Flac Surround

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dsp5500
Posts: 4
Joined: Thu Jul 08, 2010 4:57 am

Flac Surround

Post by dsp5500 »

I have been converting all DolbyHD and DTSHD audio to Flac when i ripp Blu-ray. Im now concerned that im loosing data when doing so. Specifically Metadata. Can some one tell me what isn't being copied across when converting to Flac.
Are all the channels kept and in correct order when converting to flac?
What about down mixing a 7.1 track to 5.1. is there metadata that is need in the file to do this correctly or is it done elsewhere without the need of metadata.
Romansh
Posts: 873
Joined: Sat Jan 22, 2011 7:09 pm

Re: Flac Surround

Post by Romansh »

The channel layout and channel order should be correctly preserved when converting to FLAC, as is the main audio presentation (and losslessly, of course).

However, the source audio may feature additional metadata such as:

- 7.1 to 5.1 downmix coefficients

- 7.1/5.1 to Stereo downmix coefficients

- Dialog Normalization level

- Dynamic Range Compression hints

Generally speaking, however, well over 90% of bitstreams out there appear to use the default values provided by the DTS(-ES(-HD)), resp. (E-)AC-3/TrueHD encoder, so even if those aren't transferred to the output, it may not be that important for most users.
dsp5500
Posts: 4
Joined: Thu Jul 08, 2010 4:57 am

Re: Flac Surround

Post by dsp5500 »

Thanks for getting back to me Romansh and explaining what metadata is included.

I have a 5.1 setup and don't see me upgrading to 7.1 anytime soon so i think i may have made a mistake in converting to flack.
Can you explain in more detail what would happen to a 7.1 Flac track when played thru a media player like Plex via a HDMI cable to a 5.1 processor without the metadata?
Do media players such as Plex use default values to down mix if there is no metadata? and if they send 7.1 then I guess two channels will be dropped?

Sorry i probably won't make much sense but im trying to understand the basics of down mix coefficients.
The down mix coefficients are they constant? As in do they describe in the beginning how to down mix the channels for the movie or are they variable and will they mix differently as the movie goes on?

Regards,
Rob
Romansh
Posts: 873
Joined: Sat Jan 22, 2011 7:09 pm

Re: Flac Surround

Post by Romansh »

Plex is capable of downmixing 7.1 to 5.1 without dropping channels.

The downmix coefficients provided in the DTS or Dolby bitstream could theoretically vary from frame to frame, but I don't think any encoder actually supports such a thing, so I doubt there's any bitstream that does this anywhere in the world.

Plex's decoders don't necessarily support extracting the downmix coefficients from DTS or Dolby bitstreams, BTW. The one case where such metadata may be used is when passing the audio through directly to a compatible receiver.
dsp5500
Posts: 4
Joined: Thu Jul 08, 2010 4:57 am

Re: Flac Surround

Post by dsp5500 »

Again thanks for your time and explanations.
I think at the end of the day the safest option is to extract the DolbyHD and DTSMA audio and be done with it!
Interesting also to note that just because the downmix coefficients are present that they may not be used!

Cheers,
Rob
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