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Topic: Lowpass (Read 3113 times) previous topic - next topic
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Lowpass

Hello,

I did recordings from older tape deck at 48 kHz sample rate and would like to filter (low-pass) frequencies above e.g.  approximately 18 kHz. Using SoX, is it OK to use

sox.exe in.flac out.flac sinc -18000

or  rather

sox.exe in.flac out.flac lowpass 18000 (which has a IIR Butterworth filter) ?

The first one seems to be more advanced and has no significant artifacts as far I have tried it. If I would like to use slower roll off filter (which I prefer) than default 5 per cent, can it be modified to the following ?

sox.exe in.flac out.flac sinc -18000 -t 3000

Thanks for opinions.

Jan


P.S. Thanks to the moderators for allowing me posting again. If there are any problems with my posts I am ready to solve/correct them so that they are not offensive to the members of HA.


Re: Lowpass

Reply #2
I did recordings from older tape deck at 48 kHz sample rate and would like to filter (low-pass) frequencies above e.g.
Simply get yourself a Korg recorder and re-record in dsd.
Is troll-adiposity coming from feederism?
With 24bit music you can listen to silence much louder!

Re: Lowpass

Reply #3
Your preference for a more gradual roll-off is based on what?

Good question. The high end roll off of mag tape (analog) recorders is set by either some analog notch filters designed to deal with Dolby pilot tones or tape bias signals, or the first cancellation caused by the tape head gap being so wide that the segment of tape that it covers is 1/2 wavelength long, and essentially cancels itself.

By modern digital standards either is actually relatively broad compared to the usual digital player's reconstruction filter's transition band.  There seems to be some evidence that really narrow (a few Hz) transition bands may be audible in a DBT.  (thinking of the use of such signals being for listener training in the "Typical Digital Filter"  paper.  Of course the typical digital reconstruction filter has a transition band that far wider than that.  The low pass filter in an analog tape machine is probably already wider and more gentle than that.

The mention of TOS #8 therefore seems highly relevant.

Re: Lowpass

Reply #4
Many years back me and others tried to hear a 18.5kHz lowpass with music. I still heard a 20kHz sinus pretty well back then. I failed but didn't know much about ringing. If i only had known these fancy, colored ringing illustrating pics...
Is troll-adiposity coming from feederism?
With 24bit music you can listen to silence much louder!

Re: Lowpass

Reply #5
I think it's a pretty big leap of faith to conclude that people can hear ultrasonic ringing based on a test that did not include controls to account for this.

Back to the original subject, without the ability to demonstrate an audible difference, there is no rational basis for a preference.  As such, I don't see any good reason to keep this topic open.

Re: Lowpass

Reply #6
...
sox.exe in.flac out.flac lowpass 18000 (which has a IIR Butterworth filter) ?
...

If that command uses 2nd order LPF from RBJ's paper (implemented in sox) then roll-off speed it's quite far from the 'ideal' (analog filter). This is because of BLT RBJ uses.

jpg images
(fs=48kHz, fc=18kHz, O=0.707, (case BLT (pw) = BLT with prewarped fc))

If you go with IIR then
Code: [Select]
lowpass −1 18000
should give you a bit slower roll-off.

Re: Lowpass

Reply #7
can it be modified to the following ?

sox.exe in.flac out.flac sinc -18000 -t 3000
Yes, that gives you a transition-band from 16500 to 19500 Hz (and 6dB down @ 18kHz, the mid-point).

Use the sox --plot option (with Gnuplot or Octave) to check that the filter response (either sinc or lowpass) is as expected.

Re: Lowpass

Reply #8
Thank you very much for concrete answer, bandpass, that was exactly what I was asking for! Glad to know that -t parameter can be added this way.


Re: Lowpass

Reply #9
Many years back me and others tried to hear a 18.5kHz lowpass with music. I still heard a 20kHz sinus pretty well back then. I failed but didn't know much about ringing. If i only had known these fancy, colored ringing illustrating pics...

There are approximately  an infinite different number of different ways to  make a low pass filter.

Most of us think about the reconstruction in real world DACs as an extreme case. These filters typically have transition bands a few KHz wide. This leads to mild ringing.

In  JAES 9174 "The audibility of typical digital audio filters in a high-fidelity playback system"

"
2.6. Training

The second phase of training was intended to
familiarize listeners with the filtering used and with
using the GUI. Two intervals were presented, as for
the main test, but the test interval always contained
the unaltered extract and the second always contained
the altered extract; listeners were informed
of this, with the intention that labeling the extracts
as having been processed didifferently might aid the
identification of differences . Listeners were able to
listen to as many labelled pairs of extracts as they
liked before progressing to the test. The filter used
here was an FIR filter with a frequency transition
band spanning 8-10 Hz
. This filter was chosen as it
would have been straightforward for most listeners
to identify differences introduced by its application.
"

I don't know what kind of ringing a filter this fantastically narrow causes, and I lack the facilities for simulating one quickly.

A person who is more familiar with SOX than I may be able to provide an answer and perhaps even a working example quite quickly.

In my current state of ignorance I can imagine that the ringing would be quite pronounced, to the point of this filter being more of a tone generator than a filter. If that is so, then the experiment would become one that involves detecting a high level steady-state tone in the 19 KHz range, which many of us seemed to have no problems doing, at least earlier in our lives

I'm not a good candidate for this test as  I'm currently 69 and my hearing has recently suffered from the additional effects of a 2 month course of chemotherapy which had many other obvious neurological effects including a general loss of hearing sensitivity.  The medical literature covers this and many other similar issues. The good news is that as I recover, some of these effects are starting to reverse themselves.

Re: Lowpass

Reply #10
I lack the facilities for simulating one quickly.
You may lack the specific pieces down the signal chain as well, which may have a considerable influence on the outcome.

...back on topic:
There are approximately  an infinite different number of different ways to  make a low pass filter.
Indeed and some are better suited for specific jobs than others.  In this case, who knows; listening test data should be necessary in order to avoid countless pages of hand waving(?).

Re: Lowpass

Reply #11
I hope you have a fast recovery and get well soon Arny! I know from high dosage cortisone how meds temporarely mess up senses.

I don't remember anything about the filters but the source material was standard CD and my soundcard must have been a Santa Cruz. It was related to chosing an effective lowpass for lame.

Is troll-adiposity coming from feederism?
With 24bit music you can listen to silence much louder!