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AAC: Lossy Encoding Is Getting Better – AAC Format Summary

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AAC: Lossy Encoding Is Getting Better – AAC Format Summary

AAC Music File Format

At the time of writing this article, the MP3 codec is over 23 years old. So as not to repeat myself with the article (its most recent version), which already describes the OGG Vorbis codecs (and again hello to the Xiph organization, this is also its development), MPC (Musepack), WMA (Windows Media Audio) and AAC, I will briefly describe the format here. AAC in terms of technologies that until recently were at the forefront of lossy coding.

In my humble opinion, AAC (Advanced Audio Codec) is one of the most advanced formats in the field of data encoding. I will describe the main features of this format, starting with the popular profiles that can be represented by a matryoshka (see the figure below):

– Low Complexity Advanced Audio Coding (LC-AAC)

Low decoding complexity is great for implementing a hardware codec; The hardware requirements for CPU and RAM are also low, which has gained a lot of popularity for this profile. It encodes the 96 kbps signal efficiently.

– High Efficiency Advanced Audio Coding (HE-AAC).

The HE-AAC profile is an extension of LC-AAC and is complemented by patented SBR (Spectral Band Replication, thick – “spectral repetition”) technology. It is spectral repetition technology that allows you to “preserve” high frequencies by encoding at low bit rates.

Why is “save” in quotes? Because the king is not real: the codec leaves room for additional information that is used by the codec synthesizer to restore the high frequencies, but since these frequencies are synthesized, that is, they are recreated by the codec, they are, in fact, a rough copy of the high frequencies that existed in the original file. … In practice, a signal encoded at 48 kbps will sound, for example, as mp3 at 98 kbps if it is supported by the decoder; otherwise, the file will simply be played without restoring the high frequencies and its bit rate will correspond to its mp3-like quality.

– High Efficiency Advanced Audio Coding Version 2 (HE-AACv2)

This profile is relatively young (described in 2006), it was created for a more efficient audio coding in low bandwidth conditions.
The second version of the profile is an extension of the first profile, the changes are in the addition of PS (Parametric Stereo) technology. The principle is somewhat similar to SBR technology: the codec also makes room for recovery information from the stereo base, sacrificing precision.

The operating conditions for this profile are the same as for the HE-AAC described above; The lack of profile support from the decoder will make the recording sound in mono.

– AAC-LD (advanced audio coding – low delay)

The AAC-LD profile has advanced coding algorithms to reduce delays (up to 20 ms);

– AAC-ELD (Advanced Audio Coding – Enhanced Low Delay)

This profile, which inherits all the capabilities of HE-AACv2 (analogous SBR and PS technologies are used, but designed for low latencies);

– AAC main profile

This profile was introduced as MPEG-2 AAC or HC-AAC (High Complexity Advanced Audio Coding). Not compatible with LC-AAC;

– AAC-LTP (Advanced Audio Coding – Long Term Prediction)

This profile is more complex and resource intensive (but also of higher quality) than all the others. It is also not compatible with LC-AAC.

That’s all I wanted to write about this codec. I put the greatest emphasis on the technologies that are used in various AAC profiles (which, by the way, generate a lot of abbreviations: AAC, LC-AAC, eAAC +, aacPlus, HE-AAC, etc.), as I will compare them with the from Opus, but the codec does its job: it is widely used in Internet radio, as well as in digital radio transmission technologies: DRM (Digital Radio Mondiale) and DAB (Digital Audio Broadcasting) (you can see these technologies here), YouTube , as an audio track for many videos in mp4, mkv, etc.

2. Introduction to Opus: description of the format

On December 21, 2017, Xiph announced the beta version of the Opus audio codec version 1.3. I will not go into important matters when I describe this codec, since such information is freely available (for example, here, here, and for those who know English, here and here). The release notes for this beta version can be found here. Here I will point out that this codec is an excellent candidate to replace other codecs. It has many advantages:

bit rate from 6 to 510 Kbit / s;
sampling frequency from 8 to 48 KHz;
support for constant bitrates (CBR) and variables (VBR);
support for narrowband and wideband audio;
support for voice and music;
support for stereo and mono;

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How AAC works

AAC Music File Format

Like MP3, AAC is a lossy music storage format. As in the case of the MP3 format, AAC encoding algorithms cut the highest and lowest frequencies, and also actively use the characteristics of a person’s hearing to remove information about sounds that a person cannot easily distinguish when listening to music. . As a result of compression, AAC files do not sound identical to CD-quality files, but in general the algorithms compress the music efficiently enough that most people overlook the fact that it has been deleted. resulting sound information.

AAC Audio Format

Like MP3, the quality of an AAC file is measured by its bit rate. Common AAC bit rates include 128 kbps, 192 kbps, and 256 kbps.

Without going into technical details, here are the reasons why AAC files sound better than MP3s:

AAC encoding uses samples with a wider frequency coverage from 8 to 96 kHz; mp3 encoding uses samples with coverage from 16 to 48 kHz
Up to 48 channels can be encoded, while MP3 encodes 2 channels in MPEG-1 mode and up to 5.1 channels in MPEG-2 mode
Arbitrary bit rates and variable frame lengths are used

Higher algorithm efficiency and simpler filter banks (instead of hybrid MP3 encoding, AAC uses pure MDCT method)
Greater precision is provided when encoding AAC sound using smaller sample sizes, which are equal to 128 or 120 units, while 192 units are used for mp3 encoding. In this case, the smaller sample size allows you to code more precisely, since the original sound wave is an analog signal, and for a more accurate description, it needs to be measured as often as possible. Therefore, a smaller sample size is better because it allows you to more accurately describe the analog nature of a sound wave in digital form.

Sound frequencies above 16 kHz are processed more efficiently. Let me remind you that in mp3 format these frequencies are simply erased
More progressive algorithms are used in stereo binding mode. Let me remind you that stereo stitching allows you not to store both channels separately, as in CD-quality recording. The common signal for both channels is stored and only the difference between the channels is stored separately from each other.
Additional modules were added to improve compression efficiency: TNS, Inverse Prediction, PNS and others
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How AAC works with iTunes

Apple adopted AAC as its primary audio format. All songs stored in iTunes Store and all songs streamed or downloaded from Apple Music are encoded in AAC format. All AAC files in the Apple ecosystem are encoded at 256 kbps.

Audio file format AAC

AAC files are designed to replace MP3 files. Lossy compression gives you better sound at the same bit rates. AAC files are standardized by ISO / IEC as part of the MPEG-2 and MPEG-4 file families (originally they were members of the MPEG-2 Part 7 filegroup). AAS files contain a higher number of sample rates (compared to MP3) and up to 48 channels. Significantly improved encoding efficiency, including an increased number of filter sets. Enhanced transient signal coding accuracy. Like MP3 files, AAS files suppress sound at frequencies that a person cannot hear. This way you can reduce the file size. Compared to MP3, AAS files are much smaller.

AAC Format

Technical details of AAC files

MPEG-2 Part 7 files are represented by three types of files: Low Complexity AAC-LC, Main Type (AAC Main), and Variable Rate Files (AAC-SSR). AAS files allow temporary noise generation, uneven sampling, and repeated generation of the bitstream format (for 16 stereo channels, 16 mono channels, 16 low-frequency channels, and 16 channels for single bitstream annotation). In 1999, the MPEG-2 Part-7 format was incorporated into the MPEG-4 Part-3 format, allowing the introduction of audio object types as well as constant noise replacement technology. The AAC format is currently described in ISO / IEC 14496-3. Lossy compression uses sound masking to remove unnecessary information while maintaining quality.

Aac format

Additional information about the AAC format

Until now, the AAC format has not yet achieved mass distribution in audio media. However, in a number of parameters it exceeds all existing types of audio compression, which means that it deserves our attention.

What is this?

Let’s start with the definition: AAC is the proprietary (proprietary) option for compressing an audio file. At the same time, the quality loss when encoding is less than with MP3 at the same bit rate. Furthermore, the AAC format is a wideband algorithm for encoding audio that uses two fundamental encoding principles to greatly reduce the amount of data required to transmit high-quality digital audio. This solution is recognized as one of the highest quality implemented using lossy compression technology. The format is compatible with most modern devices, including laptops. It should be noted that ringtones in AAC format can be purchased from the iTunes store and that this store presents music that is only compressed with this solution. It should also be said that the AAC format was originally created as a successor to MP3, which can offer improved encoding quality. The solution was published in 1997 as the new seventh part of the MPEG-2 family.

All the details

Interestingly, the AAC format has a sample rate in the range of 8 to 96 kHz, as well as the number of channels in the range of 1 to 48. MP3 uses a set of hybrid filters. AAC, in turn, refers to the modified discrete cosine transform with an expanded “window” size that reaches 2048 points.

Therefore, compared to MP3, AAC is much better suited for encoding audio with complex pulse stream as well as square wave signals. The format was given the ability to dynamically change MDCT block lengths within 2048-256 points. In the case of a short or simple movement, a small 256-point “window” is used to achieve better resolution. In this case, a default 2048-point “window” is used to maximize encoding efficiency. AAC has several advantages over regular MP3. Noteworthy is the implementation of a large number of audio channels (up to 48), significant coding efficiency at constant and variable bit rates, and sample rates from 8 Hz to 96 kHz (for MP3 from 8 Hz to 48 kHz). and a more flexible special mode called Joint Stereo. The solution is “AAS +”, a codec designed for a low bit rate. It is a combination of SBR and AAC

Differences Aac plus vs MP3

When deciding which compression format to choose to take full advantage of own resources such as those of the listener in streaming broadcasts, one of these two alternatives is generally considered.

MP3 or MPEG-1 Audio Layer 3:

It is a standard compressed digital audio format with loss, the loss of information from the mp3 format is not audible by the human ear, therefore we will not distinguish the difference between an uncompressed audio file and an mp3 file.

In addition an mp3 file manages to reduce the size of the sound file without influencing its quality, approximately 1 minute of audio in mp3 format occupies 1 MB with a quality almost equal to the quality of Cd.

These advantages have achieved that the mp3 format can be played in almost all audio players, which is the format par excellence for the exchange through the internet, one of the best options at this time to store music with good quality, and also The audio format that is most used in portable players is a standard and therefore compatibility with all media is guaranteed.

The mp3 audio format allows you to select the quality of the audio that we are going to compress, the quality of cd would be equivalent to 128 Kbps (Bit rate), but we can select the compression between 8 Kbps and 320 Kbps taking into account that the higher the transmission of data (Kbps), the greater the use of broadband resources for both the broadcast and the reception by the listener.

AACplus: MP4 / aacPlus (HE-AAC) Enconder v 1.31; Stereo
As in the mp3, in the aacplus there is a solution for the little width for the channels. This is what is known as “Parametric Stereo” and corresponds to what is known as HE-AAC v2. Unlike what happens in the case of mp3, it is not convenient to use this option in high bitrates, in this case over 48kbps.

Considerations:

Quality assessments are expressed in percentages, with 100% being the overall quality of the original file.
The valuations are merely estimates but are intended to faithfully reflect the differences in quality between the different formats to the different bitrates.
The best alternatives for each format have been marked with red, according to the relationship of quality and consumption of own resources when emitting and resources of the listeners to correctly receive the audio.
Conclusions:

The first and most important regarding the mp3 is that, even in its highest quality, it is already obsolete. In any bitrate it is surpassed by the others so it will always be preferable to perform compression to any other of the subsequent formats. Ogg Vorbis and mpc are better quality formats that far exceed the mp3 but we have not included because they are not widely used. There are also several other formats that offer better compression.
For the mp3 the best bitrate is at 128kbps or 192 kbps, with a minimal difference in quality. Above that the difference is imperceptible and inferior to it is widely discernible.
At 72/74% the AAC PLUS format beats its competitors widely in the 48 / 64kbps being the best choice in the range. It will depend on the original file, as well as the greater or lesser preponderance of the high, medium and low. The decrease in fidelity is easily noticeable but it is not too much and many people will have no problem accepting it.
Under 128kbps, the quality of the mp3 drops considerably, while in the AAC PLUS format the decrease is not as pronounced. In the latter, it can be seen that between 128 and 48kbps the decrease in quality is minimal, with a difference of only 7%. Therefore, it is not worth compressing at 128kbps or at 96kbps having the option of doing so at 48 or 64 with a minimal difference in quality.

Advantages of AAC with respect to MP3

The AAC format supports multichannel sound which is an appropriate audio codec for the surround sound of 6 or more speakers, although it is currently not the most supported codec for it. However, MP3 only allows, in its base format, stereo sound.
It can reach a sampling frequency of 96khz, used in recording studios. MP3 is designed for playback only and to reach up to 48khz.
The sound quality is considerably superior to the same birate. This means that it is possible to encode an audio file in AAC with a bitrate lower than the equivalent in MP3 while maintaining the same sound quality. An audio file in HE-AAC encoded at 48kbps can offer a quality similar to an MP3 encoded at 128kbps.
Even at very low bitrates (32kbps or less) the sound suffers little loss compared to MP3. This could be