Audacity Forum

Hi guys,

Here's what I want to do: I have a FLAC version of a song and the same song converted to 128 kbps MP3 with Audacity. I want to invert the soundwaves of the 128 kbps and overlap them on the FLAC waves so that the sound cancels out. This way, all that will be left will be the extra sound added by the MP3 conversion process (noise/artifacts). After repeating this process with 192 and 320 kbps MP3s, I'll be able to see exactly how much noise each bitrate adds, which should better help my decisions in the future about ripping media to MP3.

However when I open both versions of the song in audacity, copy the MP3 track and paste it onto the FLAC track, it overwrites the FLAC song, rather than overlapping with it. How do I make it so I can invert the MP3 soundwaves and make they cancel out with the FLAC ones instead?

Thank you very much!

I have never done that with Audacity, but there is a serious flaw in the technique! The sound of the difference (subtracted files) is NOT the difference in the sound! Weird, eh?

MP3 makes phase & timing changes that may not be audible, but they show-up in difference testing...

It's true, that if you subtract two files and you get absolute silence, the two files are mathematically identical, and of course they will sound identical. And, we already know that MP3 is lossy, and we know that low bitrates are more-lossy than high bitrates...

But for example, if you add 10 milliseconds of silence to the beginning of a file it will sound identical to the original. But, if you subtact the two files you will get a comb-filtered file showing a drastic difference that bears no resemblence to a tiny delay... The sound of the difference is not the same as the difference in the sound.

Another example - If you invert the phase the file will sound identical (or very-nearly identical). Now if you "subtract" that file from the original, you are of course actually adding the two files together (subtracting a negative), resulting in a difference-file that's twice as loud as the original, but otherwise identical (assuming no clipping).

After repeating this process with 192 and 320 kbps MP3s, I'll be able to see exactly how much noise each bitrate adds, which should better help my decisions in the future about ripping media to MP3.

There is software available for performing blind ABX tests to determine if you hear a difference in your program material.

It turns-out that program materal has a lot to do with it... Some songs easier to compress than others, and the variable bitrate LAME "V" settings try to take this into account, using a higher bitrate when necessary and a lower bitrate when possible. So rather than choosing a constant bitrate like 192, it's usually best to choose a "V" setting that averages-out around 192.

P.S.
Like I said, I've never done this with Audacity, but take a look at Mixing. Mixing is done by addition (both in analog mixers and digitally), so inverting and mixing with the original will give you the result you are looking for.

Thank you for your great reply. Since you say the MP3 conversion process has timing changes, what if instead of doing 128 kbps MP3 to FLAC, I do 128 kbps MP3 to 320 kbps MP3, both derived from that FLAC? Will there still be timing changes between 128 and 320 to invalidate this process?

Furthermore, I have followed your advice and downloaded fubar2000, as well as the ABX test plugin for that player. It actually uses two unknown samples: X and Y, while A is the lower bitrate version and B is the higher one. I converted the FLAC of a relatively "busy" (lots of instruments going on) song (the first track of this incredible EP: http://ramesesb.bandcamp.com/album/inspire-ep) to 320, 128 and 56 kbps MP3 versions. While I can tell the difference between 320 and 52 kbps with 100% accuracy after 12 tests, since the instruments lose alot of their clarity, I have found that I cannot reliably tell apart 128 kbps from 320 kbps, even when doing A vs. B alone (rather than X & Y vs. A & B). I wonder if I'm that bad of a listener, or it's my equipment! I have an ASUS p8p67 Deluxe motherboard with its inbuilt sound card and Razer Carcharias headphones, which are supposed to be quite quality.
To test if it's me or the equipment, I'll try asking friends to do this test on my machine

Thanks again.

Oh, I tried it again with a violin piece, I could reliably tell 128 from 320 in a part with a transition from a low pitch to a high pitch in the violin. I got 9/9 right, meaning a chance of me being guessing of only 0,2%.

192 vs. 320, however, was indistinguishable.