Help. How to evaluate a Mp3's quality? Like noise existed in the f Options

[MaiTiano]

As mentioned in the title.

Now I am evolving into a project. In this project, we have a 128kbps mp3 library and we need to find which files have the inferior quality, such as noise existed, volume is too small etc.

Any advice? It would be better if someone know some open-source project which already has the ability to detect the mp3 files with bad quality.

many thanks.

[Dynamic]

You've defined your problem a bit better.

You might want to look into SoX audio toolkit. It can be run from the commandline or a batch script, making it possible to deal with a whole bunch of files. It's also open source, like Audacity, which you used to plot that waveform.

It has switches to detect silences, which you could modify to make it detect or split that file:
http://sox.sourceforge.net/sox.html
... scroll down to the silence section.

You could either modify the source code to do what you want, or run Sox to decode an appropriate file and see if it produces more than one silence

Your use case is not normal, as few people who care about quality will have such bad files because they'll have chosen secure or database-verified ripping or listened to the output with their ears.

I'm sure you'll need to do some coding yourself.

[MaiTiano]

You might want to look into SoX audio toolkit. It can be run from the commandline or a batch script, making it possible to deal with a whole bunch of files. It's also open source, like Audacity, which you used to plot that waveform.

It has switches to detect silences, which you could modify to make it detect or split that file:
http://sox.sourceforge.net/sox.html
... scroll down to the silence section.

Many thanks, Dynamic.
I have a problem about your advice. As can be seen from the figure, actually, those horizontal wave which I highlighted in the red box is not on the 0 dB line. In other words, the sound track may not be silence in these area, right?

[saratoga]

I have a problem about your advice. As can be seen from the figure, actually, those horizontal wave which I highlighted in the red box is not on the 0 dB line. In other words, the sound track may not be silence in these area, right?

Since you can't hear a DC level, that is still silence.

[MaiTiano]

Since you can't hear a DC level, that is still silence.

Thanks. Actually, I think it is approximate DC

[Dynamic]

I'd assumed, perhaps wrongly, that you were displaying Waveform (dB) rather than linear, in which case those were probably only a fraction offset from zero, and setting silence detection to -80 dB might even be OK.
The other approach is to plot a spectrogram - which SoX can export as a PNG image. The silences, whether or not they contain a DC offset as shown, would appear as black in all frequencies. That might be easier to interpret or to analyse graphically or numerically, particularly if converted to an ASCII-encoded graphical format like PPM or PGM (part of PNM or Netpbm format family).

[MaiTiano]

I'd assumed, perhaps wrongly, that you were displaying Waveform (dB) rather than linear, in which case those were probably only a fraction offset from zero, and setting silence detection to -80 dB might even be OK.
The other approach is to plot a spectrogram - which SoX can export as a PNG image. The silences, whether or not they contain a DC offset as shown, would appear as black in all frequencies. That might be easier to interpret or to analyse graphically or numerically, particularly if converted to an ASCII-encoded graphical format like PPM or PGM (part of PNM or Netpbm format family).

I would like to give you a even clear figure which includes the Y axis and even zoomed in view in it.



For the advice of using Sox to draw a DC spectrogram, I have three questions:
[1] I can get the spectrogram PNG image by Sox, but is it standing for the spetrum features of the whole song long?
[2] How can I get the correct part of these silence area? For example, 57.25s ~ 57.36s is silence part and it would be more likely to see the all black in all frequencies right within this time period. In other words, if I analysis the waveform between 57.15s~57.36s, maybe the spectrum is not all black in all frequencies, right?

Hope to get your help. Many thanks.