Skip to main content ↑ ↓

Notice

Please note that most of the software linked on this forum is likely to be safe to use. If you are unsure, feel free to ask in the relevant topics, or send a private message to an administrator or moderator. To help curb the problems of false positives, or in the event that you do find actual malware, you can contribute through the article linked here.
Topic: iTunes AAC files (Read 8821 times) previous topic - next topic
0 Members and 1 Guest are viewing this topic.

iTunes AAC files

I usually notice that iTunes AAC VBR produced files don't have a lowpass... Curiously the Nero AAC VBR do have some kind of 16kHz lowpass, as well as  LAME VBR 0, in the realm of 19kHz. I know that neither of this means actually something audible, but howcome iTunes AAC does not follow the same principle when encoding very high frequency?

iTunes AAC files

Reply #1 –
The files I encode at 128 VBR are lowpassed qt 17.5KHz consistently. I use Audacity to plot the spectrums.

iTunes AAC files

Reply #2 –
I believe the iTunes/QuickTime AAC encoder won't enforce a lowpass at 224kbps settings and above.  Curiously though, it used to apply a lowpass even when using the highest true VBR setting.  But I believe that has changed so that settings in the 224kbps range won't apply a lowpass either.  Different encoders do things differently.  There isn't one right way for every encoder and format to handle data.  Nero is going to implement something different than the Lame devs and they are going to be different that what Apple does.

iTunes AAC files

Reply #3 –
Yes, the 256kbps VBR AAC files don't have a lowpass enforced.

iTunes AAC files

Reply #4 –
Is this mean that analyzers such as Audiocheker and Tau Analyzer will report upconverts from such AAC files as CDDA?

iTunes AAC files

Reply #5 –
Quote from: AcidDan on
Is this mean that analyzers such as Audiocheker and Tau Analyzer will report upconverts from such AAC files as CDDA?

Why not try some and report back.

iTunes AAC files

Reply #7 –
Ok, I ripped Jan Hammer's "Crockett's Theme" from my CD to FLAC format.
Encode resulting file to MP3 and AAC formats with different encoders using foobar2000 and iTunes.
Then convert lossy files back to FLAC and analyze them with auCDtectTaskManager 1.5.1 RC4.
Results are surprising: this analyzer report that upconverts from AAC files encoded with highest settings is actually CDDA track
But bitrate in this cases are insane



FLAC


MP3 @ 252 kbps


AAC @ 278 kbps


AAC @ 278 kbps


AAC @ 349 kbps


AAC @ 251 kbps


AAC @ 401 kbps

iTunes AAC files

Reply #8 –
How did you do this with Sox, can you tell?
Looks so cool !

iTunes AAC files

Reply #9 –
Quote from: krafty on
How did you do this with Sox, can you tell?
Looks so cool !

It's not me  - it's auCDtect Task Manager program.
After analyzing - double click on row in table with results and in opened dialog select "Spectrogram" tab.
In this tab generated image with option to save it to file with button click.
Unfotunately, i don't know what parameters passed to Sox from this program.

iTunes AAC files

Reply #11 –
This wasn’t elusive:
Quote
spectrogram [options]
Create a spectrogram of the audio; the audio is passed unmodified through the SoX processing chain. This effect is optional - type sox −−help and check the list of supported effects to see if it has been included.

iTunes AAC files

Reply #12 –
Quote from: greynol on
What's the point?

I was just curious how analyzers will detect upconverts from AAC files without lowpass filter applied.

iTunes AAC files

Reply #13 –
I've actually "tested" music I purchased from iTunes* and have seen auCDtect consistently identify it as CDDA. I did this out of curiosity as the spectograms always looked like CDDA. (I admittedly know far too little about this...)

*iTunes files identified as AAC @ 256 Kbps.
The Loudness War is over. Now it's a hopeless occupation.

iTunes AAC files

Reply #14 –
They only look like lossless sources inasmuch as they are not completely lowpassed. However, the AAC files (even that with the highest-bitrate) still have some quite visible cutting of the very highest frequencies.
 

iTunes AAC files

Reply #15 –
Quote from: kornchild2002 on
I believe the iTunes/QuickTime AAC encoder won't enforce a lowpass at 224kbps settings and above.  Curiously though, it used to apply a lowpass even when using the highest true VBR setting.  But I believe that has changed so that settings in the 224kbps range won't apply a lowpass either ...


That seems about right. I haven't tested CBR below 224 but I'm guessing it still applies a lowpass like it did before. With XLD and it's true VBR setting, QuickTime applies a lowpass at the setting 95 (~195) and below while it doesn't at 96+ (225+)