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Topic: low pass frequencies in iTunes AAC (Read 6073 times) previous topic - next topic
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low pass frequencies in iTunes AAC

Does anyone know what the low pass frequencies are for each of the encoding bitrates when using iTunes 4.1.1 + QT 6.4? I just loaded an AAC file in Cool Edit and my 192kbps encodes are low passed at about 17.5k. I expected a little bit higher, to be honest, especially when compared to LAME aps...

low pass frequencies in iTunes AAC

Reply #1
I think most encoders do low pass the spectrum. For example the FHg Mpeg4 AAC encoder would also cut off the upper frequency components.. At 96kbps, it actually downsampled to 32Khz sampling rate.. At 128kps, the cut-off freq is about 18 kHz.. 

However, at 192kHz, I don't think you need to lowpass at all..

low pass frequencies in iTunes AAC

Reply #2
At 128 kbps, QT will lowass to little bit less that 16 kHz (probably 16 kHz and less is the filter property)

FhG usually has more conservative lowpass strategies,  earlier it was 15.5 kHz for their professional encoders, and now it is as same as QT - very near 16 kHz.

At 192 kbps, QT lowpass is set up to 17750 Hz

This parameter is because encodings are CBR, and during the encoding psymodel/bit allocation calibration process, the cut-off is being set in order to equalize all artifacts (bandwidth loss, noise loudness, etc..) to some "best" value from the engineering point of view.

low pass frequencies in iTunes AAC

Reply #3
Quote
At 192 kbps, QT lowpass is set up to 17750 Hz


What about higher bitrates, like 224 and 256, will the lowpass happen at higher frequencies?

low pass frequencies in iTunes AAC

Reply #4
224 kbps - 18-18.1 kHz

256 kbps - 20-20.1 kHz

Results are for 44.1 kHz, Stereo, QT6.4

low pass frequencies in iTunes AAC

Reply #5
There's little real-world music where a 17.5 lowpass is audible. Go try to find any.

low pass frequencies in iTunes AAC

Reply #6
Hey Ivan. What FhG encoder are you talking about? The new one that is aviable for download at their site?
I wonder if it's the old AACdemo 3.0.

In Liquifier Pro beta 24 one can set the desired cutoff self, at any bitrate. This program uses FhG encoder.

low pass frequencies in iTunes AAC

Reply #7
No, from what I saw that encoder is their "slow" (professional) encoder they earlier called "AAC 2.x" in the MPEG-2 days , that codec is  very similar to one used in Liquifier 4.0 and 5.0 - but probably more advanced and bugfixed and, of course with MPEG-4 tools (PNS, LTP, ..  but I am not sure if they are enabled).

They probably do not allow custom lowpass frequencies because encoder parameters were tuned for smallest possible distortion for each bit rate preset. Changing the lowpass would probably make encoder work in "non-optimal" mode and they prolly have a reason why they do not allow that.

Basically, their faster encoder (v3.x) has two modes - fast,  and "fastest",  fast one uses same lowpass as "professional" encoder, and fastest uses little bit less bandwidth IIRC.

low pass frequencies in iTunes AAC

Reply #8
At 160 kbps, QT lowpass is set up to ~ 17000 Hz against ~ 19250 Hz for the FhG encoder (144 kbps) you can get for download.
(according to my recent (quick) test).

I don't know if this encoder is the old 2.x or if this is the profesional version or not but concerning preecho (and BTW the short block test), it sucks... and Dolby AAC is much better.

to cbope:
I don't think a high lowpass is the most important, can you really hear a difference between a lowpass 17 kHz and a 19 kHz one ?
I understand that you may be a little disappointed to know that Lame APS goes higher in frequency but i'm sure QT will produce better quality at the same bitrate. 

low pass frequencies in iTunes AAC

Reply #9
Well - it's not that bad to rate it as "sucks", certainly there are better developments nowdays  But not that bad, especially not at pre-echo handling,  most echoing problems are due to over-TNSing  which is pretty common to FhG's encoders. 

Anyway it is very slow.

low pass frequencies in iTunes AAC

Reply #10
Quote
Well - it's not that bad to rate it as "sucks"

I agree with you Ivan. 

but the fact is this encoder is worst than Lame or even FhG MP3 when it comes to handle pre echo... 
but i'm sure the profesional version, used in Sorenson Squeeze 3.5, should produce better results.

low pass frequencies in iTunes AAC

Reply #11
Can you upload some clips which were sounding that bad - I'd like to check them out because they might trip out some other codecs as well.

Such a high lowpass (19.250 kHz) for 144 kbps is little bit strange..

low pass frequencies in iTunes AAC

Reply #12
Quote
Can you upload some clips which were sounding that bad - I'd like to check them out because they might trip out some other codecs as well.

Such a high lowpass (19.250 kHz) for 144 kbps is little bit strange..



Ivan, there are available @:
http://nick.vallet.free.fr/samples/
cheers !

low pass frequencies in iTunes AAC

Reply #13
Quote
Can you upload some clips which were sounding that bad - I'd like to check them out because they might trip out some other codecs as well.

Such a high lowpass (19.250 kHz) for 144 kbps is little bit strange..

Ivan, there are available @:
http://nick.vallet.free.fr/samples/
cheers !

low pass frequencies in iTunes AAC

Reply #14
Ivan,
i've noticed that the recent Fraunhofer encoder fails quite badly with a sample taken from the beginning of David Bowie - Rebel Rebel song. Even at 144 kbps there are a lot of dropouts which causes a very annoying ringing artifacts. A spectral view showed that the frequencies between 9.5 kHz and 13.5 kHZ are throwed away. With Apple QT @128 the sample is very good. Sorry i can't test for the recent version of your encoder, i've an old version (aac.dll v2.5.6.0) and there is still a slight ringing at 128 Kbps (clearly better that Fhg but not as good as QT).
The same problem is audible (and visible) at 128 kbps with the mp3 Fhg encoder bundled with CEP2.1, LAME is much better.

You can download the original sample and see some sprectral views of encodings here: http://members.xoom.virgilio.it/fofobella/Rebel/
WavPack 4.3 -mfx5
LAME 3.97 -V5 --vbr-new --athaa-sensitivity 1

low pass frequencies in iTunes AAC

Reply #15
Quote
Quote
Can you upload some clips which were sounding that bad - I'd like to check them out because they might trip out some other codecs as well.

Such a high lowpass (19.250 kHz) for 144 kbps is little bit strange..

Ivan, there are available @:
http://nick.vallet.free.fr/samples/
cheers !

WOW that sample doesn't work right with replaygain!  Once i encode it to iTunes AAC and then replaygain it in foobar the AAC file and wav file have obviously different volume levels.  Any idea why that would be?