AAC

AAC is the abbreviation for Advanced Audio Coding, a lossy digital audio data compression and encoding system developed by the MPEG group, as the basis of MPEG-4 and other Internet, wireless, and digital broadcast systems, and intended as the successor to MP3.

Claimed improvements over MP3 include more (and higher) sample frequencies, up to 48 channels, increased coding efficiency, improved high-frequency response over 16 kHz, and more. A wide variety of hardware and software devices support AAC, most notably Apple’s iTunes and iPods.

AAC has been standardized by ISO and IEC, as part of the MPEG-2 and MPEG-4 specifications. Part of AAC, HE-AAC (AAC+), is part of MPEG-4 Audio and also adopted into digital radio standards DAB+ and Digital Radio Mondiale, as well as mobile television standards DVB-H and ATSC-M/H.

Welcome to the world of digital audio, where various coding standards are employed to ensure that we experience high-quality sound on our devices. One such standard is the Advanced Audio Coding, more commonly known by its acronym, AAC. But what is AAC, exactly?

At its core, AAC is a method used for digital audio compression. It falls into a category referred to as “lossy,” meaning some audio information is lost during the compression process to reduce the file size, making it a preferred choice for streaming and storing music. However, despite this loss, AAC manages to retain impressive sound quality.

Designed as the successor to the widely recognized MP3 format, AAC was introduced to enhance sound quality while maintaining or even reducing file size. It achieves higher sound quality than MP3 encoders at the same bit rate, making it an ideal format for delivering high-quality audio in limited bandwidth or storage scenarios.

The AAC format is standardized by the ISO and IEC as part of the MPEG-2 and MPEG-4 specifications, making it a widely accepted and used standard in the digital audio industry. It’s commonly found across numerous platforms and devices, from your iPhone and other Apple devices to YouTube Music, digital radio standards, and even some gaming consoles.

This versatility, combined with superior sound quality, is why understanding AAC is crucial for anyone involved with digital audio, whether you’re a music producer, a tech-savvy musician, an audio engineer, or simply a music enthusiast looking to better understand the technology behind the sound.

In the sections that follow, we’ll delve into the workings, history, improvements, and applications of AAC, shedding light on why this format holds such a crucial position in the realm of digital audio. Welcome aboard, and let’s decode the fascinating world of Advanced Audio Coding together.

How does Advanced Audio Coding (AAC) work?

Before we can fully appreciate the role AAC plays in delivering high-quality audio, we need to delve into the mechanics of how it functions. Fear not, we’ll break this down in a way that makes this complex process easy to understand, regardless of whether you’re a seasoned audio engineer or a curious newcomer.

The central concept in Advanced Audio Coding – and, in fact, in all digital audio – is the process of audio compression. But what exactly does this mean?

The ABCs of Audio Compression

Imagine you have an uncompressed audio file. It’s vast, taking up a significant amount of storage space. Transmitting, streaming, or even simply storing this file is no easy task. That’s where audio compression comes into play. Compression, specifically ‘lossy’ compression (as used by AAC), reduces the size of this audio file by removing some of the audio information.

Lossy compression, however, is a delicate art. If not done right, it can drastically degrade the quality of the audio. The genius of AAC lies in its ability to compress audio in such a way that the reduction in file size doesn’t noticeably impact the listening experience. How? By eliminating parts of the audio that our ears are less likely to notice.

Under the Hood of AAC

The heart of AAC’s efficiency lies in its use of a tool known as the Modified Discrete Cosine Transform (MDCT). This algorithm, adopted exclusively by AAC, is more efficient than the hybrid algorithm used by MP3, which is part MDCT and part FFT (Fast Fourier Transform).

AAC’s approach focuses on eliminating redundancies and irrelevancies in the audio data. Redundancies are instances where the same data is repeated, while irrelevancies refer to sound data that is typically inaudible to the human ear. This is based on psychoacoustic models that reflect how we perceive sound, ensuring that while data is lost, the perceived audio quality remains high.

The efficiency of AAC is further exemplified when handling stereo audio. In a process known as ‘joint stereo,’ AAC can combine the similar parts of left and right channels, resulting in smaller file sizes without a perceivable decrease in audio quality.

Flexibility and Scalability

Another vital aspect of AAC is its ability to support a multitude of channels. AAC can include 48 full-bandwidth audio channels in one stream, plus additional channels for low-frequency effects and ‘coupling’ or dialogue channels. This capability makes AAC not only a good choice for music but also for surround sound in movies and TV shows.

Furthermore, AAC provides a high level of flexibility and scalability. Different encoding profiles can be combined, resulting in varying levels of compression efficiency to fit specific needs. It means AAC is adaptable, catering to a wide range of applications, from low bit rate applications like streaming audio to high-definition audio for professional use.

What are the origins of AAC?

To truly grasp the significance of Advanced Audio Coding (AAC), it’s crucial to understand its roots and the course of its development. The journey of AAC traces back to the quest for improved sound compression techniques, a path paved by pioneers in audio technology.

Prelude to AAC: DCT and MDCT

Our story begins in the early 1970s with the invention of the Discrete Cosine Transform (DCT). This groundbreaking technique for lossy compression was proposed by Nasir Ahmed in 1972, with subsequent development carried out by Ahmed along with T. Natarajan and K. R. Rao, culminating in their published results in 1974.

Fast forward to 1987, the Modified Discrete Cosine Transform (MDCT) was proposed by J.P. Princen, A.W. Johnson, and A.B. Bradley. The MDCT, an advancement of DCT, set the stage for what would eventually become the encoding mechanism used by AAC.

The Birth of AAC

The actual development of AAC was a collective effort by some of the biggest names in tech. A consortium of companies including Bell Labs, Fraunhofer IIS, Dolby Laboratories, Sony Corporation, and others, collaboratively worked on this project. Their work took a leap forward when Lars Liljeryd introduced a method that drastically reduced the information needed to store digitized audio – an essential development towards what AAC would become.

In April 1997, the Moving Picture Experts Group (MPEG), a working group of authorities set up by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC), officially declared AAC as an international standard. AAC was specified as Part 7 of the MPEG-2 standard and later included in Part 3 of the MPEG-4 standard.

Evolution of AAC

The initial standardization of AAC, known as MPEG-2 Part 7, outlined three profiles: Low-Complexity profile (AAC-LC), Main profile (AAC Main), and Scalable Sampling Rate profile (AAC-SSR). Over time, AAC was continually refined, with several enhancements introduced.

In 1999, AAC became a part of the MPEG-4 family of standards, known as MPEG-4 Part 3 or MPEG-4 Audio. This update brought several improvements, notably the addition of Audio Object Types, which enhanced interoperability with a diverse range of other audio formats. It also introduced Perceptual Noise Substitution (PNS), further enhancing compression efficiency.

AAC’s evolution didn’t stop there. Over the years, new audio object types and profiles were added to the standard, providing a richer and more flexible toolkit for audio encoding. This continuous refinement of AAC speaks to the format’s adaptability and the commitment of its developers to delivering superior audio quality.

Today, the current version of the AAC standard is defined in ISO/IEC 14496-3:2009. Also, AAC+ v2 is standardized by the European Telecommunications Standards Institute (ETSI) as TS 102005.

Why is AAC considered an improvement over MP3?

In the world of digital audio, AAC (Advanced Audio Coding) is often lauded as a successor to the popular MP3 (MPEG-1 Audio Layer 3) format. But why is this so? What makes AAC superior to its well-known predecessor? Let’s delve into the specific areas where AAC shines and transcends MP3.

Expanded Sample Rates and Channel Support

One of the first noticeable improvements AAC brings to the table is its support for a wider range of sample rates. While MP3 supports sample rates from 16 to 48 kHz, AAC broadens this range, accommodating sample rates from as low as 8 kHz to as high as 96 kHz. This range makes AAC highly versatile, suitable for various audio types and recording conditions.

In addition to sample rates, AAC also boasts superior channel support. MP3 supports up to two channels in MPEG-1 mode and up to 5.1 channels in MPEG-2 mode. In contrast, AAC supports up to 48 channels, providing ample space for immersive, multi-channel audio experiences such as those needed for high-quality surround sound.

Enhanced Coding Efficiency and Accuracy

When it comes to the technical nuts and bolts of audio compression, AAC exhibits several advantages over MP3. One significant factor lies in AAC’s usage of a pure Modified Discrete Cosine Transform (MDCT). This differs from MP3′s hybrid coding, which partly uses MDCT and partly FFT (Fast Fourier Transform). The pure MDCT used by AAC leads to higher efficiency and a simpler filter bank.

Further, AAC demonstrates improved coding efficiency for stationary signals and enhanced coding accuracy for transient signals. These improvements are achieved by utilizing larger block sizes for stationary signals (1024 or 960 samples compared to MP3’s 576 sample blocks) and smaller block sizes for transient signals (128 or 120 samples, allowing for more accurate coding than MP3’s 192 sample blocks).

Additional Modules for Increased Compression Efficiency

AAC also features additional modules or “tools” designed to increase compression efficiency. These include Temporal Noise Shaping (TNS), backwards prediction, and Perceptual Noise Substitution (PNS). Such tools can be combined in different ways to create different encoding profiles, adding a layer of customization and adaptability to the format.

Superior Handling of High Frequencies and More Flexible Joint Stereo

AAC handles audio frequencies above 16 kHz more efficiently than MP3, offering a more faithful representation of high-frequency sounds. Additionally, AAC provides a more flexible approach to joint stereo, employing different methods in different frequency ranges, leading to superior stereo imaging.

What are the key features of AAC?

Advanced Audio Coding (AAC) has emerged as the go-to digital audio compression format for many producers, engineers, and audiophiles, thanks to its robust set of features. Let’s dissect the standout attributes that make AAC a powerful tool in the music production and broadcasting realms.

Increased Sample Frequencies

At the heart of AAC’s superior audio quality lies its support for a broad range of sample rates, spanning from 8 kHz to 96 kHz. This capability enables AAC to deliver high-quality audio across various scenarios, from low bandwidth applications like voice-over IP (VoIP) to high-fidelity music streaming. This broad support also makes AAC a versatile format, suitable for a diverse range of audio content, whether it’s dialogue, music, or complex sound effects.

Multi-Channel Support

Multichannel audio is no longer a luxury but a necessity in our era of advanced home theater systems and immersive audio experiences. Recognizing this, AAC offers support for up to 48 channels. This expansive channel support provides audio engineers with a vast playground to create dynamic, immersive soundscapes that enhance the listener’s experience, whether they’re enjoying a blockbuster movie, a live concert recording, or an atmospheric video game.

Improved Coding Efficiency

AAC stands out for its impressive coding efficiency, especially in comparison to its predecessor, MP3. It employs a pure Modified Discrete Cosine Transform (MDCT), which simplifies the filter bank and results in higher coding efficiency. Furthermore, the AAC format leverages larger block sizes for stationary signals and smaller block sizes for transient signals, leading to superior coding efficiency and accuracy respectively.

Better High-Frequency Response

Unlike some older audio compression formats that struggle with high-frequency content, AAC excels in this domain. It exhibits superior handling of audio frequencies above 16 kHz, delivering a more faithful representation of these high-frequency sounds. This capability enhances the overall clarity and brightness of the audio, ensuring that nothing is lost in the digital compression process.

Flexible Joint Stereo and Additional Modules

AAC also stands out for its flexible approach to joint stereo encoding. By using different methods in different frequency ranges, it ensures optimal stereo imaging across the entire audio spectrum.

Additionally, AAC includes various modules, such as Temporal Noise Shaping (TNS), backward prediction, and Perceptual Noise Substitution (PNS), that are designed to increase compression efficiency. These tools can be combined to form various encoding profiles, adding a layer of versatility to the format.

How is AAC used in Music Production?

Advanced Audio Coding (AAC) holds an integral place in music production, streaming, and distribution, and for good reasons. Let’s delve into the relevance of AAC in music production and its compatibility with an array of devices and platforms.

AAC: A Favorite Among Music Producers

For music producers, the quality of audio is paramount. They continually seek audio formats that do justice to their carefully crafted soundscapes. In this regard, AAC stands out. Its superior sound quality, as compared to MP3, coupled with its higher efficiency and increased channel support, makes it a favorite among many in the music production industry.

The flexibility of AAC’s encoding also holds significant advantages. Producers can leverage AAC’s additional modules like Temporal Noise Shaping (TNS), backward prediction, and Perceptual Noise Substitution (PNS) to enhance compression efficiency. This allows for more complex encoding strategies and, consequently, more efficient compression.

AAC in Music Streaming

In the world of music streaming, AAC shines brightly. Several leading music streaming services, including Apple Music, YouTube, and Deezer, have embraced AAC for its superior audio quality and compression efficiency. The reason for this preference is twofold: first, AAC’s better sound quality at similar or even lower bitrates than MP3 means less bandwidth consumption and faster streaming. Second, the improved sound experience elevates the overall user experience, making the listener more likely to stay engaged and enjoy the music to its fullest.

Device Compatibility and AAC

While MP3 might have the upper hand in terms of universal hardware and software support, AAC isn’t far behind. Apple’s iTunes and devices like iPhone, iPod, and iPad, prefer AAC over MP3 due to its better sound quality and smaller file sizes. Android devices, while historically favoring MP3, also support AAC. Additionally, AAC has robust support across various media players, digital radios, and gaming consoles, making it a versatile format for wide-ranging applications.

Moreover, the adoption of AAC by Digital Audio Broadcasting (DAB) and Digital Radio Mondiale (DRM) as a standard audio codec further cements its importance in the digital audio world.

Why should Music Producers and Audio Engineers choose AAC?

Music producers and audio engineers are always on the lookout for technology that can improve the quality of their output, enhance the listener’s experience, and make their jobs easier. Advanced Audio Coding (AAC) offers several such benefits that make it a worthy choice for these professionals. Let’s delve into some of the compelling reasons why music industry professionals might prefer AAC.

Unrivaled Sound Quality

AAC stands out for its exceptional sound quality. Numerous blind tests conducted since the late 1990s have repeatedly demonstrated that AAC provides superior sound quality and transparency over MP3 at the same bitrate. This superior quality makes it possible for music producers and audio engineers to deliver a richer and more immersive listening experience to the audience. In the music industry, where sound quality is paramount, this advantage can’t be overstated.

Flexibility and Efficiency

AAC is not a one-size-fits-all type of audio codec. Instead, it offers increased flexibility in its design, which allows for more concurrent encoding strategies, resulting in more efficient compression. It is more efficient in its coding for stationary signals and more accurate for transient signals, thanks to its variable block sizes. These features can be vital in optimizing the quality of various types of sounds in a complex musical composition, making AAC a flexible tool for music producers and audio engineers.

Better High-frequency Handling

AAC shines when it comes to the handling of audio frequencies above 16 kHz. This trait makes it ideal for preserving the nuances of high-pitched instruments and vocals, enabling the production of richer, more textured sounds. For music producers and audio engineers striving to capture every detail of the music, AAC offers an invaluable tool.

Greater Channel Support

AAC supports up to 48 channels, offering far more than the 5.1 channels supported by MP3 in its MPEG-2 mode. This feature can open up new possibilities for music producers and audio engineers who want to experiment with multichannel recordings and deliver an immersive audio experience to listeners.

Industry Support

AAC has been embraced by several industry giants, including Apple and YouTube, which gives it a significant advantage. When a format is supported by influential industry players, it guarantees compatibility with a broad range of devices and platforms, making the content more accessible to listeners. This wide-scale acceptance of AAC can provide music producers and audio engineers with the assurance they need to adopt this format.

Where is AAC commonly used today?

Advanced Audio Coding (AAC) has permeated various sectors and technologies since its inception. With its advanced capabilities and high-quality output, AAC has found applications in an impressive array of devices, platforms, and services in the contemporary digital landscape. Here’s a glimpse of where you might encounter AAC today.

Streaming Services

Many of today’s most popular streaming platforms rely on AAC to deliver high-quality audio to users. Platforms such as Apple Music, YouTube, and Spotify use AAC as their primary or one of their primary audio formats. The efficiency and superior audio quality of AAC make it an excellent fit for streaming, where the balance between audio quality and data usage is critical.

Digital Radio and Television

AAC is also extensively used in digital radio and television broadcasts. Its ability to deliver high-quality audio at low bitrates makes it an attractive choice for broadcasters. Digital Audio Broadcasting (DAB) commonly uses AAC, and the format is also found in Digital Video Broadcasting (DAB) and other digital broadcasting standards.

Video Games

Video game developers are always in search of immersive and high-quality audio to enhance the gaming experience. AAC, with its high audio quality and efficiency, serves this purpose well. Several video game systems, including Sony’s PlayStation 3 and Microsoft’s Xbox 360, use AAC for in-game audio.

Devices and Software

Many consumer electronics devices, from smartphones and portable music players to smart TVs and home theatre systems, support AAC. Apple’s suite of products, including iPhones, iPads, and iTunes, uses AAC as the default audio format due to its superior performance and sound quality. In addition, software like Adobe Flash and Google’s WebM project also support AAC.

Podcasts and Audiobooks

AAC’s excellent sound quality and efficient compression make it a top choice for podcasts and audiobooks. Podcasters and audiobook producers prefer AAC as it allows them to provide high-quality audio while minimizing file size, essential for listeners who download or stream this content.

Telecommunication

In the telecommunication sector, AAC finds its usage in VoIP (Voice over Internet Protocol) services, where it provides clear, high-quality voice transmission even at low bitrates. The AAC-LD (Low Delay) variant is specially designed for these applications.

Frequently Asked Questions about AAC

As we delve into the world of Advanced Audio Coding (AAC), you might find yourself curious about certain aspects of this audio format. This FAQs section is designed to shed light on the most common questions surrounding AAC. Whether you’re an audio engineer, music producer, or just someone with a penchant for understanding the technicalities behind your favorite tunes, you’ll find these answers beneficial.