Mp3 or Not???

[jcrab66]

Ok, with the recent talk of different analyzers I thought I would throw some shots up here and see what some people think.. Here are some shots using Eac, CEP and Analfreq. Is it Mp3 or not???

[jcrab66]

here are the last two shots....

[Five]

mp3

but the results sure look different from program to program!

can you show us a cep spectral analysis, a full song and also a shot zoomed on on 1.5-2 seconds?

does the cep fa stay like that when you move the cursor around the cep display?

could you please post another shot of the EAC fa with the fft size set to maximum?

thanks. this is shaping up to be an enlightening thread.

[jcrab66]

Ok, here are the 3 requested shots, def looks like mp3 with the CEP Spec but the freq was sort of questionable

[paddington]

The steep 16kHz rolloff is a dead giveaway, in MOST cases. It's possible that the microphone sucked or that type of thing.

If you get 16k roll-off, you can be pretty sure it's MP3, BUT one more thing you can check beyond that is this:

When an UGLY roll-off filter is applied, like what the MP3 compression algo does, you get audible "aliasing" at the high edge of the low-pass band. LISTEN to it, and see if your high frequencies sound "watery" instead if crisp.

If you hear that mess and show a 16k roll off, you probably have MP3. If the taper used a crappy mic with bad freq response, you could still have the 16kHz roll off (as a coincidence), but you wouldn't hear the aliasing (watery high end) because no shitty filter was applied - the mic just didn't get it at all.

If I had to guess without listening, I'd say MP3.





As an aside - 16kHz is PLENTY high enough to enjoy a recording, it's the dirty steep roll-off filter on MP3s that ruin the audio. Yes, you are missing 16-20kHz, but the only things really there are harmonics and some REALLY high stuff you usually don't miss. When the you encode in MP3, the aliasing creates jagged stepped edges around 16kHz and those resonate harmonics down much lower, effectively ruining the audio well BELOW 16kHz, where it's very audible (and irritating).

[jcrab66]

thanks, i am aware of all that stuff, i primarily wanted to show how different analysis tools can give different result opinions just because of the software alone....

[U2Lynne]

Thanks James. I'm still learning about identifying mp3s, so what you wrote helps me out and makes lots of sense.

[paddington]

Originally Posted by jcrab66

thanks, i am aware of all that stuff, i primarily wanted to show how different analysis tools can give different result opinions just because of the software alone....

oh, in that case, some of your shots don't help much because the scale needs to be adjusted. If it seems like some of them are showing different things, it's the scale displayed. Some seem to not roll as badly until you see that the audio modulation level is like -160dB or something . You generally won't hear much below -72dB or so.

Five has a thread posted stating that CEP 2.0 or Adobe Audition 1.1 do the best job of analyzing for this kind of thing and I agree. Just get the scale adjusted appropriately.

[Five]

It would be cool to see if we could get the same results from several fft programs by tinkering with settings.

this show it seems we need the sa to really see what's going on. the fa looks kinda flat in the highs but the drop isn't visible as with AnalFreq. EAC's sa you can see the drop at 16kHz at the highest fft setting, even tho it looks a little different.

the cutoff just above 16kHz in a straight line, this is the classic case of lossy source, since this is how 128kHz CBR mp3 displays. Also, the little legolike shapes visible when you zoom in with CEP indicates lossy.

I can't seem to figure out how to adjust the scale of the fa for cep... when I pull on the left bar it just moves up and down, and there's nothing in the advanced options I can see.

Oh, while I was doing this I discovered something.

I opened CEP, loaded in a short lossy-sourced .wav I had on my desktop and the fa looked like this:

[Five]

then, when I clicked on the display, the fa suddenly changed to look like this:

[Five]

I selected all, and the fa looked like this:

[Five]

So I guess the morale of the story is when you do fa with cep you need to click your cursor anywhere on the waveform display to get the fa window to come to life.

hey jcrab66, can you try this out with your file and see if you get a different wav display like I did?

here's the sa for same file as the previous three fa shots I posted:

[diggrd]

fa in CEP is always gonna show either what's selected or just where the cursor is. It also gives the matching musical 'denomination' which I find interesting. I'm still running 1.2, I wish I has upgraded to 2.0. Now if I could find the disc it would save me what $130 bucks instead of $230

[62v8]

With respect to the discussion about 15KHz and up ...
Doesn't FM and TV audio cut off at 15KHz as well?

Curious as to how do you tell the difference or does that become subjective assesment?

[Five]

diggrd, what I am suggesting is that CEP 2.0+ has a little bug where you have to click on the waveform to get the correct fa to display. Then you can select all or move the cursor and the originally displayed fa never comes back (the one with the flatline above 14KhZ).

62v8, you are correct about this. FM cuts off around the same place, but not quite so abruptly. The slope on the fa will be less steep, and in spectral view the tops of the spikes will be more uneven. Furthermore, when you zoom in you shouldn't be able to find any "blocks". 32kHz DAT cuts off in much the same way and can be checked in much the same way. TV-sourced stuff usually has the famous red stripe visible roundabout 16kHz and will cut off unevenly like FM somwhere just below that. The fa will show a spike where the red stripe is too. There will normally be at least a little bit of activity (99% noise) above 16kHz, with the possible exception of 32kHz DAT digitally transferred. These ones it's extra-important to zoom in and check for those giveaway lossy lego blocks visible in the sa.

FM, TV and 32kHz DAT all have mostly no activity above 16kHz, but they are superior to lossy sources because they are free from audible lossy aritifacts like slurring of hihats and those little noises that are like scifi movie sound fx.