What Is an Audio Codec?

An audio codec (encoder/decoder) is a software program that compresses and decompresses digital audio data. Audio codecs play a critical role in digital audio by enabling audio files to be efficiently stored and transmitted. Without audio codecs, digital audio files would be too large to download and store on devices. Audio codecs allow high-quality audio to be delivered through limited bandwidth channels by reducing the size of audio files. They are essential for applications like streaming music online, video conferencing, digital radio, VOIP, and more. This article will provide an overview of how audio codecs work and discuss the difference between lossy and lossless codecs, common codecs used today, and how to choose the right codec for your needs.

History

Audio codecs originated in the 1980s with the development of algorithms that compress audio files by selectively discarding audio data in ways that minimize perceptual loss in sound quality. Some key developments include:

In 1982, MUSICAM (Masking pattern adapted Universal Subband Integrated Coding And Multiplexing) technology was invented, forming the basis for MP3 and other popular codecs. It uses psychoacoustic models of human hearing to selectively “mask” unimportant sounds.

In 1988, the first MP3 encoder was created by the Fraunhofer Institute, based on MUSICAM. The MP3 format significantly reduced audio file size while maintaining reasonable quality.

In 1991, HQ Codec technology was developed by NTT’s Human Information Processing Research Laboratories as an improved audio compression method over MUSICAM. This was adopted into various subsequent commercial codecs.

In 1992, AT&T developed the AC-2 codec for surround sound, which was used for DVDs and broadcast television. The AC-3 codec followed in 1994.

In 1995, RealNetworks released the G2 codec for internet streaming media. Microsoft also released WMA the same year.

In 1999, MP3 became a de facto standard for digital music online. The iTunes Music Store popularized the AAC format starting in 2003.

In the early 2000s, lossless formats like FLAC and ALAC emerged, providing better quality for archival purposes.

Today, AAC and Opus are widely used for their quality and efficiency. Development continues on next-generation codecs like Ogg Opus and MPEG-H 3D Audio.

How Audio Codecs Work

Audio codecs work by compressing and decompressing audio signals through various methods of encoding and decoding. At a basic level, audio codecs take an analog audio signal, sample it at regular intervals, and convert the samples to digital values using analog-to-digital conversion. This process is called encoding. The codec then compresses the digital audio data by removing redundant and irrelevant information. Common compression techniques include perceptual coding, which removes audio frequencies that are less audible to the human ear, and entropy coding, which represents frequently repeated values with fewer bits. To decompress and play back the audio, the codec reverses the process through decoding. It reconstructs the digital values, converts them back to an analog signal using digital-to-analog conversion, and plays back the analog signal. The aim is to reduce the amount of data needed to represent the audio while retaining as much quality as possible. Lossy codecs accept some loss of quality for greater compression, while lossless codecs compress without data loss. Overall, codecs strike a balance between audio quality, file size, and processing power needed (Source).

Lossy vs Lossless

Lossy and lossless are the two main types of audio codecs. As their names imply, the key difference lies in whether they lose quality when encoding audio data or not.

Lossy codecs like MP3, AAC, Ogg Vorbis, etc. use “perceptual encoding” to selectively discard data that is considered beyond the auditory resolution ability of most listeners. This allows them to achieve much greater compression ratios compared to lossless, reducing file sizes by a factor of 10 or more. However, this loss of data also reduces audio quality. Lossy compression introduces compression artifacts audible at lower bitrates.

Lossless codecs like FLAC, ALAC, etc. retain the source audio perfectly. No data/quality is lost during encoding. However, lossless provides much lower compression ratios of around 2:1. This results in significantly larger file sizes compared to lossy formats. Lossless is used when higher quality is required, such as for archiving or editing audio.

The choice between lossy and lossless depends on the use case. For casual listening, portable devices, and streaming, the smaller sizes of lossy like AAC or Ogg Vorbis are preferred. Audiophiles and audio production professionals use lossless formats to preserve quality. Overall, lossy is more popular for consumer use while lossless finds professional audio applications.

Common Codecs

Some of the most widely used audio codecs include:

MP3 – MP3 (MPEG-1 Audio Layer III) is one of the most popular lossy formats for audio compression. It was developed in the early 1990s and allows for near CD-quality audio while significantly reducing the file size by removing sounds that are less audible to the human ear. MP3 is commonly used for digital music files, podcasts, audio books, and more. However, the compression does result in some loss of audio quality.

AAC – Advanced Audio Coding (AAC) is a lossy compression format that was designed to be the successor to MP3. It is able to achieve better sound quality than MP3 at similar bit rates. AAC is commonly supported across various devices and platforms including iTunes, YouTube, Nintendo Switch, PlayStation 4, and Android. It is also the default audio format for MP4 videos.

FLAC – FLAC (Free Lossless Audio Codec) is an open-source lossless audio format. Unlike lossy formats, it does not remove data to reduce file size, so the original audio quality is perfectly preserved. However, FLAC files are typically much larger than lossy formats. FLAC is suitable for archiving audio collections and is supported on many media players and streaming services.

WAV – Created by Microsoft and IBM, WAV (Waveform Audio File Format) is an uncompressed, lossless format for storing audio digitally. Due to containing uncompressed PCM audio, WAV files are very large but provide pristine audio quality. WAV is popular for storing studio-quality music but less suitable for distribution.

Ogg Vorbis – Ogg Vorbis is an open-source lossy audio format that competes with MP3 and AAC. It was created as a free and open alternative and is known for producing smaller files than MP3s at equivalent bit rates. However, device support is more limited compared to MP3 and AAC.

Uses

Audio codecs have a wide variety of uses and applications. One of the most common uses is compressing audio for music files such as MP3s or streaming services. MP3 and AAC are popular codecs for compressing music while retaining good audio quality (https://sisu.aalto.fi/student/courseunit/otm-07656e8b-7f4c-4b3f-b7f6-89918bf13404). Audio codecs are also crucial for video files and streaming. Video files contain both a video and audio track, and audio codecs like AAC compress the audio portion while keeping it in sync with the video. Services like YouTube and Netflix rely on audio codecs to deliver high quality audio with their video streams.

In voice over IP (VoIP) applications like phone calls or video conferencing, audio codecs compress the voice data into small packets for transmission over IP networks. Popular codecs for VoIP include G.711, G.729 and Opus. These codecs balance audio quality and low latency, which is important for natural sounding real-time communication. Audio codecs are also used in speech recognition and synthesis to code and transmit speech efficiently.

Within live sound and recording, audio codecs help convert analog signals to digital and back again. They allow audio to be transmitted between digital devices with minimal loss of quality. Overall, codecs are a crucial element enabling high quality, efficient audio across a wide range of modern applications.

Choosing a Codec

There are a few key factors to consider when selecting an audio codec for your needs:

Audio Quality: If you need very high-fidelity audio, lossless codecs like FLAC, ALAC, or WAV are best. If audio quality can be sacrificed somewhat for smaller file sizes, lossy codecs like MP3, AAC, or Ogg Vorbis are preferable. VBR (variable bitrate) lossy codecs can offer a good middle-ground.

Types of Audio: For music, AAC and MP3 are most common. For speech, Opus is optimized well. For system sounds and video game audio, ADPCM works well. Consider the main use case for choosing a codec.

Compatibility: Consider what devices or software need to support the codec. For wide compatibility, MP3 and AAC are good choices. For just PCs and Apple devices, FLAC or ALAC fit. Check compatibility needs before deciding.

Encoding/Decoding Speed: Some codecs encode and decode faster than others. Opus and Ogg Vorbis are very fast, while FLAC and ALAC are slower. This may matter for real-time applications.

Licensing: Codecs may have licensing restrictions. MP3 and AAC require licenses for commercial use. Opus, FLAC, Vorbis, and others are open source and license free.

Test a few codecs with your audio sources to see which gives the best results for your specific needs. There is no one-size-fits-all solution. Choosing the right codec requires balancing quality, compatibility, and use case.

Audio Quality

Audio codecs can have a big impact on the quality and fidelity of audio files. Lossy codecs like MP3, AAC, Ogg Vorbis, and WMA use compression algorithms to reduce audio file sizes, sometimes at the expense of quality. The more aggressive the compression, the more data is discarded and audio fidelity is reduced. Higher bitrates preserve more data and result in better quality, but also larger file sizes. Lossless codecs like FLAC and ALAC compress audio without discarding data, so the original quality is preserved. However, lossless files are much larger than lossy files at the same bitrate.

According to audio codec quality comparison tests, lossless codecs and high bitrate lossy codecs like 320 kbps MP3 provide excellent quality that is indistinguishable from the original source to most listeners (Audio compression techniques, digital audio, PCM – Educypedia). Lower bitrates introduce audible distortions,clicks, and loss of clarity in the sound. Codecs like Ogg Vorbis are more efficient than MP3, offering better quality at lower bitrates. So 128 kbps Ogg may sound as good as, or better than, 192 kbps MP3.

Ultimately, the ideal codec depends on the usage – streaming audio can benefit from more compression to reduce bandwidth, while music archiving and listening may demand lossless quality. Understanding the trade-off between audio fidelity, file size, and bitrate can help select the right codec for the desired quality and listening experience.

Encoding & Decoding

Encoding refers to the process of compressing and converting an audio signal into a digital audio format. The encoder applies an algorithm to analyze the raw audio data and identify redundant or irrelevant information that can be discarded or minimized. This allows the audio to be compressed into a smaller file size while attempting to maintain sound quality. Some common algorithms used in encoding include pulse-code modulation (PCM), MP3, AAC, Opus, and Vorbis. Lossy codecs like MP3 achieve compression by permanently removing details from the original audio that are considered beyond the auditory resolution ability of most listeners. Lossless codecs like FLAC compress the file without discarding information.

Decoding is the reverse process of reconstructing the compressed audio back into an uncompressed analog signal for playback. The decoder reads the compressed file and applies the compression algorithm in reverse to decompress the audio into a format the user can listen to. This converts the compressed digital audio data back into the original raw PCM audio waveforms. However, some loss of sound quality can occur with lossy codecs since the decoding cannot fully restore all of the detail that was discarded in the encoding process. The decoder must balance efficiency with preserving as much of the original audio fidelity as possible given the limitations of the codec and compression.[1]

Choosing the optimal codec involves balancing audio quality against file size. Lossless codecs provide the best sound quality but result in much larger files. Lossy codecs allow greater compression at the cost of potentially discarding some audio data.[2] The ideal codec for a given application depends on the acceptable tradeoff between audio fidelity, storage constraints, bandwidth limitations, device capabilities, and other factors.

[1] https://www.youtube.com/watch?v=VOxH90GXw7s

[2] https://gearspace.com/board/mastering-forum/1403114-speakers-reveal-distortion-2.html

The Future

Audio codec technology continues to evolve rapidly. Looking ahead, some potential developments include:

Lossless compression – While lossy codecs like MP3 aim to shrink file sizes by discarding audio data, lossless compression allows perfect recreation of the original file. This means comparable compression without quality loss. Formats like FLAC and Apple Lossless (ALAC) are growing in popularity.

New lossy codecs – Newer lossy formats like Opus and Ogg Opus seek to improve on MP3 and AAC, offering better quality at lower bitrates. These are open source and royalty free.

High resolution audio – With growing interest in high fidelity audio, formats like MQA aim to deliver studio quality sound in a compressed package by capturing more of the original analog waveform.

Multichannel audio – Surround sound formats are also evolving, with object-based approaches like Dolby Atmos allowing more precision in placing and moving audio elements in 3D space.

Hardware acceleration – Dedicated audio codec chips and GPU acceleration will allow more sophisticated real-time encoding and decoding, enabling immersive audio experiences.

Machine learning – AI and machine learning may help optimize compression algorithms and enable new approaches like generative audio codecs.

Overall, the future seems focused on more efficient compression, heightened realism, and more immersive sound.^{1 2}