Audio Codecs

Introduction

There are two categories of audio codec: lossy and lossless. Lossy codec will discard some information to achieve greater compression rate. Lossless codec will not discard any information to keep every detail of music. As the storage and bandwidth become cheaper and cheaper, lossless codec become more common in music industry.

There is a special category of lossy codec, called speech codec or voice codec. They use very low bit-rate to carry human voice, primarily used in telephony.

According to the creator, the audio codecs can be divided into 5 categories: ITU-T, ISO-MPEG, Microsoft, Apple, and others. Here is a brief description of commonly used audio codecs.

ITU-T

ITU-T is the Telecommunication Standardization Sector of ITU(International Telegraph Union). They published a series of voice codec.

• G.711 1972, 64kbps, 8kHz, uses μ-law or A-law to compress each sample into 8 bits.
• G.721, G.723, G.726, G.729...

ISO-MPEG

• MP3 (MPEG-1 Layer 3) is a widely used audio codec.
• AAC (Advanced Audio Codec) is a widely used audio codec.

Microsoft

• WMA (Windows Media Audio)

Apple

• ALAC (Apple Lossless Audio Codec) similar to FLAC.

Others

• GSM, a voice codec used in GSM mobilephone system.
• MPC (Musepack), open source nearly lossless codec.
• AC3 (Dolby Digital AC-3), audio codec used in cinema or DVD movies.
• DTS (Digital Theater Systems), similar as AC3, used in movies.
• FLAC (Free Lossless Audio Codec)
• Vorbis a free and open codec, replacement for MP3, usually used in ogg format.
• Opus IETF RFC 6716, is particularly suitable for interactive real-time applications over the Internet, usually used in WebM format.