what happens when I convert 7.1 to 5.1?

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Bubbabob
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Joined: Sat Apr 03, 2021 1:05 pm

what happens when I convert 7.1 to 5.1?

Post by Bubbabob »

I have a 5.1 system, and will occasionally need to convert a movie with 7.1 audio to 5.1. This is because the nature of my system will literally only play proper surround sound if the codec is ac3. So that's what I have to play with. So if I have a file with 7.1 or dts, down to ac3 5.1 it goes.

So my question is this: what happens when I mix down to 5.1? Do I still have the joy of surround sound (sounds that are supposed to be behind me coming from the surround speakers), or am I just changing the audio to all channel?

Thanks!
Bubbabob
Posts: 20
Joined: Sat Apr 03, 2021 1:05 pm

Re: what happens when I convert 7.1 to 5.1?

Post by Bubbabob »

I forgot to include that the converter I'm using is Videomass, which incorporates ffmpeg. This app allows me to convert just the audio with video pass through.
Deleted User 11865

Re: what happens when I convert 7.1 to 5.1?

Post by Deleted User 11865 »

Bubbabob wrote: Sat May 15, 2021 10:36 pm I forgot to include that the converter I'm using is Videomass, which incorporates ffmpeg.
So are you asking what happens when using HandBrake or when using ffmpeg?
Bubbabob
Posts: 20
Joined: Sat Apr 03, 2021 1:05 pm

Re: what happens when I convert 7.1 to 5.1?

Post by Bubbabob »

Rodeo wrote: Sun May 16, 2021 8:30 pm
Bubbabob wrote: Sat May 15, 2021 10:36 pm I forgot to include that the converter I'm using is Videomass, which incorporates ffmpeg.
So are you asking what happens when using HandBrake or when using ffmpeg?
Whichever one (or both) you know the answer to.
Bubbabob
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Joined: Sat Apr 03, 2021 1:05 pm

Re: what happens when I convert 7.1 to 5.1?

Post by Bubbabob »

Re-upping this. First things first, I'd like to get a really basic question out of the way: what goes into making surround sound in the first place? Is there an audio engineer determining which sounds go to which channels? I know this is an incredibly rudimentary question, but I'm starting from an incredibly rudimentary point.

And finally, if you mix down from 8 channels to 6 channels, how would a program know how to keep the appropriate sounds to the appropriate channels?
Deleted User 11865

Re: what happens when I convert 7.1 to 5.1?

Post by Deleted User 11865 »

The left surround and rear left surround are mixed together, same for right surround and rear right surround (with -3dB each).
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JohnAStebbins
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Re: what happens when I convert 7.1 to 5.1?

Post by JohnAStebbins »

what goes into making surround sound in the first place? Is there an audio engineer determining which sounds go to which channels?
Pretty much, yes. There are a variety of techniques. Microphones placed in different locations and/or pointing in different directions for live events. Sound effects added to specific channels during post-production. etc.

To answer the question of downmixing more generally, a channel that is being eliminated is mixed into the "closest" channel that is being kept by some measure of "close". I.e. left channels should mix into other left channels whenever possible, back channels to back channels, etc. An interesting example is elimination of a center channel. In this case, half the signal from the center goes left and half goes right.

This is all done using matrix multiplication. If there are N original channels and downmixing to M channels, the N channels are multiplied with an NxM matrix of coefficients that defines how much of each original channel goes into each output channel.
Bubbabob
Posts: 20
Joined: Sat Apr 03, 2021 1:05 pm

Re: what happens when I convert 7.1 to 5.1?

Post by Bubbabob »

Rodeo wrote: Thu May 20, 2021 3:50 pm The left surround and rear left surround are mixed together, same for right surround and rear right surround (with -3dB each).
Thank you. It's good to know I'm not hurting the audio (overly much) by down mixing it.
Bubbabob
Posts: 20
Joined: Sat Apr 03, 2021 1:05 pm

Re: what happens when I convert 7.1 to 5.1?

Post by Bubbabob »

JohnAStebbins wrote: Thu May 20, 2021 4:11 pm
what goes into making surround sound in the first place? Is there an audio engineer determining which sounds go to which channels?
Pretty much, yes. There are a variety of techniques. Microphones placed in different locations and/or pointing in different directions for live events. Sound effects added to specific channels during post-production. etc.

To answer the question of downmixing more generally, a channel that is being eliminated is mixed into the "closest" channel that is being kept by some measure of "close". I.e. left channels should mix into other left channels whenever possible, back channels to back channels, etc. An interesting example is elimination of a center channel. In this case, half the signal from the center goes left and half goes right.

This is all done using matrix multiplication. If there are N original channels and downmixing to M channels, the N channels are multiplied with an NxM matrix of coefficients that defines how much of each original channel goes into each output channel.
Thanks for this excellent explanation.
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