MP3/OGG Transcoding and Volume

Epsilon · Dec 24, 2007, 11:24 AM

Ok, so I know that transcoding these two formats is not recommended in terms of quality, but my question has to do with the volume. If you transcode either way, will the volume of the audio be affected? If so, how?

Thanks.

LiquidAcid · Local time: Dec 25, 2007, 11:51 PM

You almost answer your own question. The volume is of course affected because decoding the source audio and compressing this material again won't reproduce the source audio when playing back the newly compressed file. That's the only reason for a volume change. And although the change in volume is probably measurable I doubt you can say with only your ears which one of the files is louder.

If you're aiming at a different reference volume for the files then the answer is no. Assuming that both ogg and mp3 can perfectly reproduce a simple sine tone, then encoding a sine tone of +0 dB volume level should produce a mp3 and an ogg file that both output a +0 dB signal.
What is possible however (I know that only for MP3) is to change a volume scaling entry in the MPEG frame header which directly affects how much the resulting waveform (after the reconstrucing process) is scaled. That's what the permanent mp3gain process does to the files (instead of writing metadata into the files).

Personally I wouldn't be much affected even if there was some volume change because I have all my properly tagged files also replaygain processed.

Epsilon · Dec 26, 2007, 08:27 AM

Originally Posted by LiquidAcid

You almost answer your own question. The volume is of course affected because decoding the source audio and compressing this material again won't reproduce the source audio when playing back the newly compressed file. That's the only reason for a volume change. And although the change in volume is probably measurable I doubt you can say with only your ears which one of the files is louder.

If you're aiming at a different reference volume for the files then the answer is no. Assuming that both ogg and mp3 can perfectly reproduce a simple sine tone, then encoding a sine tone of +0 dB volume level should produce a mp3 and an ogg file that both output a +0 dB signal.
What is possible however (I know that only for MP3) is to change a volume scaling entry in the MPEG frame header which directly affects how much the resulting waveform (after the reconstrucing process) is scaled. That's what the permanent mp3gain process does to the files (instead of writing metadata into the files).

Personally I wouldn't be much affected even if there was some volume change because I have all my properly tagged files also replaygain processed.

Awesome. Yeah, I was referring more to the reference volume, but you answered that. Well, this'll help me a lot. Thanks!