Understanding Audio in Media
Audio tracks combine encoded sound with timing, channel information, and playback instructions.
A movie has three English soundtracks: 7.1 surround, stereo commentary, and audio description. Which one should you select? Start with what the track contains, then consider how it will play on your listening system. The most impressive channel count is irrelevant if you have selected the director discussing the film instead of the film's dialogue.
Language, codec, channels, and the selected presentation
Each audio track is a separate timed stream in the media file. Its language, title, and accessibility role help identify the intended presentation. Its codec and channel layout describe how that presentation is represented.
| Part of an example track | The question it answers |
|---|---|
| English — Main | Is this the language and presentation I want? |
| E-AC-3 | Which decoding format is required? |
| 5.1 | Which channel roles are in the compatible channel presentation? |
| 48 kHz | What sample timeline does decoded audio use? |
| 640 kbit/s | How much encoded audio data is carried per second? |
Those numbers are not different ways of expressing one quality setting. Changing 5.1 to stereo combines spatial channels. Changing 48 kHz to another sample rate changes the time grid. Changing the encoded bitrate gives a compressed-audio encoder a different data budget. Each operation solves a different problem.
A container keeps the tracks together with timing and metadata. That is how a viewer can switch language or commentary while the video remains the same. The selected soundtrack must stay aligned with the picture even when its packets and video packets represent different amounts of time.
5.1 audio on a stereo television
Suppose you choose the 5.1 film track and listen on a stereo television:
- Emby selects that track for delivery. If its codec and packaging fit the app, the server may send the stored audio.
- A decoder reconstructs audio samples for the track's channels. This might happen in the player or during server conversion.
- A downmix combines those channels into two outputs, including center-channel dialogue that would be lost by simply discarding all but front left and right.
- The television converts the final signal into sound.
On a compatible surround system, the same track can instead keep its separate channel presentation. An object-based soundtrack can additionally supply spatial information that a renderer uses for the connected speaker or headphone arrangement.
This is why the source format, Emby's delivery method, and a receiver's input display can have different labels. “PCM” at the receiver can mean that decoding already happened in the player. “Stereo” can describe the final output even though the stored soundtrack has six channels.
Use sampling to understand the numbers representing sound, channels to understand its spatial arrangement, or the playback path to locate a conversion between the file and what you hear.