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frequency response correction

Postby Ugly » 03 Jul 2017 03:33

I can put a set of filters into eq plugins in Reaper once they have been formulated.

My problem is I have no idea about the strategy of forming optimal response corrections filters for my imperfect system response. Is there some software to help me out with this? What would be nice is something like a Room Eq Wizard type application that could be used for cartridge response correction.

I was playing with my Ortofon 0002 test record which has an RIAA corrected white noise track.

This is my Ortofon concorde pro s, sty-30 tip, on my SL1210 mark 2 feeding the moving magnet input of a Cambridge 651p. The sl1210 has been upgraded with KAB's cardas litz tonearm wire and phono interconnect cable. There is no fluid in my KAB arm damper trough. The gain is pretty high compared to normal recording levels and not calibrated at all for this capture.

Notably, I have a bit of high frequency peaking going on.

Since I'm using about as low capacitance cable that will go on there for now and I'm probably not going to alter my preamp input capacitance (220pF polypropylene each input) since it is part of the emi I want to keep.

Edit: Awe crap! Sorry about that blue thing on the upper channel graphic.
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RIAA corrected white noise response.jpg
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Re: frequency response correction

Postby audiopile » 03 Jul 2017 05:33

So from 10kHz to 20 kHz you have around 3 db or so ? rise. Beyond 20 kHz I certainly don't worry about ( unless it were something truly extreme ). I would think this would sound very good -does it ? I have the same cart and OM30 stylus - works a charm on any record passing under it.Assuming a fresh/un-worn stylus I'd think you'd have to upgrade your phono section to improve things . Q: Why no fluid in Kevin's trough ?
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Re: frequency response correction

Postby Ugly » 03 Jul 2017 06:01

Why don't you think a good minimum phase digital eq filter would work?

I was reading it more like somewhere between 5-10dB at 20kHz which is getting to be fairly significant. It doesn't sound bad or anything. Just not quite right. Since I'm ripping these into a computer and planning to do some digital touch up anyway I might as well go for correct frequency response if possible. I'm hoping to come up with a bit of a batch process if possible and just crank these through once I've settled on what I need.

I just haven't had any fluid in it lately as I've been flipping the table over a lot lately playing with a few different mods. When I do recording the fluid will be there.
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Re: frequency response correction

Postby jiiteepee » 03 Jul 2017 08:40

On Reaper you have pre- and post-processing slots for plug-ins so could it be enough if you set a linear/minimum phase PEQ (enough bands or multiple instances if needed) + Voxengo SPAN (monitoring) to those pre slots and then find the correction EQ preset. Rest of your plug-ins goes to the post slots. It's easy to use because of Reaper saves the plug-in states/settings with the project file.

You could also prepare the compensation EQ as a VST plug-in or a IR file to make it simplier. Matlab or Octave for to desingn the EQ and then prepare either a VST plug-in (IIR or FIR based) or find a suitable VST plug-in for the IR. Reaper also supports scripting (EEL, Lua, Python).
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Re: frequency response correction

Postby Ugly » 03 Jul 2017 15:42

It seems there are plenty of nice looking PEQ plugin options which would work just fine if only I had the know how to utilize them correctly. My problem is my confidence in rweaking eq knobs is near zero. I have no idea how to correctly eq anything. I can easily imagine the response I want but have no clue about finding the correct filter for compensating the raw result and turn it into the desired result.

How does one even learn eq'ing? Trial and error, by ear? I am literally lost.

Ideally I'd hope to find an automated tool or technique I can use to find the correct FIR correction filter parameters to use with one of the EQ plugins that works in Reaper. The idea would be to compensate for the imperfect frequency response of my cartridge. Since I am clueless I am hoping to find some tools to help me out...

As an example, the home theater guys have an automation application they use for coming up with their FIR room correction filters that works really slick. It's really cool to watch the thing in action. When it is working well, it is amazing. Check out youtube videos on Room EQ Wizard. It's pretty cool watching it in action.

I love all the power Reaper seems to offer, that's why I ended up buying the Reaper license. I think there are probably several approaches to doing the job if I were an expert.
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Re: frequency response correction

Postby audiopile » 03 Jul 2017 16:54

In the end it comes down to do you enjoy the result ? I too am pretty impressed by the room correction devices built into modern HT units. Put even a decent speaker into a half reasonable room and the measured result is going to be appalling . Audissey and it's clones do a remarkable job of cleaning this up. Your goal is to create a very accurate archived copy of your LP/arm/cart/phono pre-amp and turntables playback. As other's have suggested - use the program -EXPERIMENT. The one thing I would suggest is apply what I call the two week rule to changes made in your system. We are all inclined to make snap judgements about sound (similar to the tasting experience with food or drink). Most yuck experiences don't change with further exposure - but the seduction of the different can be harder to evaluate. I give myself two weeks of random listening to decide if I think it's a solid improvement or does the "improvement" become irritating or simply unsatisfactory with further exposure/experience. The only caution I have in this is that despite knowing that issues with room/speaker interaction have been dealt with - personally- I don't like listening to LP's thru HT receivers. YMMV.
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Re: frequency response correction

Postby jiiteepee » 03 Jul 2017 17:18

If you get the frequency response of your playback system saved to a file as WAV or AIFF then isn't it possible to use REW features to prepare the compensation EQ (as impulse response file (IR)) or data for PEQ filter usage (coefficients, filter type/parameter values).

In generally speaking, when you are preparing a compensation EQ preset you just adjust your filters (usually peak type filters needed) to follow the response curve and then invert the EQ to get the compensation (i.e turn the gain values opposite).

I can't remember if there are any PEQ software which would allow you to load the response graph (like in your post) as background image which would help with the adjustements of filters (I have done some GUI software which lets do this but just with certain exact sized/positioned images only).

When you want to compensate cartridge output error you should remember that the error isn't constant but can variate by temperature as for an example. That's why I suggested the PEQ + SPAN combination so you be able to check if the compensation preset is correct at that time you're using it (play the test tone and make EQ corrections while monitoring in realtime).
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Re: frequency response correction

Postby Ugly » 03 Jul 2017 17:39

jiiteepee wrote:isn't it possible to use REW features to prepare the compensation EQ (as impulse response file (IR)) or data for PEQ filter usage (coefficients, filter type/parameter values).

In generally speaking, when you are preparing a compensation EQ preset you just adjust your filters (usually peak type filters needed) to follow the response curve and then invert the EQ to get the compensation (i.e turn the gain values opposite).


This is a great point. I could probably follow one of the more manual procedures for room correction filter generation and tweak the procedure to fit my phono system response correction application. Thank you for the idea.

EDit. By the way. In your experience the thermal response thing is something that comes up how frequently worst case??? this is pretty fascinating. I can investigate this stuff once I start tracking these things.
Last edited by Ugly on 03 Jul 2017 17:53, edited 1 time in total.
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Re: frequency response correction

Postby Ugly » 03 Jul 2017 17:47

As other's have suggested - use the program -EXPERIMENT.


I've tried a bit of that before posting here. I'm certain I have a lot to learn still.
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Re: frequency response correction

Postby audiopile » 03 Jul 2017 17:49

Another thing worth considering is the accuracy of your test record ? For instance - install a older Shure cart -say a M44 series -without compensating for the difference in required capacitance - you should get a dramatically different measured response ? And probably before proceeding I'd plug another phono pre-amp in briefly - just to be sure that you aren't seeing anything strange from that unit? 95% of what i enjoy in terms of equipment (carts excepted)is used - so I'm always in the "trust but verify" camp.
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Re: frequency response correction

Postby jiiteepee » 03 Jul 2017 18:14

Ugly wrote:...

EDit. By the way. In your experience the thermal response thing is something that comes up how frequently worst case??? this is pretty fascinating. I can investigate this stuff once I start tracking these things.


Page 12 gives some direction for this thermal phenomena.
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Re: frequency response correction

Postby JoeE SP9 » 03 Jul 2017 21:55

Could the problem be related to the capacitive load the cartridge is working into?
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Re: frequency response correction

Postby Ugly » 04 Jul 2017 15:35

JoeE SP9 wrote:Could the problem be related to the capacitive load the cartridge is working into?


Undoubtedly. As per many suggestions from around the web, it would appear that there is a bit too much capacitance in this case. That leaves removing capacitance as the avenue to remedy the issue in hardware.

Unfortunately, I'm using about as low capacitance cable as I can for now. The input capacitance on my preamp is part of it's emi filtering I'd rather not disturb. I'm a bit stuck in that regard.

Since I'm out of options for altering the capacitance I'm comfortable with, I feel as though the best way of dealing with this for my purpose (ripping vinyl) is to eq the high frequency peaking to flat as much as possible during my digital file cleanup processing.
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Re: frequency response correction

Postby Ugly » 04 Jul 2017 15:36

jiiteepee wrote:Page 12 gives some direction for this thermal phenomena.


Thanks, buddy. Good stuff.
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Re: frequency response correction

Postby Ugly » 04 Jul 2017 15:53

audiopile wrote:Another thing worth considering is the accuracy of your test record ?


Unfortunately, I'm quite confident in the results.

I have several layers of redundancy with respect to preamps and test records.

In this case things look fairly legit, unfortunately. I have several different avenues for "seeing" this imperfection and they all seem to be in fairly close agreement.

I only chose the Ortofon result as an example. It's a nice track in that it gives quite an easy to interpret visual which is seemingly adequate to illustrate my issue for the forum to see.
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