Re:FLAC – Format overview
by crazygringo on Monday September 05, 2022 @08:15PM
(#32731012)
Yup, it's incredibly clear in the spectogram.
You're basically looking for two things: first, an abrupt stop to frequencies above a certain point -- compression uses applies a low-pass filter to eliminate them.
Second, a bunch of rectangular-ish boxes where signal is missing. The spectrogram will look like swiss cheese in a way -- this is where the compression algorithm decides audio information can be deleted because it's "masked" by e.g. louder surrounding sounds.
(Generally speaking, the more aggressive the compression, the lower the frequency ceiling, and the more/bigger swiss cheese holes.)
On the other hand, if you run e.g. AAC compression at a very high bitrate and explicitly force no lowpass filter at all (which virtually nobody does in practice, but you can do from a command-line tool)... you won't be able to see any qualitative difference. Because you won't be able to hear any qualitative difference either. :)
You're basically looking for two things: first, an abrupt stop to frequencies above a certain point -- compression uses applies a low-pass filter to eliminate them.
Second, a bunch of rectangular-ish boxes where signal is missing. The spectrogram will look like swiss cheese in a way -- this is where the compression algorithm decides audio information can be deleted because it's "masked" by e.g. louder surrounding sounds.
(Generally speaking, the more aggressive the compression, the lower the frequency ceiling, and the more/bigger swiss cheese holes.)
On the other hand, if you run e.g. AAC compression at a very high bitrate and explicitly force no lowpass filter at all (which virtually nobody does in practice, but you can do from a command-line tool)... you won't be able to see any qualitative difference. Because you won't be able to hear any qualitative difference either. :)