There's a very simple principle when dealing with mutimedia files: Crap in, crap out. If you convert .mp3 files to .wav, it is not going to sound any better.
But some people believe it and they will send you .mp3 files converted to .wav swearing it is lossless quality. How can you guess the real quality of a file without even having to put your ears to the test?
You need a spectrogram to analyze the file. This will give you a visual representation of its content, allowing you to detect problems. Adobe Audition has an excellent spectrogram, but today, we're going to use Spek, a very good spectrogram freeware for Windows, Mac OS X and Linux. Download it here.
Just drag and drop your audio file on Spek and pay attention to high frequencies: 16kHz and more. If they appear to have been "cut" or degraded above 16kHz, the audio file has probably been compressed.
Be careful though:
1) A clean and nice looking spectrogram does not mean the file is perfect, but if it looks bad, you can be certain there's a problem.
2) There can be an artistic degradation of the file. For example, the "Telephone voice effect" cuts everything above 4kHz.
3) Some instruments do not produce high frequencies. It is very well possible to have lossless tracks with no high frequencies at all.
Here are 3 spectrograms of different versions of the same track.
A. Lossless conversion, flac
B. Lossy high-quality conversion in ogg q10
C. Lossy low-quality conversion in mp3 CBR (constant bitrate) at 128kbits/s
There are almost no visual differences between the flac file and the ogg q10 but there is a big difference between the flac file and the mp3 CBR 128kbits/s file, meaning the file has been degraded.
So here you go, this method might not be perfect, but at least it can give you an idea about how bad an audio file is.
Spectrograms can also be useful to spot out clicks or whistling noise.
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