[FFmpeg-user] AC-3 Dolby Digital 5.1 Encoding

Joshua Tidsbury Joshua.Tidsbury at bellmedia.ca
Sun May 27 17:01:21 CEST 2012

On 2012-05-27, at 10:44 AM, Chase Patterson wrote:

> Thanks everybody! It turns out, Totem (the Gnome Movie Player) doesn't
> recognize it as surround sound correctly, but vlc does.
> There was only a small difference in ffmpeg's output to the terminal
> with the different commands, that is not warning about not specifying
> a channel layout anymore, and it says "5.1" rather than "5.1(side)".

It's in the metadata (channel labelling). In most cases, set top boxes and receivers will treat them the same, but by specifying mode 63, you've forced the metadata to match the standard Dolby metadata for an ITU 5.1 program stream.

I did notice that you utilized -24 as the dialnorm value, which was in my example, but isn't necessarily correct for your program. I would encourage you to measure the long term LEqA (Dolby) or ITU-R BS1770 loudness of your program in order to determine the actual loudness value in order to be presenting valid metadata and be in compliance.

Also, I provided a downmix value of 2, which causes the program stream to suggest (note: not necessarily enforce) a preferred downmix mode of Lo/Ro, which entails a simple folddown of the content to stereo. This is as opposed to Lt/Rt (which would be option 1) which causes a 90 degree phase shift to the surround channels prior to performing the folddown, and is designed to improve later upmixing should the end consumer's receiver be in Dolby ProLogic mode, but can cause other interesting artifacts due to the phase shift.

Yes, as odd as it sounds to hear that folks would take a stereo output of a device (which is actually a downmix of a true 5.1 program) and then run it through an upmixer afterwards, it happens more often than you'd think.

I won't argue that one mode is better than the other. It tends to be highly content sensitive. I would suggest you encode in both modes and compare the stereo mixdown output to see which provides a better experience. I tend to lean towards Lo/Ro for most of the mixing I do, especially if there is a great degree of discretely mixed 5.1 music as part of the program, but I'd encourage you to listen to both so you can understand the results and ensure the greatest portability of your final mix.

Take care,

Joshua Tidsbury | CTV Television
t 416.384.7253 | m 416.433.3968 | joshua.tidsbury at bellmedia.ca

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