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Player implementation of ReplayGain (Android)
Havin_it
post Oct 8 2012, 11:53
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Hello,

I'm hoping some folks here can help me get my head around the task of adding ReplayGain functionality to the stock Android music player app. My lack of training in maths and audio science is making it hard for me to ask the right questions probably, so please bear with me if you can.

I've read through the replaygain spec (original proposal on your minisite and the updated version in the wiki) but there's still a lot that I'm unclear on. A sound engineeer friend helped me along a little further, but I think I exhausted his patience :(

Now I understand that all replaygain adjustments start from -14 dBFS, and RG treats this as equivalent to 89 dB (SPL). Where I get lost is how I'd translate that to the logarithmic scale used by the volume control function I have to work with, which takes a float value (0-1 = mute-max) to attenuate the playback volume for the app (normally it stays at 1 and is used mainly for fading; it's independent of the master volume controls for the device). It can be abstracted to a linear scale (of the type that a physical volume control would use, I believe) of 0-<number of your choice>. The value is calculated from a linear-scale value like this:
CODE
#Example to set volume to 50/100 on the linear scale
int maxvol = 100;
int newvol = 50;
float ratio = (float) (1 - (Math.log(maxvol - newvol) / Math.log(maxvol)));


But to get to that point, I need somehow to abstract that starting-point of -14 dBFS / 89 dB SPL onto that linear scale. From what I've read so far, I get the feeling there is no "pure" way to do this and I'll be required to make some educated guesses. So that's where being uneducated lets me down :(

I'd just be grateful for any suggestions, help or references in how I might understand the task better and construct a *reasonably* accurate equation for it.
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Havin_it
post Oct 9 2012, 03:24
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Group: Members
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Joined: 7-October 12
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It's not exactly that I can't increase it, just that I'd have to somehow calibrate/estimate/pullouttamyass an appropriate value between 0 and 1 to act as the reference level and increase (or decrease) from there. The volume control function I was planning to use sets the volume just for that player app, keeping it independent of the device's master volume levels (I think the idea being that it doesn't interfere with other things like the ringer, voice, notifications etc). It always starts at 1, so its purpose is really for attenuation (e.g. fading tracks in and out).

Anyway, that one's out the window unless I'm content to resort to some sort of bodge (won't deny it's tempting, but...). However, I did turn up something in the MediaPlayer API that might serve. It's possible to attach an aux effect to the player that "directs a certain amount of its energy to" that effect (potentially all of it, if I read it right). One of the available effects is an Equalizer, whose methods include setting the gain for a series of frequency bands. Sooooo, perhaps I can use that to apply the gain across all the available bands simultanaeously. More reading ahead...
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