## Topic: Exact frequency boundaries values of mp3 scalefactor bands (Read 4579 times)previous topic - next topic

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• deranger
Exact frequency boundaries values of mp3 scalefactor bands
##### 19 October, 2009, 07:25:53 PM
As far as I know, in mp3 there are 22 scalefactor bands, going from 0 up to 21. But despite its name, the last sfb (ie sfb21) has no scalefactor.. The missing scalefactor for sfb21 is said to be the reason why frequences from this band are often not encoded when there's not enough bitrate.

I've searched through the forum and didn't find any table with the exact values of frequency boundaries for mp3 scalefactor bands.
The reason I ask the question is that I want to know - does sfb21 starts exactly from 16000 Hz or not ?
I thought it was, but currently an article in Wikipedia about MP3 states that There is no scale factor band for frequencies above 15.5/15.8 kHz. Is this statement incorrect ?

If the frame of an MP3 file does not feature sfb21 - will this frame have a high-freq cut-off exactly at 16000 Hz ?

• lvqcl
• Developer
Exact frequency boundaries values of mp3 scalefactor bands
##### Reply #1 – 20 October, 2009, 12:20:52 PM
http://www.hydrogenaudio.org/forums/index....st&p=652942

So for 48000 Hz samplerate sfb21 starts from 16000 Hz, for 41000 - from 16001.56 Hz, for 32000 - from 15278 Hz (if I did my calculations correctly).

• deranger
Exact frequency boundaries values of mp3 scalefactor bands
##### Reply #2 – 21 October, 2009, 06:15:37 PM
Thank you for the info.

Is it correct to say, that if a 44100 Hz samplerate WAV file (for example, a copy of a track from a bootleg Audio CD) has a sharp high-frequency cut-off starting at exactly 16000 Hz - then it's more then possible that this WAV is actually a decomressed MP3 ?

Many MP3 encoders are known to "throw away" sfb21 frequences when bitrate is not high enough (and XING encoder cuts at 16 Khz all the time).
But are any encoders known to throw away sfb20 and lower sfb's when not enough bitrate ?

p.s. Did I get it right, that this is the table to calculate sfb range for 44100 Hz:

tableScaleBandBoundLong 44100 = [  0,  4,  8,  12,  16,  20,  24,  30,
36,  44,  52,  62,  74,  90, 110, 134,
162, 196, 238, 288, 342, 418, 576]

so sfb21 starts at 44100/2 * 418/576 = 16001,5625 Hz
and sfb20 starts at 44100/2 * 342/576 = 13130,46875 Hz
and sfb19 starts at  44100/2 * 288/576 = 11025 Hz