Does ReplayGain degrade sound quality?
Reply #9 – 2007-08-31 20:22:31
MP3Gain has two modes, as does Foobar 2000. Mode 1 is via tags. On a non-supporting player, the MP3 playback is at the original loudness. On a supporting player it's adjusted accurately. Mode 2 is by modifying the global gain factor in each frame of the MP3 file (which changes volume in 1.505 dB steps, giving ±0.7503 dB accuracy). Mode 2 also adds a tag that corresponds to the modified file, so players that support Replay Gain can: a) still choose either Track or Album Gain mode b) make the fine adjustment to the accurate Gain value, even though 0.75 dB is hardly perceptible. (In this way identical output to Mode 1 can be achieved, because no data was discarded from the MP3 file) c) apply clipping prevention if desired Non Replay Gain aware players will just output at the volume chosen when the adjustment was applied (i.e. Track or Album was chosen when it was applied, possibly with clipping prevention enabled also). In practice, any quality reduction is almost certainly inaudible. 16-bit CD audio with flat dither and no fancy tricks has about -120 dB noise floor per frequency bin, giving roughly 120 dB of audible dynamic range. Often 96 dB is quoted for dynamic range, but this is actually the Signal to Noise Ratio, which is different because it applies to the whole bandwidth, not to each frequency bin. The ear effectively divides sound up into frequency bins and is comparable to a 1024-point FFT Power Spectrum (512 bins). A professional 18 inch chainsaw is about 113 dB above the threshold of hearing at full revs without ear defenders. You'd have to play your music roughly that loud to notice the increased noise floor of applying about 7 dB replaygain on a 16-bit playback system. That's why even 14-bit would have been quite adequate for CD digital audio. In short, while it's obvious that some elements of the electrical signal will sink into the noise floor if you have 16-bit playback, and that freaks people out, none of the audible sound will suffer this fate and it's no worse than turning down the volume to get the track to a comfortable level. It may actually improve the sound for anyone whose amplifier or DAC is non-linear or otherwise misbehaves at close-to-full-scale, though such problems are rare these days.