High pass filtering - what cutoff frequency?

[Midranger]

My vinyl ripping workflow is modeled after the advice given here:
http://manual.audacityteam.org/man/Sample_workflow_for_LP_digitization#Clean_the_LP

I'm hoping someone can authoritatively elaborate on the following:
"Use Effect > High Pass Filter... with a setting of 24 dB per octave rolloff, and a cutoff frequency of 20 - 30 Hz to remove unwanted subsonic frequencies which can cause clicks when editing. If your record is warped, this will definitely generate unwanted subsonics, in which case consider a lower cutoff frequency."

I've seen people say they high pass anywhere from 20-50Hz but I have never seen a good explanation of how they came to choose say 25Hz and not 30Hz or 40Hz and not 20Hz.

Is there any consensus on exactly what the cutoff frequency should be? Can anyone offer a compelling argument for a particular cutoff frequency?

[Trackside]

There is no hard and fast rule as it depends what subsonic problems may exist in your vinyl and turntable and how much low frequency extension your playback device has. Also some phono stages already feature a high pass filter to deal with it at source.

[josephazannieri]

Yo midranger:

Basically, you pays your money and makes your choice. There is no set rule that I know of.

It's sort of that you look at the material that you are attempting to transfer and what's the lowest fundamental note likely to occur in that material, and cut off so you will hear that. In general, the lowest "e" on almost any instrrument will be in the area of 40 Hz. For stringed instruments such as double bass or Fender bass, the vast majority of the production is in second harmonic which is double the fundamental frequency, or 80 Hz. That's how they get away with those crappy open back cabinets for bass amps. If you want the fundamental, you use 40 Hz. There are some instruments such as pipe organs, that will go down to 20 Hz, and 20 Hz is regarded as the lowest audible freqency.

Warp wow is in the 5 Hz area, and most arm and cartridge resononces are in the 12 Hz area. Those frequencies should go. Some turntables have a rumble frequency in the area of 30 Hz. And there are a lot of other "rules" that people use to figure out how low to go. You can cut off at 60 Hz or so, and there are a lot of 78's that will sound just fine with a 60 Hz cutoff, since mostly below that will be just grunge on 78's.

But mostly you just listen and pick a frequency that gets rid of what you want to get rid of and leaves behind what you want to leave behind. The more you know about the relative frequency measurements of certain sounds, the better choices you will be able to make. There are a lot of books on using parametric equalizers that will tell you the frequencies of certain items.

And good luck from the old frequency guesser,

Joe Z.

[Midranger]

Thanks for the info. I suppose the best method would be to evaluate each record for program material but I was more concerned with finding one universal setting that would give acceptable results as I don't have the motivation/time to evaluate and tweak each individual rip.

To date I have been using 20Hz as the cutoff frequency and I guess I have no complaints. My reasoning behind choosing 20Hz was to keep it in line with compact disc audio's lowest frequency and also I had read that the lowest note on a piano was around 28Hz so it seemed like any cutoff should at least be lower than that.

[jiiteepee]

Warp wow is in the 5 Hz area, and most arm and cartridge resononces are in the 12 Hz area. Those frequencies

Is 24dB/oct @ 20Hz suitable setting to get those silenced ?

My normal HP filter setting is 72dB/oct @ 30Hz (Std Technics DD player/Technics MM cartridge) and sometimes up to 48dB-72dB/oct @ 50Hz (when there aren't low frequency instruments played).

jiitee

[Midranger]

Is 24dB/oct @ 20Hz suitable setting to get those silenced ?

That's a good question. Audacity's high pass filter has five choices for rolloff values: 6, 12, 24, 36, and 48dB. Why would 24dB be recommended in their ripping guide and not the maximum setting of 48db?

Also, if some phono preamps include high pass filters, what cutoff frequencies are used in these hardware filters?

[josephazannieri]

Yo frequency guessers:

If I recall correctly, Marantz used to use 30 Hz, 50 Hz and 90 Hz (12 dB/Octave, if I remember) in their preamps. 24 dB/Octave is a really sharp filter, and should be plenty to get rid of your low frequency issues. The one issue you might get with those really sharp filters is that there are some phasing issues. But your Audacity filters are "virtual" filters, so they are somewhat different from regular filters that use real electronic components. I am not sure how such a "virtual" filter would behave. I will yield to a person with real knowledge of "virtual" filters.

And good luck from that old guy mired firmly in the way distant past,

Joe Z.

[jiiteepee]

I am not sure how such a "virtual" filter would behave.

Few years ago I did some measures of software based HP filters.


(From: http://forum.cakewalk.com/tm.aspx?m=1574447)

Voxengo is based on 24dB/oct filters (24dB/oct = 4th order).

Some deeper measures: http://jiiteepee.fortunecity.com/tests/ ... _test.html

jiitee

[mmarston]

No hard and fast rule here. I developed my own workflow similar to Audacity's, but simplified... most of my records are in pretty good shape, so I don't bother with HF noise reduction, and I only use "click removal" between tracks, preferring the result from "repair" with the few clicks I usually find... but I digress.

My arm/cartridge resonance is around 8 Hz. I'd guess 90% of records I've copied excite this to some extent. Sometimes it's 30 dB below the music, sometimes much higher.

I base my choices on how much musical LF info is present vs. the strength of the unwanted rumble. As noted, most rock/pop records don't have much below 40 Hz, but a few with synth bass lines might. Pipe organs can go down to 16 Hz (some huge ones even lower) but such frequencies are extremely rare on LP. Tympani in orchestral music may reach down to 30-ish... for records I know well, I can usually tell by ear, having played them on a number of systems.

For others, the spectrum analyzer tool is your friend. It takes a little practice to interpret, but by selecting sections of audio with a lot of perceived low bass, you can get a pretty good idea where the lowest musical tones are (and see how much infrasonic info is present.)

So if the music goes down to 30 Hz, I may select 22 or 24 Hz cutoff and 36 dB/octave. If nothing much below 50 Hz, I might use a higher freq, 36-42 Hz but at 24 dB/.

Theoretically, response is 3 dB down at the cutoff freq. An octave down is half that freq, so for 24 Hz at 36 dB/, 12 Hz will be down 39 dB, 6 Hz 75 dB, etc.

The reason for the different slopes is to minimize effects above cutoff where you can hear them. In particular, smearing of bass transients may occur, although those may not be particularly noticeable. Sometimes though, filtering may counterintuitively *increase* peak signal level, which can cause clipping even if your original recording level didn't.

You basically want to use as little as you can to minimize the problem, while retaining as much of the desired audio as possible.

Sorry if this is long...

Best,
Mike

[Juhazi]

I've been digitizing some 50 LPs during the last two months. I am using audacity and a very nice Terratec iPhono ADC-riaa which gives me 24/96 via usb.

I get minimal AC hum when using my laptop but I mostly record with a PC. AC hum level is usually -60 to -75dB at 50Hz.

The maximum rumble of my turntables (NAD 5120 and Thorens TD145 MKII) is -35 to -45dB at 8-12HZ. Suprisingly the plastic-bodied NAD is better for rumble but the pickup gives more AC hum.

I use Audacitys high-pass to cut rumble. I take 3-5 5s spectrum samples to decide the cutoff Fq and steepness. Usually high pass is 24dB@32Hz.

There are remarkable differencies between records. The live recordings from '80s have lotso of signal from 20Hz up (perhaps mic-preamp noise) and I leave most of it. Modern studio recordings don't have much below 40Hz but there are ecxeptions.

Here is a sample, Led Zeppelin II vinyl, original and high-passed

(edit: what??? where did the .png piccies go??)