I meant -15.3 RMS and -.3 peak. And bingo, you were correct in your last guess. I feel dumb for not thinking of it myself. He's using a 24 bit interface, but he must not be outputting 24 bits because when I tested the file, it's only actually 16 bits of audio. Next time, that's the first thing I'm checking :)
Thanks again for all the help. I knew something about this didn't seem right... Scott On Thu, Dec 2, 2010 at 6:38 PM, Brian Willoughby <[email protected]>wrote: > > On Dec 2, 2010, at 14:53, scott brown wrote: > >> original 24/48 wav file: 264,904,968 bytes >> flac level 8: 105,992,780 bytes >> >> dithered 16/48 wav file:173,885,996 bytes >> flac level 8: 108,700,948 bytes >> >> truncated 16/48 wav file: 173,885,996 bytes >> flac level 8: 105,224,448 bytes >> >> RMS level of original 24 bit: -15.3dB with peaks at -0.3dB >> >> if I normalize the original file to a max of 0.0, the resulting flac file >> is 192,798,482 bytes. weird.... >> > Normalizing will probably always make the FLAC larger. Of the many > components of the FLAC algorithm, one is to use differential values rather > than absolute, and another is to use variable-length coding (Rice coding). > Quieter files have smaller differential sample values, and thus compress > more. > > I am surprised that a mere 0.3 dB normalization would add over 80% to the > FLAC size. Perhaps you missed something? Can the RMS level actually reach > -0.3 dB without serious distortion? ... or did you mean -15.3 dB RMS and > -0.3 dB PPM? > > Comparing the 24-bit WAV to the 16-bit WAV, it looks like as much as 4 MB > of non-audio data is in the file. You may be looking at waveform overviews > or other extra chunks in the WAV file which are usually discarded by FLAC. > But this only explains a small part of the surprising numbers. > > One thing that stands out to me is that your original 24/48 WAV may not > actually have 24-bit samples in it. I wrote a program which could detect > this situation (16-bit samples in a 24-bit file), but I do not know of any > other available tool to test this. It seems very suspicious that the 24-bit > FLAC and truncated 16-bit FLAC are within 0.73% of each other (i.e., less > than 1%). I have a suspicion that your friend is recording 24-bit files > from a 16-bit A/D converter interface, or at least the software is set for > 24-bit files while the interface is set to 16-bit mode. You should have the > recordist double-check all settings. A mistake here would easily explain > the strange FLAC compression ratios. > > Brian Willoughby > Sound Consulting > >
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