AUDIO and VIDEO FILE FORMATS

Claudia Rohwetter
U Bielefeld
7 October 1997

INTRODUCTION

When dealing with multimedia in the WWW it is necessary to know at least a little something about audio and video formats used in the WWW. For this purpose the more frequently used ones (having a glimpse at e.g. CNN or MTV) shall be listed below.
An overview concerning Plug-Ins for audio/video systems is given in brief, audio/video tools for Unix Workstations are also mentioned.

GENERAL FACTS

Audio and video information is very complex indeed, and that is why audio and video files always contain a lot of data and thus need quite an amount of space when stored.
Attempting to solve this problem some file formats work with compressed data, to prevent files from becoming too large. The critical point is that compressing data for storing usually leads to a loss of quality in playing .

AUDIO BASICS

An audio file is a collection of digitized sound data which is characterized by the following three factors.

sampling rate : Like movies, an audio file is recorded in a discrete manner. At a fixed time interval, the recording machine captures the sound data which is called a sample. The number of samples that are recorded per second is called the sampling rate. The higher the sampling rate, the higher the sound quality that can be achieved.

sampling bits : Each sample is represented by digitized data which occupies certain bits.

channels : Sound data can be recorded or played on more than one channel. Mono sound data use one channel. Stereo sound data use two channels.

AUDIO FILE FORMATS

AIFF
AIFF is an audio format developed by Apple for high quality sound and musical instrument information. Players for AIFF sound files can be easily found on PCs and MACs. For Unix workstations, xplay on the Sun SPARC can be used or soundfiler on the SGI. AIFF files have the extension *.aif or *.aiff .

WAVE
This file format follows the RIFF (Resource Information File Format) specification. It was developed by IBM and Microsoft as a counterpart of AIFF on PCs. WAVE is the standard sound format for Windows 3.1 and it contains pure uncompressed sound samples. Within this audio-system the original sounds, including speech, are digitized. This is done with a rate from 8000 to 44000 times per second. The extension for WAVE files is *.wav .

MPEG
MPEG is an international standard multimedia file format, which uses a compression scheme for encoding and thus MPEG audio files are considerably smaller than other audio files. This way of encoding achieves a good compression but leads to a slight loss of quality. MPEG files can be found (up to now) in three different versions, called layers 1-3. Layer 1 files have the extension *.mpeg , layer 2 files *.mp2 and layer 3 files *.m3u or *.mp3 . Layer 3 is the latest version and is said to be the best developed audio file format (e.g.streaming-live). The three formats can be played with the latest player, which needs a pentium system for sensible use.

MIDI
This audio system takes the length of notes as the base for data. These data are converted into sounds with the help of a sound card (stereo or mono). The advantage is that even long pieces of music can be stored with a small amount of data, the disadvantage is that the playback of one sample sounds different on different sound cards. Furthermore the transmission of singing and speech is not possible. MIDI files (extension *.mid) are usually produced with a midi-keyboard and standard sound cards.

Real-Audio
Real-Audio was developed by Progressive Networks . It works with compressed audio-data, files having the extension *.ra . These files are not stored on the users computer before playing, but are played `realtime`. Stereo and live transmission is possible. Necessarily there must be a link to the data source, this means the server must support Real-Audio.

SUN Audio
The SUN Audio system also uses compressed files (extension *.au). It was developed by SUN and is used with JAVA- or SUN- Applets.

MOD
A MOD file is a bank of sample data and a description of how to play these samples in a certain order, pitch and distortion on four channels. Although the samples (file extension *.mod) are called instruments any sound can be a sample. This is a big advantage in comparison to e.g. MIDI files. Compared to WAVE files, MOD files occupy much less space than WAVE files do. The enormous drawback of the MOD format is that its sound data can be stored only with 8-bit resolution and the maximum possible sampling rate is about 32 KHZ, which makes high quality sound impossible.

VIDEO FILE FORMATS

avi
Video/Audio format for Windows, where a file is downloaded first and then displayed. Usually avi files are recorded with an analog video camera and digitized afterwards.

VXtreme
Highly developed video format, used by e.g. CNN for video technique in the WWW.

QuickTime
Video format developed by Apple. The files are downloaded first and then displayed. QuickTime is the standard video format for MTV in the WWW.

MPEG
MPEG files contain highly compressed video data and should be played with a MPEG video card (hardware), although there are MPEG files available without using those MPEG video cards.

Real Video
Real Video is based on the RealAudioPlayer. Although it has the advantage of being a streaming-live video format (video and sound is played during the process of downloading already), it is only recommendable for high-speed WWW connections. Otherwise the quality of the displayed video is low and the sound often interrupted.

vdo
Vdo is another streaming-video format. Due to this the possibility of a live-transmission is given and thus vdo is used for e.g. news in the WWW.

INLINE PLUG-INS FOR VIDEO/AUDIO (Netscape)

QuickTime Plug-In by Apple Computer,Inc.
A Plug-In for using QuickTime animation, music, MIDI, audio, video and VR panoramas and objects directly in a Web page. A `fast-start`feature allows to display the content while downloading.
Available for: MacIntosh 68K, Power Mac, Windows 3.x, Windows 95, Windows NT

Beatnik by Headspace
This Plug-In manages the playback of e.g. RMF (Rich Music Format), MIDI, MOD, WAV, AIFF, AU files (within Web browsers). It has high-fidelity sound quality comparable to high-end sound cards and sounds the same across multiple platforms. Beatnik can be used for interactive music (play after mouse-click etc.) due to a comprehensive set of JavaScript functions.
Available for: MacIntosh Power Mac, Windows 95, Windows NT

CineWeb by Digigami
A Plug-In for standard movie and audio files, e.g. avi, mov, mpg, wav, mid, mp2
Available for: Windows 3.x, Windows 95, Windows NT

UMP by Umpire
UMP is a Unix Plug-In for Netscape Navigator that uses Timidity to play MIDI files on many popular Unix platforms.
Available for: IRIX, SUN OS, HP-UX, OS F1, LINUX

VDOLive by Vdonet
VDOLive compresses video images without compromising quality on the receiving end. The speed of the connection determines the frame delivery rate. Available for: MacIntosh Power Mac, Windows 3.x, Windows 95, Windows NT

ViewMovie QuickTime by Ivon Cavero Belaunde
ViewMovie is a Netscape Navigator 2.0 Plug-In that supports viewing and embedding QuickTime movies in Web pages. Available for: MacIntosh 68K

VivoActivePlayer by Vivo Software
This is a streaming video Plug-In for Netscape Navigator. It is a simple and fast way to get video clips on a Web page. Video data for Windows avi files are compressed up to 250:1, the resulting VIV files can be viewed with the VivoActivePlayer as they download. VivoActive Videos are transmitted using HTTP. Available for: MacIntosh Power Mac, Windows 3.x, Windows 95, Windows NT

Web Theater Client by Vxtreme
The Web Theater Client is a streaming video Plug-In for Netscape Navigator. It includes VCR-controls that allows viewers to fast-forward, stop, pause, skip or jump backwards in the video.
Available for: MacIntosh Power Mac, Windows 3.x, Windows 95, Windows NT, SUN OS

07.10.1997 - Claudia Rohwetter Uni Bielefeld - claudia@spectrum.uni-bielefeld.de