Audio and Video File Formats: An Introduction

FILE FORMATS

There are several audio file formats in common use. There are fewer video file formats, but audio video file extensions can be confusing. This is just a brief description of what the more commonly used audio and video file formats and systems are.

AAC:Advanced Audio Coding This is the audio file format used by Apple for the iTunes Music Store, and it may appear with the M4A filename extension. It is better than MP3 for sound quality. It was developed as part of the MPEG4 group owned by Dolby (see below).

AU: This audio file format is the standard used by Java, Sun and Unix.

MPEG: Moving Pictures Expert Group There are a number of MPEG types now, described below.

MPEG-1: This is used in digital cameras and camcorders for small video clips. VHS quality playback can be expected from MPEG-1.

MPEG-2: Used for digital satellite TV, professional movie recording and recording of home DVD recordings. Provides provision for multi-channel surround sound recordings.

MPEG-3: MPEG-3 was propose as an entity, but eventually merged into MPEG-2.

MPEG-4: This is the newest MPEG system and is used for streaming internet content. It is also used in portable video recorders and for internet downloads. Required for DivX. It improves digital broadcasting and interactive graphics and multimedia.

MP3: Digital audio files, most commonly used to store and playback music. It compresses the files to about 10% of a normal audio file, and a normal music track will be about 5 -6 MB in size. MP3 stands for MPEG-1 Audio Layer 3, not MPEG-3 as many people think. A typical MP3 audio file is near CD quality.

OGG: An audio file format supporting a variety of codecs, the most popular of which is the audio codec Vorbis. However, MP3 files are much more broadly supported than Vorbis.

RA: Real Audio This format is designed for streaming audio over the Internet. It is a self-contained file format with all the audio information stored within the file itself.

WAV: The simplest of the audio file formats, developed by Microsoft and IBM, and built into Windows 95. It is an uncompressed audio file format with large file sizes (10 x MP3), and does not need further processing to play. The WAV file consists of three blocks of information: The RIFF block which identifies the file as a WAV file, The FORMAT block which identifies parameters such as sample rate and the DATA block which contains the actual data, or music sample.

WMA: Windows Media Audio A digital system invented by Microsoft, and is used in portable digital audio players. Using WMA, a file can be programmed so that it cannot be copied, and can be used to protect copyright.

WMF: Windows Media Format These are audio-video files comprising WMA and video codecs. They provide high quality and media security for streaming and download and play applications on computers.

WMV: Windows Media Video Used in the Windows media Player, this is used to stream and download and play audio and video content.

When dealing with audio and video file formats, you will sometimes notice the term 'codec'. A codec is simply short for encoder-decoder (or compressor - decompressor). A main function of a codec is to compress audio or video data streams so that transmission of digital audio samples and video frames can be speeded up and storage space reduced.

The objective of all codecs is to reduce the file size to a minimum while maintaining audio and video quality. A quick indication of the codec's place in the path of transmission and reception is:

Video device (e.g. camcorder) - video capture card - video digitized - codec (compresses digital info) - result (MPEG2, AVI, WMV etc) - codec (decompress) - video frames - display device.

Between the two codecs the compressed result is transferred to the display device transmitted, stored on file, etc). So to condense the flow even further, we could basically describe it as:

raw data - codec - transmit - codec - play

This is simplistic, but it shows where the codecs are used. Therefore, in order to play a movie, video or piece of music of a certain format, you need a codec in your computer to allow you to decompress the file and play it.

Here is some free software which checks what codec a video system uses, and what codec your system needs to play it:

Codec Tool.

The different video file formats are required to meet the requirements of various video devices. Similarly, audio file formats are designed to meet the needs of the specific delivery methods and storage and playback devices introduced by large corporations such as Microsoft, Sony and Apple. Thus, Sony invented the UMD (Universal Media Disk) for the PSP (Playstation Portable), and Apple introduced AAC (M4A) for its iPod.

Peter Nisbet is the author of many articles on audio and video file systems, their transmission, playback and conversion. More information is available on his websites http://www.legalandfree.com and http://www.online-free-movies.com

A Guide to Video File Formats

Identifying and converting digital video file formats can be confusing and frustrating because of the large number of file types used in the industry and the strange names used to identify them. Here is an easy-to-use guide to help you deal with the 40 or so types of formats that today's digital videos can be saved in.

It's not widely known, but digital video files actually have two layers. The outer layer is called the container. The container is also called a "wrapper," because it wraps together all the different subfiles that work together as a video. The inner layer of subfiles is called the codec. Codecs usually consist of a video file plus an audio file. When you play back the video on your computer, personal music player, cell phone, or DVD player, the codec files work together inside the container to give you a seamless viewing experience.

The names of the five most common container formats are probably familiar to you. They are AVI, MPEG-4, ASF (which is actually WMV), Flash, QuickTime, and 3GPP. There are many more obscure container types, but most of the time you'll be dealing with one of the "Big 5."

Usually, you only need to know what type of container file your video is stored in to tell you whether you can play it on your playback device. Occasionally, however, one of the codec files inside the container will be written in a format that your player software doesn't recognize. When this happens, your playback device will reject the video file even though it is supposed to play that type of file. If you run into this problem a lot, your best bet is to purchase video converter software that supports the highest number of container and codec types you can find, and use it to convert the files into a format that your player recognizes.

Here is a look at the "Big 5" container formats and their pros and cons:

* AVI - These files have the.avi extension. AVI is a very common format in video cameras, particularly inexpensive models such as the Flip. AVI works acceptably well for playing back on a personal computer because it's widely used, so most playback programs support it. AVI's biggest problems are needing a large file size to maintain quality, and wild variations in the codec formats inside the AVI container. It's very common for a player to reject an AVI file, even though it's supposed to play AVI files, because one of the codec files inside the AVI container is written in a code that the player doesn't recognize.

* MPEG-4 - These files have the.mpg or.mp4 extension. There are many types of MPEG files, but MPEG-4 is the most common. MPEG-4 is the best format for playing back files on a personal computer because it's such a common format and is supported by virtually all playback software programs. Although the quality is similar to that of AVI, the file size is much smaller than an AVI file. Codec problems are must less common in MPEG files than AVI files. MPEG is the closest thing to a universal video format on the market today.

* ASF (WMV) - ASF files have the.wmv extension, and are more commonly known as WMV files, even though WMV is actually a codec name, not a container name. WMV stands for Windows Media Video and is a proprietary file format owned by Microsoft. This means you will either need Windows Media Player installed on your computer, or you will need to convert the WMV file into a format that your player recognizes. Windows Media Player is available for Windows or Mac free of charge. ASF/WMV files are suitable for computer playback or for uploading to the internet.

* Flash - These files have the.flv extension and are used almost exclusively for uploading to the internet. If you've ever watched a YouTube video, you've seen Flash video in action. The file size is tiny, it streams over the internet into your browser quickly, and the quality is usually poor. The playback software for Flash is a proprietary program by Adobe that comes pre-installed on most browsers like Internet Explorer and Firefox. Codec problems can occur with Flash, but they are usually easy to fix by updating your browser to the latest version.

* QuickTime - These files have the.mov extension. They require the proprietary QuickTime player software developed by Apple, which is available for both Mac and Windows free of charge. The MPEG-4 container is a non-proprietary version of the QuickTime container, so if a codec works in MPEG-4, it generally will work in QuickTime too. QuickTime is a virtually trouble-free video format that works well for playback on your computer or streaming over the internet.

* 3GPP - These files have the.3gp extension and are formatted exclusively for playback on a handheld device like a cell phone, smart phone, or personal desk assistant. 3GPP works great when recorded on a handheld device and then played back. The video quality will vary depending on the quality of the device that recorded it, and can range from poor to excellent, similar to the differences in photos taken with cell phone cameras. QuickTime will play back 3GPP files on a computer, but very few other player programs will handle 3GPP files. Conversion software is usually needed to transfer videos between a computer and a handheld device in a playable format.

Dustin H. Pitcher is a freelance writer, technology specialist, and graphic designer with ten years of experience in print and multimedia. Visit his blog and learn more about the AVS4you Video Converter.

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