Help transcoding AC3 25 to 23.976

[nimrodim]

I need help transcoding an ac3 file from 25 to 23.976.
I have already got 5.1 channels at 23.976 using Besweet.
I now am trying to recode the AC3 using softencode.
The original AC3 is ripped from a dvd.
So:
1. What dialoge normalization do i use? (only the center channel?)
2 Should i use any filters? add or subtract dB from the LFE or other channel?
3. Do i need to use any dynamic compression - as this is already original material?

Thanks in advance

[keithmac]

I`m experimenting with this myself, as the original AC3 is ready to play I think the waves will already be the corect volume, altering them with SoftEncoder again will make the centre channel too loud? (I`ve time corrected and output to .wav with besweet)

I`ve used the -1db setting for the centre/vocal channel and unchecked the suround channel attenuation (set at 3db by default)

Just by looking at the wave disply in softencode you can see the centre channel is louder than left and right, and the rear channels are quieter.

I`m going the other way to you, from film to pal, only 1fps faster but what a headache!

[nimrodim]

How are you determining the dialoge normalization value?
What are you doing about Dynamic compression?

[keithmac]

How are you determining the dialoge normalization value?

I have set this to -1db (can`t set to zero) as i belive it should be exactly the same as the original ac3 (it has already been done when the original ac3 was written)

What are you doing about Dynamic compression?

As far as I can see, the dvd player will do this itself, I haven`t used any dynamic compression, I have read over the past few days that you should use a maximum of light compression anyway?

Basically as I see it the only thing I need to alter about the AC3 is the actual "framerate" all the other settings should be the same as the original AC3, which will be present in the wav files BeSweet outputs (mainly volume)

I`m going to play my new AC3 back to back with the original to see if the levels have changed at all (hope not!)

[nimrodim]

I have set this to -1db (can`t set to zero) as i belive it should be exactly the same as the original ac3 (it has already been done when the original ac3 was written)

I think this is a mistake as the average volume level should be -31 according to the sticky - and you want the center channel to be at that level.
Hence i think u have to measure the average RMS of the center channel first and set dialoge normalization to that value.

[nimrodim]

Have you set any "Input filtering" - is this necessary?
And what about surround channel processing? - any need for anything from here?

[keithmac]

Interesing about the dialogue normalization, I will have to look into that, have you measured your centre channel and how (what program?) I`ve read the help file from Soft Encode and set the input filtering as per their instructions although I have made sure the surround channels haven`t beeen altered.

To be honest I`ve only been looking into this for a few days, got stuck for a while with ac3enc method with besweet and gave up as there`s a major flaw with ac3enc.dll which is a shame as it could be a nice fast method.

Going to be trying a few different setups, would be good to hear how to get on :)

[keithmac]

Just read through the guide again and your right about the Dialogue Normilization, looks like it`s a flag for the dvd player to use to ajust the volume, may try a few different settings for this as I find normall dvd`s on the quiet side anyway..

[nimrodim]

You could measure the average RMS value with a program like Adobe Audition/cooledit - but i think you should measure the center channel.

As to "surround channel processing" i found the following in the sticky:
"The 90° phase shift is not essential for decoding through a Dolby Digital 5.1 system, but it is if your mixed might be played back on a Dolby Pro Logic or surround system. These expect the surrounds to be phase shifted by 90°.
The 3dB attenuation is usually necessary for files that will end up being played back on consumer home theater equipment. "

[keithmac]

"The 90° phase shift is not essential for decoding through a Dolby Digital 5.1 system, but it is if your mixed might be played back on a Dolby Pro Logic or surround system. These expect the surrounds to be phase shifted by 90°." Only problem is have they already been shifted once already i.e. is it already in the wav, from whay I can see they surround channels already look quieter.

"The 3dB attenuation is usually necessary for files that will end up being played back on consumer home theater equipment" From what I can see they surround channels already look quieter i.e. they`ve already been attenuated in the orignal AC3.

What we`ve got to work out is what parts relate to values read by the dvd player/equiptement and whats actually altered in the streams themselves while encoding (you don`t want to do this twice).

[nimrodim]

I would expect that "Surround channel processing" actually changes the sound data.
But dialoge normalization is just meta info.

[keithmac]

Just checked the centre channel wav with Soundforge, comes up with mean RMS value of -30.9, so I should use this as my normalization value?

Edieted 1cause I`m stupid!

[nimrodim]

Yes that means you need to use -31 as your dialoge normalization value

[keithmac]

Just played the softencode output though Powerdvd and it`s exactly the same volume as the original which is nice!

Funny thing is if I just double left click on the ac3 output by softencode it crashes my system? anyone any ideas why?