MPEG Audio Layer II (MP2)
MPEG Audio Layer II is a lossy audio coding format used in broadcast systems and older media recordings.
A television recording can contain MPEG-2 video and MPEG Audio Layer II, commonly abbreviated MP2. The names share a standards history, but the video and audio are separate coded streams.
Layer II is a lossy subband audio codec used in broadcast systems and older media. Conventional DAB uses Layer II; DAB+ instead uses the AAC family.
Allocate precision across subbands
Layer II divides the signal through a filter bank into frequency subbands. The encoder estimates how much precision each needs, then describes their sampled values with the selected quantization and scaling information.
The decoder reads those choices, reconstructs the subband values, and combines them through the synthesis filter bank into the audio waveform. Giving one subband more precision can reduce its coding error, while the total bitrate constrains the available allocation.
The subbands are frequency regions within audio channels. They are not separately selectable language tracks or physical surround speakers.
Decode the channel mode as well as the samples
| Layer II mode | Intended relationship |
|---|---|
| Mono | One presentation signal |
| Stereo | A related left/right presentation |
| Joint stereo | Shared coding treatment for suitable channel information |
| Dual channel | Two independently intended channel signals |
Dual-channel material can carry two language programmes. Treating those as left and right of one stereo soundstage can make both languages play simultaneously. Track selection and channel interpretation are therefore different operations.
Keep the broadcast context while converting
A transport-stream recording supplies programme selection and timestamps around the coded audio. The MP2 decoder reconstructs the sound; the systems reader keeps it associated with the correct programme and timeline.
If an Emby client accepts the recorded video but needs a different audio codec, a Direct Stream path can retain compatible video and create a suitable soundtrack. That operation must preserve the selected programme, intended channel presentation, and timing.
The converted audio starts from the Layer II reconstruction. Repackaging the original MP2 into another supported wrapper instead retains its coded representation. The choice follows whether the target needs another wrapper or another audio decoder.