"Don Pearce" wrote in message
...
On Wed, 6 Feb 2008 10:07:37 +0000, Stimpy
wrote:

On Tue, 5 Feb 2008 23:25:36 +0000, Don Pearce wrote
On Tue, 5 Feb 2008 21:01:32 +0000, Stimpy
wrote:

I've been sent some .mp3 files that claim to be 320 kbps but they sound
pretty ropey. Is there any way I can detect if they've been re-encoded
from a lower bitrate?

Do an FFT- the maximum frequency they contain will be a clue. If they
are native 320k, they should go all the way to 20kHz; if they have
been 128k at some point, they will probably stop somewhere around
16kHz.

That's good advice, thanks. I'm afraid I'm a bit dumb when it comes to
technical stuff, what's a FFT? Is there a tool you'd recommend I use to
check the mp3 in question?

Thanks again

Ah, ok. Get yourself a copy of Audacity (freeware) and use it to look
at the files. It is a good general purpose audio editor that you will
find useful for all sorts of other stuff too.

Go to the "Analyze" menu, and select "Plot Spectrum". You will get a
graph showing what frequencies are there.

FFT is short for Fast Fourier Transform. What it means is that you can
analyse the spectrum of the music rather then just looking at the
waveform and trying to figure out what it is doing. An FFT is what the
plot spectrum function in Audacity does.

You're entirely too cool. I've been familiar with the term FFT for a while,
but while I can even claim to be able to pronounce it properly, I've never
quite wrapped my mind around what it _is_. I've also wondered about
detecting reencoded MP3s, too. Of course, I don't make MP3s
anymore....since I learned that my iPod plays AAC (but sadly, not HE-AAC).