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Topic: Formula to convert dB to AAC global_gain? (Read 9268 times) previous topic - next topic
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Formula to convert dB to AAC global_gain?

I'm an experienced programmer but total newbie regarding digital audio. I don't have access to the ISO specs for AAC, but am more or less figuring them out by reading the FAAD2 source code.

As a way of learning more, I'm trying to build the "aacgain" program described in this thread. I've been able to hack FAAD2 to alter global_gain on each individual_file_stream and write out an updated mp4 file. Works great!

It looks pretty simple to port the replaygain code from either mp3gain or foobar_2000 into FAAD2.

The way I read the FAAD2 code is that in AAC,
scalefactor=POW(2, global_gain/4). Is this correct?

The replaygain algorithm gives results in dB, and my recollection is that
scalefactor=POW(10, dB/10). Is that correct?

So my question is: given a replaygain in dB, what is the function that converts a replaygain in dB to an adjustment for global_gain?

Many years ago I could have done this, but my brain cells that used to know algebra have been filled with other stuff not to be mentioned:-)

Thanks for the help!

Dave

Formula to convert dB to AAC global_gain?

Reply #1 –
Quote
So my question is: given a replaygain in dB, what is the function that converts a replaygain in dB to an adjustment for global_gain?[a href="index.php?act=findpost&pid=240183"][{POST_SNAPBACK}][/a]


After fighting with this a bit more, I came up with:

global_gain adjust = 4 * LOG2(POWER(10, replaygain/10))

Can some kind soul confirm if this is correct?

I'd also appreciate some opinions if DSPGuru's approach in the previously referenced post is correct. I have no idea what he meant my just having to adjust only the 1st global_gain; I needed to adjust all of them in each ICS to make it work.

Thanks again!

Dave

Formula to convert dB to AAC global_gain?

Reply #2 –
AFAIK an increase of 1 of the global_gain value is an increase of 0.25 dB in gain.

So if you know how much the gain needs to be increased you know the required change in global_gain.

Menno

Formula to convert dB to AAC global_gain?

Reply #3 –
Quote
AFAIK an increase of 1 of the global_gain value is an increase of 0.25 dB in gain.

So if you know how much the gain needs to be increased you know the required change in global_gain.

Menno
[a href="index.php?act=findpost&pid=240197"][{POST_SNAPBACK}][/a]


Wait a minute.. I thought an increased of the G_Gain by 1 is an increase of

20log10(2^0.25) dB at the decoder time domain samples?? .. .. which is log10(32).. about 1.5 dB ??? log10(32) = log10(10 * 3.2) = log10(10) + log10(3) = 1 + 0.4771 which is about 1.5dB  or 5 * log10(2) = 5 * 0.301 = 1.505 dB..

Formula to convert dB to AAC global_gain?

Reply #4 –
davelasker: Great that you are doing this! 
Juha Laaksonheimo

Formula to convert dB to AAC global_gain?

Reply #5 –
Well it's always fun to get conflicting advice:-)

Does global_gain in AAC have the same units as in MP3?

In the MP3GAIN source code,

global_gain =db*1.505,

while I interpreted wkwai's post to say that

db = global_gain*1.505.

Which is correct for AAC?

What is the relationship between 20log10(2^0.25) and the FAAD2 scalefactors?

Thanks for helping educate me!

Dave

Formula to convert dB to AAC global_gain?

Reply #6 –
Quote
Well it's always fun to get conflicting advice:-)

Does global_gain in AAC have the same units as in MP3?

In the MP3GAIN source code,

global_gain =db*1.505,

while I interpreted wkwai's post to say that

db = global_gain*1.505.

Which is correct for AAC?

What is the relationship between 20log10(2^0.25) and the FAAD2 scalefactors?

Thanks for helping educate me!

Dave
[a href="index.php?act=findpost&pid=240436"][{POST_SNAPBACK}][/a]


Hmm, you might be right

Of course if you are already able to change the global_gain it is easy to prove it empirically.

Menno

Formula to convert dB to AAC global_gain?

Reply #7 –
Quote
Of course if you are already able to change the global_gain it is easy to prove it empirically.
[a href="index.php?act=findpost&pid=240439"][{POST_SNAPBACK}][/a]


Please tell me how. I have no audio test equipment (other than my PC).

Thanks again!

Dave

Formula to convert dB to AAC global_gain?

Reply #8 –
Quote
Quote
Of course if you are already able to change the global_gain it is easy to prove it empirically.
[a href="index.php?act=findpost&pid=240439"][{POST_SNAPBACK}][/a]


Please tell me how. I have no audio test equipment (other than my PC).

Thanks again!

Dave
[a href="index.php?act=findpost&pid=240529"][{POST_SNAPBACK}][/a]


If you have an editor to view the replayed waveform,  you should see that adding 4 to the global
gain doubles the amplitude of the replay.

There is one possible problem with the technique: the individual scalefactor gains are differentially
coded from the global gain, but also limited to be between 0 and 255. If you change the global
gain too much, you could run individual gains outside limits. How any individual decoder would
behave in that case is unpredictable.

I think this is probably just a theoretical problem, because the gain range is so large that you
will probably never exceed those limits.

Tim.

Formula to convert dB to AAC global_gain?

Reply #9 –
Quote
If you have an editor to view the replayed waveform,  you should see that adding 4 to the global
gain doubles the amplitude of the replay.[a href="index.php?act=findpost&pid=240542"][{POST_SNAPBACK}][/a]


Hi Tim

Yes I can see that adding 4 to globbal_gain will double the amplitude just by reading the faad2 code. What I am trying to verify is the relationship between dB and global_gain. Do you know how to measure that?

Thanks for the help!

Dave

Formula to convert dB to AAC global_gain?

Reply #10 –
Quote
Quote

If you have an editor to view the replayed waveform,  you should see that adding 4 to the global
gain doubles the amplitude of the replay.[a href="index.php?act=findpost&pid=240542"][{POST_SNAPBACK}][/a]


Hi Tim

Yes I can see that adding 4 to globbal_gain will double the amplitude just by reading the faad2 code. What I am trying to verify is the relationship between dB and global_gain. Do you know how to measure that?

Thanks for the help!

Dave
[a href="index.php?act=findpost&pid=240587"][{POST_SNAPBACK}][/a]


Doubling the amplitude of a signal is equivalent to an increase of 6dB, hence the 1.5dB
cited above for each increment of global gain.

Tim.

Formula to convert dB to AAC global_gain?

Reply #11 –
Quote
In the MP3GAIN source code,
global_gain =db*1.505
[a href="index.php?act=findpost&pid=240436"][{POST_SNAPBACK}][/a]


Just to set the record straight, I was looking in the wrong place in the mp3gain code when I made the above rash statement. File mp3gain.c line 2304 says:

dblGainChange = dBchange / (5.0 * log10(2.0));

which is consistent with wkwai's post.

Thanks again to all for your help!

Dave

Formula to convert dB to AAC global_gain?

Reply #12 –
Hi Dave. Can we expect this app to become available soon?
Juha Laaksonheimo

Formula to convert dB to AAC global_gain?

Reply #13 –
Quote
Hi Dave. Can we expect this app to become available soon?
[{POST_SNAPBACK}][/a]


Yes but it will take a while. I have a full time job, so I only get to work on it a couple of hours a week. It's taking me a bit longer than [a href="http://www.hydrogenaudio.org/forums/index.php?showtopic=14322&pid=147583&mode=threaded&show=&st=&#entry147583]DSPGuru's 3 hour estimate.[/url]

As a first step I will post a patch to faad2 that allows manual adjustment of gain, similar to the /g option of mp3gain. Hopefully this will be done by next weekend.

After that I need to integrate the ReplayGain algorithm. That will take a bit longer .

Does anyone know if faad2 patches should be posted here, on audiocoding.com, or on sourceforge???

Thanks...

Dave

Formula to convert dB to AAC global_gain?

Reply #14 –
Thanks for the info.  Finally this kind of tool arrives

Faad2 patches shouldn't be a problem. Just contact Menno (or maybe Menno contacts you) either here or on Audiocoding.
Juha Laaksonheimo