Re: I need help trying to understand ReplayGain and its metrics
Reply #1 – 2024-03-22 07:54:47
There is another thread here in which the OP is saying that 89dB is too much for specific tracks, and that 85dB is better. What I'm having trouble understanding is what these numbers are derived from; what they are in relation to. I think you're making it more complicated than necessary. Everything is relative. The "89dB" in this case refers to the figure in the settings which provides user control to tweak the RG-normalised volume level. So setting this to 85dB instead simply reduces the overall gain in the system by 4dB. The reason this might be important is if a track with low overall loudness has brief high-loudness passages in it, then boosting the volume of the whole track could digitally clip the peaks. Digital clipping is when the numeric value of an input digital sample, when multiplied by the scaling factor, then exceeds the maximum value the processing or the DAC can handle. From the point of view of digital audio processing, dBFS is vitally important because 0dBFS is the onset of clipping. So far, the above is all about actual signal levels, measured as a digital value or a voltage. The confusion starts because RG and LU are weighted according to human acoustic response, so contributions in the high and low frequency range are less important than the middle frequency range. That means a single frequency sine wave at (say) 500Hz and -10dBFS might be -10dBLUFS, but 2kHz -10dBFS might be -20dBLUFS. Thus there is no simple relationship between signal voltage and LU, it depends on spectral components. So you have an RG tag value generated using a psycho-acoustic model to assess the overall loudness of a track relative to some arbitrary "RG standard loudness", best expressed in LU, followed by a user gain fudge factor because if the RG-adjusted output were standardised at a level which accommodates highly-dynamic tracks (eg classical) without risk of clipping, listeners of constant volume pop music would complain it was coming out too quiet (if they're listening directly to the PC output instead of feeding it into an amp with a volume control!). The "89dB" is the fudge factor. It's arbitrary because of the lack of a simple arithmetic relationship between LU and signal voltage (and therefore clipping). It might have been better represented and meaningful, but it is what it is.