**I. Overview of AAC**
AAC stands for Advanced Audio Coding, a digital audio compression standard introduced in 1997. It was originally based on MPEG-2 audio coding technology and was developed by several companies, including Fraunhofer IIS, Dolby Laboratories, AT&T, and Sony, with the goal of replacing the MP3 format. In 2000, as part of the MPEG-4 standard, AAC incorporated additional technologies such as Spectral Band Replication (SBR) and Parametric Stereo (PS). This evolution led to different versions of AAC, such as HE-AAC and HE-AACv2, which are often referred to as MPEG-4 AAC.
AAC is a lossy audio compression technology that offers higher efficiency compared to its predecessors. It uses advanced coding techniques like SBR and PS to create three main variants: LC-AAC (Low Complexity), HE-AAC (High Efficiency), and HE-AACv2 (High Efficiency Version 2). LC-AAC is typically used at medium to high bit rates (≥80 kbps), while HE-AAC is optimized for lower bit rates (≤80 kbps), and HE-AACv2 further improves performance at even lower bit rates (≤48 kbps). Most encoders automatically enable PS technology when the bit rate is set below 48 kbps, making it equivalent to HE-AACv2.
**II. AAC Specifications**
AAC has nine different specifications tailored for various applications:
- **MPEG-2 AAC LC (Low Complexity)** – Simpler, no gain control, better coding efficiency.
- **MPEG-2 AAC Main** – Full feature set, best sound quality.
- **MPEG-2 AAC SSR (Scalable Sample Rate)** – Supports variable sampling rates.
- **MPEG-4 AAC LC** – Commonly used in mobile devices and MP4 files.
- **MPEG-4 AAC Main** – Contains all features except gain control.
- **MPEG-4 AAC SSR** – Similar to the MPEG-2 version but under the MPEG-4 standard.
- **MPEG-4 AAC LTP (Long Term Prediction)** – Improves efficiency for certain types of audio.
- **MPEG-4 AAC LD (Low Latency)** – Designed for real-time applications.
- **MPEG-4 AAC HE (High Efficiency)** – Optimized for low-bitrate environments.
Among these, LC and HE are the most widely used. The Nero AAC encoder supports LC, HE, and HEv2. HE-AAC combines AAC (LC) with SBR, while HEv2 adds PS technology to further enhance efficiency.
**III. Features of AAC**
1. AAC offers a high compression ratio, significantly better than older formats like MP3 or AC-3, while maintaining CD-quality sound.
2. It uses a more advanced filter bank and transform coding technique, allowing for better compression without sacrificing quality.
3. AAC incorporates modern technologies such as Temporal Noise Shaping, Backward Adaptive Linear Prediction, Joint Stereo, and Huffman Coding, improving both efficiency and sound quality.
4. It supports a wide range of sample rates, up to 96 kHz, and bit rates, with support for up to 48 channels and 15 low-frequency tracks.
5. AAC also includes multi-language support and can embed up to 15 data streams within a single file.
6. Unlike MP3 or WMA, AAC preserves both high and low frequency components more accurately, resulting in clearer and more natural sound.
7. Its decoding algorithm is highly optimized, requiring less processing power and making it ideal for mobile and embedded systems.
**IV. AAC Audio File Formats**
AAC files are typically stored in two formats: ADIF and ADTS.
- **ADIF (Audio Data Interchange Format)** – This format allows for direct access to the beginning of the audio stream, making it suitable for disk-based storage. It has a single header at the start of the file, followed by raw audio data.
- **ADTS (Audio Data Transport Stream)** – This format is designed for streaming and includes a sync word in each frame, allowing decoding to start at any point in the stream. It is similar to the MP3 format and is commonly used in real-time applications.
The structure of ADIF and ADTS headers differs. ADIF has one global header, while ADTS includes a header in every frame, enabling random access and easier parsing.
**V. AAC Header Information**
The ADIF header contains metadata about the audio stream, such as the number of channels, sampling rate, and bit rate. It is located at the beginning of the file.
In contrast, the ADTS header is split into fixed and variable parts. The fixed header contains essential information that remains consistent across frames, while the variable header provides details that may change between frames, such as frame size and error protection settings.
**VI. AAC Element Types**
AAC data blocks are composed of several elements:
- **SCE (Single Channel Element)** – Represents a single audio channel.
- **CPE (Channel Pair Element)** – Combines two channels using joint stereo techniques.
- **CCE (Coupling Channel Element)** – Used for multichannel audio and stereo coupling.
- **LFE (Low Frequency Element)** – Enhances bass response.
- **DSE (Data Stream Element)** – Carries non-audio data.
- **PCE (Program Configuration Element)** – Describes the channel layout.
- **FIL (Fill Element)** – Adds extended information like SBR or dynamic range control.
**VII. Processing Flow of AAC Files**
1. Determine if the file is in ADIF or ADTS format.
2. If ADIF, extract the header and proceed to the raw data.
3. If ADTS, locate the sync word to find the start of a frame.
4. Decode the frame header to extract necessary information.
5. Perform error checking if enabled.
6. Deblock the data to prepare for decoding.
7. Parse the element information to reconstruct the audio signal.
This structured approach ensures efficient decoding and playback of AAC audio across a variety of platforms and applications.
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