Identifying Lossy Material

[techtuts]

Firstly I recommend downloading iZotope RX5 Advanced Register to view links. You get a free 10 day trial and after that saving is disabled and you are limited to 30 seconds of continuous playback. This doesn't matter since you only need it for spectral analysis.

The first thing to do is to go to View - Spectrogram Settings and use these settings;


You can then open a file and zoom in to a smaller time range and zoom into the frequency range around the cut off.
A typical MP3 will look like this (you can see the blocks and holes at the top):

or this:


A file with a similar cut off (but without the blockiness) can be a 32kHz recording;

You can use EQ match on the bright/clear midrange preset to make the cut off easier to see.
You will notice that the frequencies below 16kHz seem to be mirrored above, this an artifact due to upsampling from 32kHz to 44.1kHz.