the same names.

The most popular codecs

The most popular video codecs currently are Theora, VP8, MPEG-2, h.264, Xvid, and Divx. Regarding audio codecs: AAC, FLAC, MP3, Vorbis (ogg) and WMA are the most abundant.

The most used containers are AVI, Divx, Matroska, Flash video, MP4, MPEG, Ogg, Quicktime, MXF and WebM. Since containers are intrinsically linked to files, each one also specifies one or more possible file extensions:

AVI -> .avi
Divx -> .divx
Matroska -> .mkv, .mk3d, .mka, .mks
Flash video -> .flv, .f4v
MP4 -> .mp4
MPEG -> .mpg, .mpeg
Ogg -> .ogg
Quicktime -> .mov, .qt
MXF -> .mxf
WebM -> .webm

And to open a container and see its contents?

Containers are interpreted through a media player application, such as Windows Media Player, Quicktime (application with the same name as a container), VLC, Plex, RealPlayer or Winamp, etc.

A good option is the VLC player, since it offers compatibility with all formats under almost any platform and even allows you to play incomplete files (useful feature if you want to start playing audiovisual content that has not yet finished downloading, as long as it is be progressive). In addition, it is freely distributed.

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Codecs: How an audio or video codec works

To understand in a simple way what a codec is and how it works, the first thing we have to keep in mind is that the human voice is a continuous (analog) waveform, while the information that circulates through a data network is discrete ( digital). This means that if we want to send packets with data that contain the information needed to reproduce a voice fragment, we will have to digitize the voice beforehand.

The process of converting an analog signal to a digital signal is done through an element called an encoder. If the encoder is also capable of performing the opposite process, that is, moving from a digital signal to an analog signal even if only approximately, then we will have an encoder-decoder. An encoder-decoder is also called a ‘codec’.

Although it may seem that we would only be interested in selecting one or two codecs taking into account the voice quality (MOS, Mean Opinion Score) and bit-rate parameters that each codec needs, the truth is that we also have to face our network Look closely at the size of the packets that we are going to send since each voice packet is also made up of bits that are used for routing and error correction issues. As they add load, they saturate the network and make the entire system go slower. Do we reduce the number of packages we have to send making them larger? It is not always a good idea because the loss of a large package will have a greater impact on the output, even a cut in communication! What is recommended in these cases is to have a certain compromise between the number of packets that are sent to the network and the protection we want to have. The reference size should be about 20ms of voice per package.

How codecs work

The codecs reduce the information of the clips to facilitate and enable their publication and viewing through the Internet. There are two methods of compression, the so-called spatial and temporal compression.

In the first one, reduce the information by compressing the existing one inside each frame. Instead of describing the pixel-to-pixel image, pointing out for example the position and color of the pixels, the compression codec generalizes describing similar areas and their light and color characteristics. For example, instead of reproducing a blue sky pixel by pixel, it would be described as an area with similar light and color characteristics. In this sense, the less varied details an image presents, the codec can generalize and compress more easily. Creating videos with simple backgrounds facilitates compression and reduction, just as working with a tripod instead of a camera in hand means stabilizing the backgrounds and therefore facilitating subsequent compression.

The other compression method is temporary, where the information between consecutive frames is compared and only the details that vary are stored. The reference frames from which the differences are analyzed and the subsequent ones are supported are called keyframes and contain the complete image. On the contrary, the frames that reflect the differences are called “delta frames” and only contain the information of the areas that vary with respect to the previous images.

In general, videos that show few changes between frames are compressed better and this necessarily affects the realization. At present, both the television and the cinematographic realization tend to use the camera in motion. However, the compression of dynamic videos is more problematic than the cases of more static images.

Video Formats: Everything You Should Know

There is currently a wide variety of video formats available to store multimedia content in a file and play it on our devices. Depending on this format (and other multiple factors) the video will basically have a different video quality and a smaller or smaller file size.

However, if you are not up to date on these issues, you may get lost since we enter concepts such as formats, codecs, containers or related terminology such as AVI, MP3, MP4, DIVX, XVID, OGG, MKV, OGG , FLV, MPG, VP9, HEVC, AAC, H.264, AVC and much more.

Guide to convert video formats

In this brief but detailed guide, our objective will be to explain the difference between all these concepts, often confusing, clearly separating terms such as “container video format” and “video codec”, in addition to learning along the way a simple and practical way of Convert videos to different formats.

Outline of a container format

To begin, we must be clear about the difference between these two concepts that are often confused very often: the container format and the video codec. The first one is really the format of the file, the one that surrounds everything (audio, video, etc …), while the second is the format directly related to the video part:

Container format

But let’s see a description of each concept to make it even clearer:

Container format: A video file usually has a specific container format, such as AVI, MP4 or MKV, for example. As the name implies, a file with this format contains several elements (usually and at least, video and audio). However, it is possible that it only includes one of them, or for example multiple audio channels (to have several languages of the same file). Some container formats also allow you to include other elements such as images or integrated subtitles, without the need for external files.

Video codec: It is often confused with the video format, but it really is not the same, since it is related to the video content and not to the container format. It is not a mandatory element, since if a codec is not specified, the video is contained in RAW (uncompressed), taking up a lot of space (for example, 200MB about 10sec). For that reason, a codec is usually chosen to compress the video. Depending on the codec’s choice, better or worse quality will be achieved, and in turn, larger or smaller.

Video specification: Each video codec is usually based on a specific video specification (H.262, H.263, H.264 …), which is nothing more than a set of rules that define how it should behave. This can guide us to know if one codec is more or less recent than another.

Audio codec: In the same way as with the video, we have possible audio channels included in the file. They are also usually compressed with a certain codec to save space: MP3, OGG Vorbis, WMA …

Once this is clear, we will review the most popular container formats and the video codecs that they usually use (this does not imply that they cannot also be used in other container formats):

AVI format (Microsoft, 1992)

Perhaps, in all likelihood, one of the best known container formats is the AVI (Audio video interleave). It was developed by Microsoft in 1992 and is one of the most popular formats, perhaps because of its wide use on Windows platforms. Being one of the first container formats it is the most basic and only allows audio and video channels (although multiple of them can be included, if desired).

If we focus on video codecs, the AVI format accepts almost any VFW-compatible codec, but the most commonly used are the well-known DivX or XviD (which is a bifronte of the previous one).

DivX codec

DivX became popular in version 3.11 and was nothing more than a modified version of a Microsoft codec. Subsequently, and due to its success, they rewrote the code so as not to violate Microsoft patents. However, the DivX project became proprietary, generating a tremendous difference of ideas and causing the birth of a free brother codec: XviD.

Related to DivX was the Stage6 platform which was a kind of YouTube that allowed uploading high quality videos for the time. However, it did not have the necessary financing and failed economically.

XviD codec

XviD became popular as a result of the detriment DivX experienced, surpassing it due to its good quality, efficiency and its open source character. In addition, it also began to be supported on a large number of homemade DVD players, where perhaps, one of the first that stood out was the KISS DP-450.

Although there are currently technically better codecs, XviD is still updated and is still used due to its popularity, compression speed and good performance.

MKV format (Matroska, 2002)

MKV format (Matroska)
MKV format (Matroska)
Matroska, better known by MKV, the extension used in its archives, is an open-source container format that bases its name on the classic Russian Matrioskas dolls that fit inside each other. In addition to the MKV extension (used for video, audio and possible subtitles), there are other extensions used such as MKA (audio only), MKS (subtitles only) or MK3D (3-dimensional video).

Codec x264

This format usually uses some H.264 implementation (codec specification) as a video codec, such as x264. On the other hand, for the audio section you usually use the Vorbis audio codec.

WebM format (Google, 2010)

One of the most recent container formats is WebM (WebMovie), a project slightly based on Matroska. Initially it was developed by the company On2 Technologies, but in 2010 it was purchased by Google, which released the project as a source code, orienting it to the objective of using it with HTML5 as a free standard.

The format has excellent performance and, together with the VP9 codec, make it a very interesting option that seems to have a lot of future.

VP9 codec

The video codec used by WebM is known by VP8, developed by the company On2 Technologies (after a long evolution from TrueMotion S, used in games like Final Fantasy VII). Originally, WebM used VP8 video codec alongside Vorbis audio codec, however, it currently uses VP9 video codec alongside Opus audio codec.

URL | VP9

MPG format (ISO, 1993/1995)

Probably, MPEG-1 and MPEG-2 are one of the most complex concepts to understand. First of all, it must be made clear that these are specifications, and not a format in itself, so each specification is made up of several different concepts:

MPEG-1 / MPEG-2 specification
MPEG-1 / MPEG-2 specification
MPEG-1 Specification (ISO, 1993)

The first one, MPEG-1, is the one used to define the first container format (MPEG-PS) and the VCD storage format (VideoCD, based on H.261). Subsequently, some modifications compatible with VCD that used MPEG-2, such as SVCD (SuperVCD) or KVCD, would also appear.

As a curiosity, mention that contrary to what many people believe, the famous MP3 audio files are not MPEG-3, but MPEG-1 and are defined in the third part of the specification.

MPEG-2 Specification (ISO, 1996)

The second specification, MPEG-2, is the one used to define the container formats (MPEG-PS / MPEG-TS) and the new DVD storage format (H.262). Some MP3 features were also improved and AAC (Advanced Audio Coding), an advanced audio format with numerous improvements over MP3, was introduced.