5.1 to 7.1 - eac3to? Or just duplicate the sides?

[Asterra]

Working on a project which involves multiple clips, some in 5.1 and some in 7.1. In the past, it has sufficed to simply duplicate the surround channels (or ignore the extra two channels altogether when they're not called for). But I'm trying to be a little more particular this time.

The mix will be 7.1, to accommodate the clips that do have eight channels. So when it comes to 5.1 clips, it seems to me I have a few choices.

1) Ignore the extra channels and pipe the 5.1 clip's surrounds through two speakers only. (Which? Sides or rears?)
2) Duplicate the 5.1 clip's surrounds so they play through all four surround channels of the 7.1 mix.
3) Modify the amplitude of the surrounds and then duplicate them to play in all four channels. (By how much?)

I have spied hints that option #3 may be what I'm supposed to be doing, but I'm uncertain as to how much the surround channels should be attenuated. If there's a command line I could feed to eac3to, that would be ideal.

Thoughts appreciated. Thanks.

[Bonie81]

There is a standard not publicly published.

But, as such There is NO STANDARD when it comes to audio listening, as every human ears have different response to different frequencies. Taste of Audio Listening differs from Human to Human. I have a friend who likes stereo over surround and always go for option 1, in-spite he owns very professional high-end audio system. Personally I prefer PCM over AC3 or DTS for the audio channels more than six, simply unmatched sound quality. My one full songs BD contains only 18 to 20 songs.

2) Duplicate the 5.1 clip's surrounds so they play through all four surround channels of the 7.1 mix.

Normal and easy practice would be just duplicating SL --> SBL and SR --> SBR

3) Modify the amplitude of the surrounds and then duplicate them to play in all four channels....

May produce better sound.

3) ... (By how much?)

still it is a matter of personal taste of Audio Listening.

[Asterra]

3) Modify the amplitude of the surrounds and then duplicate them to play in all four channels....

May produce better sound.

3) ... (By how much?)

still it is a matter of personal taste of Audio Listening.

I actually figured this might be the case, so I suppose I should rephrase that to ask for opinions. The only reason I figured there might be something akin to a generally accepted attenuation value is the perhaps somewhat obvious fact that if one plays two channels over four identical speakers, the result is going to be at least a little louder than if they get played over just two of those same speakers. How much louder might depend on the room, but even then, I figured there's bound to be a value which serves as a staple in consideration of the typical home theater.

Problem is I don't even really have a good guess. Somewhere between 50% and 100% is about the best I'm willing to surmise.

I agree with you on the PCM thing. As it happens, this will be my third BD that uses a 7.1 PCM mix. Though I'd go for TrueHD/DTS-HD MA if they were options.

[Bonie81]

How much louder might depend on the room, ...

true and agreed. for normal living room or bed room just 5.1 channels surround is more than enough, i guess
i do not see a valid reason to throw extra bucks on 7.1 system and end-up paying more every time.

but even then, I figured there's bound to be a value which serves as a staple in consideration of the typical home theater.

if your 5.1 mix is good enough. Normal, standard and easy practice of duplicating SL --> SBL and SR --> SBR will produce good 7.1 mix for almost all listening environments. When it comes to mixing matrix, there are end-less possibilities. You can see and varify this in doom9 forum for BeHappy post - there are many mixing matrices posted by different members and all produce reasonably good mix. Going through the details of mentioned post might be a good hint to start with.

Cu Selur - Author of Hydrid contributed collection of sox profiles from Doom9 forum in my guide, you can see and generate more than 3 to 4 matrix. (I do not use any GUI, I use straight CLI encoders like sox, aften....so on.)