AAC-LC Audio
AAC-LC is the Low Complexity form of Advanced Audio Coding used for lossy mono, stereo, and multichannel audio.
“Low Complexity” in AAC-LC names a coding configuration, not a low-quality music grade. Advanced Audio Coding's LC form can represent mono, stereo, and multichannel material using a transform-based lossy codec.
Consider a music passage containing a sustained note followed by a sharp drum hit. Those two events illustrate why its tools are useful.
Sustained detail and sudden attacks ask different things of a window
A transform represents a section of waveform through frequency components. A longer analysis window helps describe frequency detail over that section. A shorter window localizes changes more closely in time.
For a steady tone, a longer window can be efficient. At the drum attack, spreading the section's coding error across a long window can make error audible before or around the hit. AAC supports switching to shorter windows to handle that transient. The encoder selects the window sequence and sends the information the decoder needs to reconstruct it.
This is not switching sample rate or making the music play faster. Several shorter transforms describe the corresponding span of samples within the same timeline.
Encode the representation, then reconstruct it
The encoder selects precision for the frequency representation and codes the resulting values and side information. The decoder reconstructs those values, performs the defined inverse processing, and combines the overlapping sections into PCM.
| Stage | What it contributes |
|---|---|
| Window and transform | A timed frequency-domain description |
| Precision allocation and quantization | The main lossy approximation |
| Symbol and side-information coding | An efficient description of the chosen representation |
| Inverse processing and overlap | A continuous decoded sample sequence |
Other AAC tools manage relationships between components or channels. Encoders can make different decisions while producing streams conforming to the same supported LC tools.
Place LC in the AAC family
HE-AAC adds spectral-band replication to an AAC core; HE-AAC v2 can add parametric stereo. Those names identify additional reconstruction jobs. HE-AAC explains why its core and output sample-rate reports can differ.
AAC-LC reconstruction does not perform those extra tasks. A profile or audio-object-type description therefore tells you more than the bare “AAC” family label when comparing sources.
Supply the decoder configuration
An AAC track needs coding configuration such as its audio object type, sample-rate arrangement, and channel configuration. MP4/M4A can store that information with its track description, while other AAC framing formats provide it through their own structures.
For an Emby source, this separates two operations. Repackaging compatible LC audio can preserve the coded sound while translating its framing/configuration. Re-encoding changes the audio representation itself. The useful choice follows whether the obstacle is supplying the existing LC bitstream correctly or reconstructing a format the destination does not support.