[mad-dev] Replay Gain
Tue, 24 Feb 2004 16:22:41 -0800
I've been asked to elaborate a bit on Replay Gain and the issues
surrounding it, as they relate to MAD and LAME.
The Replay Gain proposed standard was set forth in 2001 by David
Robinson and can be viewed here:
First it's important to understand that Replay Gain has absolutely no
bearing on the libmad proper; it doesn't affect the actual decoding
process whatsoever, and libmad has no knowledge of Replay Gain
structures or semantics.
By cooperative agreement with encoders and other tools, applications
can use Replay Gain information to adjust the volume of decoded audio
to achieve approximately equal loudness across an entire collection of
music. madplay does this when enabled with the -G (--replay-gain)
The proposed standard describes a method for computing Replay Gain
adjustments, which in any case are just decibel scalefactors.
Essentially the energy of the encoded signal is calibrated against a
reference level, and the difference is stored as the Replay Gain
adjustment value. The reference level set forth in the proposed
standard is the SMPTE-sanctioned 83 dB SPL, representing a comfortable
average listening level.
When Replay Gain is enabled, players are encouraged to supply a default
pre-amp gain of at least +6 dB in addition to the Replay Gain
adjustment. This is so pop music for example does not sound too quiet,
but the pre-amp might be reduced or disabled for other types of music.
madplay supports this with the existing -a (--amplify, --attenuate)
option, which now has a default value of +6 dB when Replay Gain is
Replay Gain information can be stored in a variety of possible
locations, for example in an ID3v2 tag. Recent versions of LAME
automatically compute Replay Gain data and store it in the LAME info
When enabled, madplay reads and honors Replay Gain from either of these
So what was the purpose of the changes in madplay 0.15.2b?
It turns out that versions of LAME since 3.95.1 do not strictly follow
the Replay Gain proposed standard. Instead of using the specified 83 dB
SPL reference level (as LAME did in version 3.95) LAME now uses a
reference of 89 dB SPL. That means the Replay Gain adjustment computed
by LAME is now 6 dB greater than what is specified by the proposed
standard, and players that implement the recommended pre-amp gain on
top of that will amplify the signal too much -- and often cause
Why did LAME do this? According to Gabriel Bouvigne, one of the LAME
developers, "we had a discussion about it with David (ReplayGain
author), and reached the conclusion that as the vast majority of
implementations are using 89dB, we should take this infortunate
situation into consideration and update the standard to 89dB." (But to
my knowledge the proposed standard has yet to be updated.)
The unfortunate effect of this heresy is that players have to know what
reference value the Replay Gain adjustment was computed relative to,
for it can no longer be assumed to be the 83 dB SPL specified in the
proposed standard. Accordingly, special logic was added to madplay to
detect LAME tags written since 3.95.1.
And there you have it.