Audio troubleshooting
Background Audio Information
To understand how XBMC processes audio you must first have a basic understanding of how audio in the digital domain works.
Pulse Code Modulation (PCM)
When analog audio is converted into the digital domain the near universal format used is Pulse Code Modulation (PCM). All manipulation of audio in the digital domain is always done on audio in the PCM format, therefore functions in XBMC such as volume control can only be done on PCM data, therefore if audio is in one of the Encoded Formats XBMC must decode the audio to PCM in order to manipulate it for volume control.
Encoded Formats
There are two types of encoded formats, these are lossy (compressed) and lossless (uncompressed).
Lossy
These formats take the PCM audio data and use encoding algorithms to throw away (hence term lossy) audio information that the human ear cannot hear in order to save space. Examples of lossy formats:
- Dolby Digital AC3
- DTS
Lossless
These formats differ from the Lossy formats in that they don't chuck data away but instead compressed the PCM data with highly algorithms that allow the original audio to be reconstructed (hence lossless), this is very similar in principle to the way zip & rar works on documents in that the original source can always be reconstructed. Examples of Lossless formats:
- DTS-HD MA
- Dolby TrueHD
- Flac
Number of Channels
When number of channels is referred to, it will nearly always be the number of PCM channels that is meant. Since PCM is uncompressed audio this is a frequently use way of expressing the raw bandwidth for a particular audio path. Thus when you see the "Number of channels" setting in XBMC, this is the number of PCM channels.
Why 5.1 is possible when number of channels is 2.0?
When audio is in a lossy format is it so much smaller than the original uncompressed PCM data that it's still possible to send 5.1 channels even when "Number of channels" is 2.0
Take SPDIF as an example, this type of audio path can only handle 2 PCM channels however a format such as DTS can still be passed as 5.1 channels because once encoded into DTS the data is so much smaller than what is required for 2 PCM channels.
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