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Spek
If it is upscaled from an MP3, it would be truncated at 15- 16 khz.
If it is loseless, it would extend beyond 20 khz.
Its a great piece of software and I ran a few files for comparison and found that upscaled MP3 or MP3's converted to flac does not give any benefit (other than increasing the file size) and remains a lossy format.
Whereas, a true flac will give the complete bandwidth and extend beyond 20 kHz as you have stated.
For this I've attached a comparison of a song from the film Murder. I own the audio cd as well a downloded MP3 and an upscaled MP3.
MP3...
Upscaled MP3...
True Flac...
It is clearly evident from the above spectrograms that true flac (ripped from the original audio cd) gives an uncompressed audio quality with full frequency band.
The MP3 and the upscaled graphs are truncated at about 18 kHz and show that the lines spread throughout are thin and transparent compared to the flac graph and are 99.9% same and there is no enhancement in music when heard.
Whereas, the true flac graph shows a complete band upto 22 kHz and its spread with thick lines throughout.
Its very simple... garbage in ... garbage out.
If one wants uncompressed, quality digital music, audio cd's/online stores who sell uncompressed audio are the only way around.
Note: I'm aware that the faint red lines at the bottom at the 0 kHz are for the bass, but I'm curious to understand more about reading these graphs in a proper way.