[LAD] Replaygain for video?

[Adrian Knoth]

On Sat, Dec 10, 2011 at 01:10:38PM +0100, Philipp Überbacher wrote:

> >    2. Players like VLC have a normalize function. I don't know if it's
> >       one-pass (on the fly, more like automated gain control) or two-pass,
> >       but it basically solves your problem of audio levels at playback
> >       time.
> 
> Counter argument:
>     If normalization would be sufficient, there wouldn't have been a
>     need for replaygain in the first place. Explanation on
>     http://www.replaygain.org/.

Huh? I cannot find anything on this website that says "normalization is
not sufficient":

   "Although music is encoded to a digital format with a clearly defined
    maximum peak amplitude, and although most recordings are normalized
    to utilize this peak amplitude, not all recordings sound equally loud.
    This is because once this peak amplitude is reached, perceived loudness
    can be further increased through signal-processing techniques such as
    dynamic range compression and equalization"

I have to admit I'm not too familiar with the details of replaygain, but
I'm not aware that it actually does compression or equalization. In
contrast, they explicitly state:

   "The player reads the corresponding gain metadata value from the file
   and scales the audio data as appropriate. Scaling the audio data simply
   means multiplying each sample value by a constant value."

That's gain, nothing more, nothing less.

According to the wiki, they store four values:

  1. Peak track amplitude
  2. Peak album amplitude
  3. Track replay gain
  4. Album replay gain

We can directly forget about the album values, since we're talking
videos here.

Let's assume we're brave and normalize to 0dBFS, then this is obviously
the resulting peak track amplitude.

Asking for additional replay gain would simply cause distortion.

Long story short: only the peak track amplitude could be a useful
information if you don't want to apply automatic gain control or
read the entire file every time you play it just to determine the
correct gain for normalization.


> > There are special tools. If nothing helps, vlc and mplayer can do this.
> > mplayer with -dumpvideo and -dumpaudio, vlc with the transcode commands.
> Right, this should do. Is there an API for that?

Sure, libavformat from ffmpeg or libav, whatever you prefer.


> > > After that, the scanning process should work as with any audio file.
> > > Afterwards the calculated replaygain values have to be added to the
> > > metadata of the file.
> > 
> > Provided that the audio track supports meta data. Depending on the
> > container, you can embed almost everything from pcm to ogg, mp3, mp4/aac
> > and so on.
> >
> > Depending on the container, it might be possible to add the meta data to
> > the muxed file.
> Ah, I didn't think of adding metadata to the audio itself, another
> possibility. However, adding it to the container would probably be more
> universal.

It's probably not that simple. If the container doesn't provide the
possibility to add sane metadata, you'd be lost. Likewise, there might
be multiple audio streams (stereo, multichannel, different languages).
You'd need a way to relate to those substreams from within your global
metadata.

It could be easier to work on the audio streams directly, but this would
require re-muxing the file, causing subtle problems like A-V sync or the
necessity to write arbitrary containers (MPEG, MPEG-TS, MP4, ogv...).
ffmpeg/libav might help.

Either way, it's complex. ;)

[jackd connection handling]
> Alex Stone, I guess :)

Exactly.

> If you have a good idea, please tell me. I'll have to find a team on
> monday and it might help to have some good proposals.

I'm still looking for somebody to rewrite hdspmixer. ;) While we're at
it, how about an HTML based approach: you fire up your browser, either
have a matrix mixer for everything or can select individual output
buses, and then have the input/playback faders for this destination
only. (I have a couple of details, if you want to go this route)

Other idea: a P1722 streamer, no idea if Christoph Kuhr is still working
on that.

Or ask Paul if he needs some help with jack3. ;)


Cheers

-- 
mail: adi at thur.de  	http://adi.thur.de	PGP/GPG: key via keyserver