Audio, like video, is rarely uncompressed. The exception being CD-ROM audio. Audio can be very large is file size, so compression schemes have been made. The simplest is RLE, but this was designed to be a low level compression scheme that operated through sound card hardware. Sound can also be compressed with lossy compression. Like video, there are certain features that humans can not detect. In sound, most humans can not hear sounds below 16Hz or higher then 25KHz. This range of sound is included in uncompressed recordings, but is removed by almost all lossy audio compressions.

MP3
The most popular sound compression is MPEG Layer 3. This is the sound portion of the MPEG video compression format. This compresses by allocating a certain number of bytes for each group of sound samples. This is called its bitrate. What is does with those bits is assign them to the most important sound changes in the sample first, and then progressivily records the rest with the remaining bits. With low bitrates, most small sound changes are removed, leaving only the more prominent.

Video files are extremely large when uncompressed, so there have been numerous different video codecs designed. Of these, the MPEG series tend to offer the best quality / performance. Almost all video compression formats are lossy, to make sure files sizes stay low. In order to keep good compression while maintaining quality, some formats, such as MPEG-2 are very complex. MPEG-2 requires dedicated hardware for decompression on computers slower then 300MHz, and requires expensive dedicated hardware for real-time compression, on even most high-end workstations.

MPEG-1 Compression
This was the first video compression scheme designed by the Motion Pictures Expert Group. This was designed for all purpose video compression, and like JPEG compression, it is lossy and allows for quality scaling. It was designed to compress full broadcast quality video at rates of 3MB to 5MB. MPEG is more lossy then MPEG2, but it is also much quicker to compress and decompress. It is everything you would expect from a stardard designed for all around use.

MPEG-2 Compression
A lossy compression standard that is used to store audio-visual at a much higher quality than MPEG-1. MPEG-2 is capable of encoding at rates of 3Mbps to 10Mbps, and encoding full broadcast quality 720x486 resolutions with AC-3 audio.

MPEG-4 Compression
This is the latest codec which is specifically designed for the low bandwidth of the internet. It tries to balance between quality and file size, allowing movies to be streamed, instead of having to be completely downloaded beforehand. It can range between 10kbps to 10Mbps, but because it is optimized for low quality, it will not have the quality that MPEG-2 has even at its highest setting. For this codec, there have post processes which will smooth edges, remove small artifacts, and attempt to reconstruct what is lost in compression.

MJPEG Compression
This is a codec that compresses each frame individually as a series of JPEG images. This allows for quick editing, but doesn't allow for the 3 dimensional compression that other continuous compressions have.

Audio Video Interleave Compression
AVI is a standard made by Microsoft. AVI can use many codecs and also can save video feed in uncompressed form, called full frames. AVI video can range in colour depth, from 8-bit to 32-bit.

Lossless Versus Lossy | Data Compression | Image Compression | Audio Video Compression