[LAU] Rolling off high frequencies when mastering?

Niels Mayer nielsmayer at gmail.com
Wed Apr 21 05:46:59 UTC 2010

On Tue, Apr 20, 2010 at 6:34 PM, Ken Restivo <ken at restivo.org> wrote:

> Kind of a general mastering question, but obligatory Linux screenshots of
> JAPA are included, I promise.
> I've noticed with some professional cd's/oggs/mp3s I have, the high end is
> rolled off at around 20Khz.
> Some roll off hard core:
> http://storage.restivo.org.s3.amazonaws.com/rolloffs/hardrolloff.png
> Some have a softer, gentler rolloff, but they still roll everything off:
> http://storage.restivo.org.s3.amazonaws.com/rolloffs/softerrolloff.png

The roloff is done to prevent Nyquist artefects from being audible -- the
44.1KHz CD sampling rate means you can't have anything above 22.05Khz or
you'll hear it as aliasing:
http://en.wikipedia.org/wiki/Nyquist_frequency#The_aliasing_problem . LPF's
aren't perfect, and can cause other annoying artifacts such as phasing or
ringing as they progress to the theoretically ideal brickwall filter (
http://en.wikipedia.org/wiki/Brickwall_filter#Brick-wall_filters ). So some
recordings might choose a gentler slope, rolling off at a lower frequency in
order to hit the drop-dead at 22.05Khz. This may account for the
observations made.

The other issue is that MP3 encoders themselves have filters built-in in
order to prevent aliasing artifacts at low bitrates. For example, in
Fedora&Ubuntu the "stock" sound-juicer/rhythmbox/etc settings cut-off at
16Khz, even if you up the bitrate. I fix this by setting
 'gconf-editor /system/gstreamer/0.10/audio/profiles/mp3/pipeline'  to:

audio/x-raw-int,rate=44100,channels=2 ! lame name=enc mode=0 vbr=4
> vbr-quality=0 quality=0 vbr-min-bitrate=192 vbr-max-bitrate=320
> lowpass-freq=20500 ! id3v2mux

Note the lame(1) arguments that affect the LPF:

       --lowpass freq

              Set a lowpass filtering frequency in kHz.  Frequencies above
> the

              specified one will be cutoff.

>        --lowpass-width freq

              Set the width of the lowpass filter.  The default value  is
>  15%

              of the lowpass frequency.

Note that many built-in settings in LAME automatically set the HF roloff
frequency. Particularly, with VBR, the issue is that the bitrate is
changing: if you have VBR from 128k-320kbps, you need to set the filter for
15.5Khz.in order to prevent aliasing at the lowest bitrates: (
http://lame.sourceforge.net/gpsycho.php )

Lowpass filtering based on the compression ratio. For high compression
> ratios, low pass filtering will improve the results. The exact amount of
> filtering needed depends on the music and personal preferences - the formula
> to decide how much lowpass filtering to use may need some tuning. At 256kbs,
> no filterings is done. At 128kbs, the lowpass filter is around 15.5khz.

Note that I set the min VBR at 192k so I can move my filter up to around
20Khz. As the default 15.5K cutoff sounds nasty.

 -- Niels
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