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Topic: Which AAC encoder to use when downsampling? (Read 2078 times) previous topic - next topic
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Which AAC encoder to use when downsampling?

Hello!

I want to convert all my lossless music from Bandcamp / CD rips to AAC for my main folder and leave the original lossless files in another folder as an archive.

When converting, I used XLD (version 20210101) on MacOS Mojave to create 44.1kHz True VBR AAC files at 127 (~320kbps).

For anything that was at CD quality (16/44.1), XLD, iTunes at 320kbps VBR AAC and the iTunes Master Droplet all gave identical files after bit comparing them in foobar2000.

Now the problem or rather, question, I had was about down sampling anything higher than 44.1kHz to 44.1kHz.
When using XLD or iTunes to convert to 44.1kHz from a 48kHz file I noticed the frequencies get cut off at around 19kHz in the spectrograph. Then I tried the same with the official unedited iTunes Master AAC Droplet (https://www.apple.com/in/itunes/mastered-for-itunes/) and there was no cutoff.

So I assume the iTunes Master Droplet software apple provides gives a higher quality resample.
As per my findings I also noted that the iTunes Master Droplet gives the exact same file as the default "iTunes Plus" setting in iTunes when there is NO resampling involved.

I read that the iTunes Master Droplet first converts to a .caf (core audio file) from a WAV or AIFF then to the final 44.1kHz AAC, I am not sure if this is better or worse than directly converting to AAC for files that are not at 24bit/(more than or equal to 96kHz) like iTunes and XLD do, since Apple recommends only using 24 bit and sample rates as high as possible when submitting to iTunes.

Basically I wanted to know whether converting with XLD or iTunes was fine or if I should use the iTunes Master Droplet since it resamples without adding a sharp cutoff. Mainly because XLD is more convenient, keeps my tags so I don't have to import them again from CSV and XLD can let me output at True VBR 320 vs the 256 from iTunes and the iTunes Master Droplet. (all just for peace of mind and to have the highest quality AACs, not exactly for audible differences)

I admit that I can't hear the difference between any of these files (after multiple blind tests), but would still like to know the ideal solution for my lossless to AAC collection.

Thank you!

Re: Which AAC encoder to use when downsampling?

Reply #1 –
Also, I am curious as to how the files are the same when not resampling when using either iTunes Plus in iTunes or the iTunes Master Droplet, since they both convert differently (?)

Re: Which AAC encoder to use when downsampling?

Reply #2 –
The stuff above 19k is probably going to be mostly noise and clipping, and anyway you won't be able to hear it.  If you want to keep it anyway, use a better resampler before encoding to AAC.  It'll run a little slower but otherwise there isn't much downside.