AAC: Lossy Encoding Is Getting Better – AAC Format Summary
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;
