Advanced Audio Coding Advantages And Disadvantages

WHAT IS ADVANCED AUDIO CODING?

Advanced Audio Coding is an audio coding standard for lossy digital audio compression. Designed to be the successor of the MP3 format, AAC generally achieves higher sound quality than MP3 encoders at the same bit rate. AAC has been standardized by ISO and IEC as part of the MPEG-2 and MPEG-4 specifications

Advanced audio coding (AAC) is a technique used for compressing and encoding scheme digital audio files.

AAC technology can be used for coding audio files at medium to high bit rates. AAC is built to be the logical successor to MP3 (ISO/MPEG Audio Layer -3) and claims to offer better sound quality than its predecessor at the same bit rate.

AAC technique involves exploiting two primary coding strategies in order to minimize the amount of data required to impart high-quality digital audio. Signal components that are irrelevant are discarded. Redundancies in the coded audio signal are wiped out.

The encoding of digital audio files involves the following steps:

  1. Modified discrete cosine transform (MDCT) is used to convert the signal from time-domain to frequency-domain. Filter banks are used to convert an exact number of time samples into frequency samples.
  2. The frequency domain is quantized using a psychoacoustic model and is then encoded.
  3. Appropriate internal error correction codes are applied.
  4. The signal is stacked or transmitted.
  5. Luhn mod N algorithm is used for each frame to avoid sample corruption

AAC can sample frequencies ranging from 8Hz to 96kHz and up to 48 channels. It is also able to compress audio that contains streams of complex pulses and square waves better than MP3.

AAC is an international standard used by some major companies like Dolby Laboratories Inc., Sony Corp. and Nokia Corp. AAC is also used as a default audio codec for .m4v format by Apple in iTunes Store video files.

What is Advanced Audio Coding used for?

Advanced audio coding (AAC) is a technique used for compressing and encoding scheme digital audio files. AAC technology can be used for coding audio files at medium to high bit rates

What it does:

Advanced Audio Coding (AAC) is a multi-channel perceptual audio coder that provides excellent compression of music signals while achieving transparent quality relative to a stereo Compact Disc original for audio material when coded at 128 kb/s.

What it is for:

AAC is appropriate for applications involving storage or transmission of mono, stereo or multi-channel music or other audio signals where quality of the reconstructed audio is paramount.

Description of MPEG Advanced Audio Coding

Introduction

Advanced Audio Coding (AAC) is a wideband perceptual audio coding algorithm that provides state of the art levels of compression for audio signals.

AAC is primarily available as MPEG AAC Profile technology, which comprises the AAC Low Complexity technology.

Motivation

The MPEG-2 AAC standardization effort was focused on providing EBU broadcast quality compression performance for 5-channel music signals at a total bit rate of 320 kb/s. In order to achieve this level of performance, the work item was not constrained to be in any way backward compatible with MPEG-1 audio technology.

MPEG-4 AAC incorporates MPEG-2 AAC, with the addition of the Perceptual Noise Substitution (PNS) tool.

Overview of technology

AAC achieves coding gain primary through three strategies. First, it uses a high-resolution transform (a 1024-frequency-bins) to achieve redundancy removal. This is the invertible removal of information based on purely statistical properties of a signal. Second, it uses a continuously signal-adaptive model of the human auditory system to determine a threshold for the perception of quantization noise and thereby achieve irrelevancy reduction. This is the irretrievable removal of information based on the fact that it is not perceivable Third, entropy coding is used to match the actual entropy of the quantized values with the entropy of their representation in the bitstream. Additionally, AAC provided tools for the joint coding of stereo signals and other coding tools for special classes of signals.

The following figure shows a block diagram of the AAC encoder, in which those modules that provide the primary coding gain are highlighted.

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Target applications

AAC has seen considerable adoption by industry.  It has application in compression for PC-based and portable devices, compression for terrestrial digital audio broadcast, streaming of compressed media for both Internet and mobile telephone channels. Additionally if functions as a core coder for other MPEG technology (i.e. HE-AAC) that has application in both satellite-delivered digital audio broadcast and mobile telephony audio streaming.

Advanced Audio Coding AAC Benefits, Advantages

While by no means lossless, particularly at lower bit rates, AAC nonetheless, provides a virtually transparent encoding / decoding audio compression mechanism. Indeed, AAC facilitates rapid loading and playing of AAC compressed audio files very quickly.  This means that AAC is an excellent stop-gap measure for storing very decent quality recordings of your music library.  Until hard disks, flash memory, become cheap enough and eliminate the need to compress your audio files, AAC is the way to go, for the following reasons.

More Devices Understand AAC Today

Most smart phones like recent Nokia and Apple iPhone models, as well as the Windows, Android, and iOS-based tablet computers know how to play AAC and AAC+ audio files and streams these days.  The same is true of popular portable media players such as the Sony Walkman, Apple iPod, Creative Zen, and Samsung Galaxy Player models.  Fortunately, though the ubiquity of the previous (MP3) standard seriously slowed the progress from it into AAC during the first ten or fifteen years after they standardized AAC, current media players and computers are about as likely to offer AAC playing capabilities as they do MP3.

Smaller Encoded File Sizes

For a given sound quality level, AAC encoded files generally tend to be significantly smaller than MP3.  This means that you can store more songs on that iPod.

Standardization Achieved

A pretty mature compression system these days, the basic AAC standards layout was finalized in 1997.  The players, devices, and codecs that implement AAC have become quite reliable over the past decade plus, and the compression system has become virtually ubiquitous among popular media players, smart phones, tablets, laptops, and desktop computers.

Better Sounding than MP3

Particularly at lower bit rates (below 64 Kbps), such as often used on speech and old-time radio show files, AAC provides significantly greater fidelity than MP3.  Or, if less hard drive space consumed is more your thing than better fidelity, you can get the same MP3 fidelity you’re used to, but with a lower AAC bit rate.  In fact, they wrote the AAC standard to overcome many of the known limitations of MP3.

Advanced Audio Coding AAC Short Decode Times

AAC has been optimized to require very little time (under half a second in most devices), to begin playing an audio file, though the decoding may require slightly more CPU while the file plays.  Though it requires significantly more time to encode (to create the AAC file in the first place), these files start playing upon loading about as quickly as MP3 files.

Widely Available

Media player software, often free or very low cost, like iTunes, Winamp, and QuickTime, is capable of creating as well as playing AAC-encoded files on suitably provisioned computers.

Low Streaming Bandwidth

Not only does AAC produce smaller sized audio files for a given fidelity of sound, but it also cuts data rates in streaming audio applications such as Internet radio.  Thus, with all else equal, AAC saves data costs when streaming to smart phones and tablets where the user is charged for the gigabyte of data received.  For great sound with the least data usage, seek out those AAC audio streams.

Picture of the Sangean WFR-20 Wi-Fi Internet Radio and Network Music Player, Front View. This internet radio plays both AAC and MP3 streams and media files. Advanced Audio Coding AAC Pros and Cons.
Sangean WFR-20 Wi-Fi Internet Radio and Network Music Player, Front View. This internet radio plays both AAC and MP3 streams and media files.

AAC Less Lossy

AAC encoders and decoders, for a given bit rate, appear more aurally transparent than MP3; particularly for lower bit rate streams. However, even for 128 Kbps and higher bit rates, typical AAC encoders produce files that sound crisper and more full than what you can achieve with typical MP3 encoders.

Advanced Audio Coding AAC Better High Frequency Response

According to Wikipedia (see the links below), Advanced Audio Coding better “handles” audio frequencies of 16 KHz and higher.  We’ve found in our tests using audio spectrometers, that AAC decoders reproduce more of these frequencies, with greater loudness, crispness, and clarity than MP3; particularly at lower decoding bit rates.

Native Variable Bit rate (VBR) Support

How AAC implements VBR is part of the AAC standard.  VBR was less well-defined in the MP3 standard, and some dispute that MP3 officially supports VBR at all in the standard.  So it’s been reported that some players that were supposed to be able to play MP3 files, could not play some variations of MP3 VBR files.  Since how VBR is to be accomplished in AAC is clearly laid out, this problem is likely to happen less often among various AAC media players.

Wide Device Compatibility

The various “flavors” of AAC implementations generally tend to be compatible with one another, so long as the AAC standards version is the same.  That is, an AAC file encoded with the Apple QuickTime AAC encoder will play on virtually any other device  or app that supports AAC, such as Winamp, Windows Media Player, et al.

Large Range of Encoding Options

Bit rates (up to 320 Kbps).  Sampling rates (up to 96 KHz). Number of audio channels (up to 48). Encoding quality levels (minimal, good, better, and best).  All these choices highlight AAC’s many encoding options.

Advanced Audio Coding AAC  Problems, Concerns, Limitations

Takes Significant Time to Encode

A typical four-minute stereo WAV file can take up to ten seconds to convert to AAC.  So a batch of several thousand files, as we often convert as DJs, can require more than a whole day.

High CPU Utilization During Encoding

We do lots of batch WAV file conversions to AAC here, using either Winamp or Apple’s QuickTime player.  During conversion, QuickTime produces a sustained 100 percent CPU utilization throughout virtually the entire encoding time per file.  We’ve observed similar behavior in the Winamp encoder.

Advanced Audio Coding AAC Slightly Higher CPU Utilization During Decoding

Since the compression techniques are more complicated with AAC, generally speaking, it takes a bit more processor work to decompress the audio data while playing.  This can result in slightly faster battery draining on mobile devices while streaming AAC, than while playing similar bit rate MP3 streams.  However, this overhead is VERY small and thus, may not be noticeable with today’s ultra fast mobile processors and higher capacity batteries. Plus, a device with built-in native AAC hardware support may experience little or no increase of internal resources usage.

More Complicated File Organization

Typical AAC files utilize the MP4 container file organization standard, which contains size fields of the various regions within the file (boxes).  These length fields, if incorrectly calculated by the encoder or corrupted by subsequent file copying or media device failure, can render the entire file unplayable.  MP3 however, is more tolerant of these problems, because it relies much less on such size fields to find the audio data.

Less, but STILL Lossy

Though quite transparent, AAC is still not a completely lossless compression algorithm.  It still throws away some audio components from the original recording that it deems as “irrelevant.”  However, unless you have a top-grade sound card along with top grade headphones and headphone amplifier, you’ll be hard pressed to perceive these losses when comparing the original recordings to their AAC-encoded versions.

Advanced Audio Coding AAC Less Hardware Support

While AAC has become available on most of the popular media player platforms today, you still may encounter hardware / firmware / software, particularly in older devices, that do not support it.  Tip: If you’re planning on buying a new media player, and you want it to play AAC files, be sure that the model you’ve selected can actually play the files and properly display your file tags, BEFORE you buy it.  Fortunately, this problem either has, or is quickly morphing into a non issue due to the high degree of vendor support and embrace that the AAC standards have received in the 21st century so far.  So eventually, you will be able to buy a device knowing that it will indeed play AAC.

Not much Sound Improvement on Higher Bit Rate Audio Streams

For example, 256 Kbps (and higher) MP3 streams seem to sound and measure about as well as 256 Kbps (and higher) AAC streams.  So if you play higher bit rate streams, you might be better off sticking with MP3, for the moment anyhow. Given the greater hardware support for MP3 along with MP3’s lower CPU and battery utilization, MP3 may be a better choice at present.  But AAC improvements really shine on lower bit rate audio (below 192 Kbps).

File Tagging Not Always Compatible Across Devices

For example, you may tag your AAC files (M4a) in Winamp.  But the tags might not correctly display when you pull up those files on an IPod or Creative Zen.  We’ve encountered no problems with this personally, though others reported it several years ago.  Again however, we expect these anomalies to be corrected as universal device support and AAC ubiquity increase over time.

Lots of Different File Extensions

AAC can appear in files with the following extensions.  .3gp, .aac, .m4a, .m4p, and .mp4 among others.  But MP3 files feature just one extension: .mp3.  The plethora of AAC extension can create support questions and problems on the various media player devices.  Not all of these extensions are supported on all devices, even when the device itself contains the hardware and software to play AAC streams.  The file extension may be the only factor that prevents playing the file, even when the player could otherwise play the data.

Advanced Audio Coding AAC MP3 Still More ”Standard” than AAC

MP3 is still universally available and works well in every device we’ve ever tested that claims to support it.  AAC on the other hand, while more advanced, better sounding, and memory saving, may not always play on every device you’re likely to encounter.    Further, MP3 will likely remain a mainstay for decades to come, since there are simply too many MP3 files out there to convert practically, if anyone would want to convert them at all.  Why wouldn’t they?  Because with lossy compression systems, you slightly degrade the file’s audio fidelity when you convert it.   With each generation of conversions, the sound quality loss worsens.  So people may not wish to incur that loss in their audio files just to get them into AAC.

AAC Encoding from Lossless Original Files May Be Too Costly

Plus, even if folks still had the first-generation lossless files (WAV, FLAC, AIFF, et al), so they could avoid these cumulative losses of repeated conversion (second, third,… generations of conversion), they might not be able to devote the months of time and computer resources required to transcode thousands of recordings, originally converted to MP3, into AAC.  To get the original quality recording in order to make a first generation conversion to AAC, might require re-ripping their CDs or downloading them again from online music providers, and perhaps paying for some of them again as well.  Thus, some of these files will remain MP3 for the foreseeable future, and so, we expect virtually every media player device to support MP3 for the rest of this generation’s life and beyond.

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