Quote:
Originally Posted by range_hood
Looks like a lowpass at around 9kHz and adding the above in some strange way. Mirroring is present, but in a (for me) unusual way.
Seems like the frequencies (4kHz to 9kHz) were copied to 9~14kHz - and above they were mirrored.
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Yes, that's basically how mp3pro (and he-aac) work: they make a "normal" encode of the lower part of the spectrum where human hearing excels, spending almost their entire available bitrate there. Additionally, they generate a low-bitrate stream (maybe 2kbps) of auxiliary information that enables the decoder to artificially synthesize the treble range.
This encoding tool - usually referred to as SBR (Spectral Band Replication) - enables these formats to spend most of their bits on the critical parts of the spectrum, thereby improving the perceived quality of relatively low-bitrate encodes.
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