Software options for Tuning - Page 26

netchris · 01-25-2007, 11:03 AM

Thanks for the compliments guys!

I've learned a bit more, and now realize that those GVST filters probably aren't the best option out there. I don't know what kind of filtering algorithm they use (Butterworth, etc) or what order they are. During my journey through the vast sea of VST plugins, I found a high and low pass Butterworth filter with selectable 12dB/octave to 36dB/octave slope. (That's 2nd order to 6th order, right?)

http://home.earthlink.net/~rocksonics/FreePlugs.htm

We would still have to figure out something for a band pass filter though (I'll keep searching!). Also there is a phase inverter at the bottom. Maybe we could use that somehow.. I found another plugin called Phase Bug that allows us to have full 360 degree control over the phase of the audio, so maybe that one would be better.

I'm still a little bit confused about the phase topic though.. If all of our slopes are the same order then we don't need phase correction right? Anyway, if we do need it, it seems like there is an abundance of free phase plugins out there, so not to worry!

Back to the crossover discussion though.. I know that FA can do 42dB/octave, but I don't know the type of filter, (is it Butterworth? I need to look that up..) and I also don't know if a 42dB/octave slope is that much better than a 36dB/octave slope (my guess is that there's not much difference in practice).

Oh and I've been making one assumption here.. Butterworth is the type of filter we want to use, right?

durwood · 01-25-2007, 11:32 AM

Quote: Originally Posted by netchris

Oh and I've been making one assumption here.. Butterworth is the type of filter we want to use, right?

Pretty much yes. It's what most audio crossovers use just because of this reason:

Quote:

The Butterworth is the only filter that maintains this same shape for higher orders (but with a steeper decline in the stopband) whereas other varieties of filters (Bessel, Chebyshev, elliptic) have different shapes at higher orders.

Compared with a Chebyshev Type I/Type II filter or an elliptic filter, the Butterworth filter has a slower roll-off, and thus will require a higher order to implement a particular stopband specification. However, Butterworth filter will have a more linear phase response in the passband than the Chebyshev Type I/Type II and elliptic filters.

This is what FA is using as well. It's the one of the easier filters to design as well. It's pretty straight forward in mathmetical terms as compared to some of the others.

Quote:

I'm still a little bit confused about the phase topic though.. If all of our slopes are the same order then we don't need phase correction right? Anyway, if we do need it, it seems like there is an abundance of free phase plugins out there, so not to worry!

That is another indepth topic. Time delay is easier to explain whereas phase delay is a whole other monster. Phase is dependent on frequency. Since the audio signal has varying frequencies, phase changes at each frequency. Time delay ignores frequency and works strictly in the time domain rather than in the frequency domain.

http://sound.westhost.com/pcmm.htm
http://www.libinst.com/tpfd.htm

With that being said, focus on crossovers, EQ, and time delay and you will be fine. Phase correction (or linear phase)is just icing on the cake that you can aim for later as it's very hard to achieve. Also, some EQ's affect phase as well unless you use one of the linear phase EQ's.

netchris · 01-27-2007, 12:50 PM

Here is a quick update for everyone!

On the topic of phase
I read the links durwood posted, and while it took a while for everything to sink in, I now understand what is going on. I don't understand why the crossover causes the audio to go out of phase, but I suppose all that matters is that the audio is out of phase, and we need to do something about it!

My understanding of FA (non lite) has expanded, and I now realize that if it really does what it says it does, it is the end-all solution to our problem. However, at $149 it's not an inexpensive piece of software!

I don't see why we couldn't use this "Phase Bug" plugin (SS attached at bottom) to manually correct our phase. That would cut out the price of PA, which would be nice.

On the topic of crossover
I contacted the author of the rocksonics filters I linked to in an earlier post, and sent him this email:

Quote:

First let me say thanks for making such versatile VST filters and then offering them for free.

I'm a member of a 'carputer' community (www.mp3car.com). We put desktop computers in cars! This allows us to apply whatever VST effects to our audio, and I'd like to apply some high/low/band pass filters to my audio so the computer will function as an active crossover. Your HyperFilter and SubFilter plugins are great because they offer a selectable slope and are perfect for creating a band pass filter by running them in sequence, but they don't offer the range to use them as a high and low pass filter.

I was wondering if it would be very hard to extend the range of selectable frequencies so HyperFilter could be used as a LPF and SubFilter used as a HPF. I suppose changing the cut filters to a pass filter would also get the job done. I am also willing to take a crack at making the modifications to the source code myself, if you would be kind enough to let me have it (I would give you full credit for your work, of course). I understand that you are busy and don't have any obligation to assist with this, but I assure you everyone in the car audio community would be VERY appriciative of your contribution if you were to lend a helping hand!

To which he replied:

Quote:

Chris;

Thank you for the email.

>>I was wondering if it would be very hard to extend the range of selectable frequencies so HyperFilter could be used as a LPF and SubFilter used as a HPF
Actually, they already are! Hyperfilter is a LPF (cuts the highs) and Subfilter is a HPF (cuts the lows).

Hyperfilter and Subfilter were mainly designed to solve studio problems (rumble, hiss, etc.). They are not the best choice for crossovers. In fact they will not sum properly even if you align the cutoff points. They are not designed for that purpose (the "Q" factor).

There are better filters that are very well suited (perfect, in fact) for audio crossovers. These filters are known as Linkwitz-Riley and are implemented in software using two stages of biquad IIR filters. They have a 24dB/octave slope and sum to a flat response (highpass and lowpass outputs add up to all pass, flat). These filters are used almost universally for audio crossovers, both electronic and software versions.

If you want more than a 2 band crossover you can cascade the above filters but then you need some phase compensation code (all pass filters) to keep the output phase shifts relatively aligned (and again, sum to a flat frequency response).

Another way to build a software crossover is with FIR filters. These filters can be VERY steep - as much as 200+dB in less than 1 octave. These filters take more computing power and are not easy to adjust on-the-fly. These filters work best if the cutoff frequencies are fixed at design time. Again, they sum to a flat frequency response (when designed right). The advantage of linear-phase FIR filters is that the transient response is excellent - what goes in a square wave comes out a square wave (vs. Linkwitz-Riley filters which produce phase shifts and ringing).

Any of the above filters can be used for lowpass or highpass. Bandpass combines a LPF and HPF as you so note.

If you want to take a stab at writing your own plugin, search Google for "Audio EQ Cookbook". It has simple, great sounding algorithms for designing 2-pole IIR Highpass, Lowpass and other variant filters. That's a great place to start. Almost every VST designer has this cookbook in their bag of tools. The VST development SDK is available from the Steinberg website.

As a final choice, I can design a plugin specifically for you if you would like (including a GUI interface if desired). That would likely save you a lot of time and headaches but I'm afraid it would not be for free -- that we would have to work out.

Whatever you decide, best of luck with mp3car!

What I have learned from this:
-It doesn't look like we'll be able to use separate filters to do crossover. It seems that we'll need a single plugin that does it all to maintain a level frequency response.

-A 24dB/octave Linkwitz-Riley filter is really two 12dB/octave Butterworth filters in sequence. Durwood stated that FA uses Butterworth filters, so I suppose enabling 2 of them would be like a 24dB/octave Linkwitz-Riley filter.

Anyway, we still have a few options.
1)Just buying FA, since it does what we want.
2)Paying the above guy to make a linear phase FIR crossover. Maybe if there is enough interest, we could organize a collection to pay for this. If everyone chips in a little bit, it might not cost too much!
3)Have my friend make a linear phase FIR crossover. My friend is a computer programmer and has been interested in audio programming for a while now. I gave him a link to the VST SDK and told him what we're looking for. We'll see if and when anything develops from that..
4)This probably won't work.. BUT! check out the screenshots of LinearPhaseEQ2 below.

As you can see, it goes all the way down to -96dB, AND it's a linear phase FIR filter! Because of that last point, I'm definitely using it as my EQ, but could it also be used to do crossover? My guess is probably not, since there would probably be summing problems, but damn, SO close!

Now it's time for me to get outside and start putting sound dampening material in my car while the weather is still good.

[edit]
Regardring crossover option 4 above, it might be more of a realistic option than I had originally thought. Upon re reading a previously linked thread, I noticed that a linear phase FIR equalizer was used to create the crossovers. It does look like a RTA mic was necessary. I saw a $70 RTA mic at the beginning of this thread, and if that is truly the case, I would rather pay $70 for a RTA mic that I can do linear phase crossover with and do DRC with instead of paying $60 FA Lite or $150 for FA.

Another thing to consider.. crossover with a linear phase FIR filter might be less CPU intensive than an IIR crossover with phase correction. I remember durwood saying that FA with PA would eat ~50% of his CPU! I will do some tests in the next day or two comparing the linear phase FIR EQ method to the FA with PA method and note the CPU load of each configuration.

netchris · 01-28-2007, 08:57 PM

Well guys I think I may have found a way to make a transient perfect crossover with per-channel time delay! (and even DRC if you want it) The best part? It uses 100% freely available software!! I used all linear phase FIR filters too, no IIR so no need for PITA phase correction. (and no RTA mic necessary like I thought in the last post, either)

I manually made the crossovers with LinearPhaseGraphicEQ2, and then tested them by running a sweeping sine wave through the VST plugin chain with the demo version of Voxengo Curve EQ at the end. I then tweaked my crossovers until I ended up with a perfectly smooth frequency response line.

I'll play around with everything a bit more and then report back re: CPU usage.

[edit]
Here are my results:

-EQ with a bunch of random settings (to ensure max CPU load)
-3 way crossover at 85Hz and 2450Hz (2 tweets, 2 mids, 1 sub [but the sub crossover is still outputting stereo right now haha])
-2ms time delay on both left channels and 3.5ms delay on the sub

Average of 25% CPU load (30% including winamp and all my other running processes) on a modest 2.2GHz Celeron! Nice!

I'm pretty sure this gives us everything we need or want.

mezz64 · 01-28-2007, 11:49 PM

So it's been a few weeks since I've looked at this thread, and I come back and see all this... Amazing! Thanks for taking the initiative netchris, it seems like you've come up with a nice free tweakable solution.

durwood · 01-29-2007, 07:00 AM

Red and I had discussed an option for having a program specifically designed for our needs. She had contacted vb-audio similar to what you were asking the other people at rocksonics. I was thinking along the same lines as a group buy might help development costs. I don't know how it would be worked out regarding liscensing and such as if we all invested, how many copies would they give us to use?

Anyway, good work netchris. I am glad you are finding other options as well. I haven't had much time to devote to this as I wanted recently.

netchris · 01-29-2007, 08:30 AM

I haven't had any further contact with rocksonics, and since I made a working TP crossover with all free software (well except for VAC) I wasn't planning on pursuing a non-free solution. I suppose I could ask how much they would charge if there is still interest, though.

Also, I think I'll do a writeup of the whole crossover process once I get my coursework out of the way. My host seems to be giving me some major trouble when trying to connect to FTP though, soo could anyone host some screenshots?

[edit]
I did some looking into RTA but I don't know a whole lot about it. I looked at some of the links on the first page and deduced that we need a special mic and a preamp with phantom power. I found a solution for ~$100.. does this look good?

ART Phantom III Power Supply
Behringer ECM8000

I wonder if I could find any of this stuff used, or if I could borrow this equipment from someone in my area.

[edit again]
If I did buy a RTA mic, I wonder if I could mount it somewhere in the car and use it for phone calls after I'm done using it for tuning..

mezz64 · 01-29-2007, 09:24 PM

Quote: Originally Posted by netchris

Also, I think I'll do a writeup of the whole crossover process once I get my coursework out of the way. My host seems to be giving me some major trouble when trying to connect to FTP though, soo could anyone host some screenshots?

I can help you with the hosting, email me what you need at mezz@superbluecrv.com

RedGTiVR6 · 01-30-2007, 11:16 AM

ok - since there is so much information in this thread I've created a wiki page: http://www.mp3car.com/wiki/index.php...g_via_Software

PLEASE read through it and add any screen shots, information, data, links, etc. that would make this easy for people to learn about this process.

spoonraker · 01-30-2007, 11:50 AM

Awesome job GTi, that's hugely helpful!

scott_fx · 01-30-2007, 11:55 AM

very cool! thanks

durwood · 01-30-2007, 12:21 PM

Fabulous RED, it was alot to sift through for someone who didn't follow it from the begining. I know we don't have a true all-in-one solution, but I kind of liek the customizeability of the VST plugin method with console.

kudos!

RedGTiVR6 · 01-30-2007, 12:45 PM

durwood - I saw your page that you created pertaining to the VST plug-ins. if you wouldn't mind linking to it in that page it would be great.

also, netchris, since you're taking a bunch of screen shots, please feel free to add those.

To add an upload, there's a link on the bottom left side of the page. Once added you can link to it.

To find out about the code necessary to format a wiki, see here: http://en.wikipedia.org/wiki/Wiki

I'd really like to make it a one stop shop for folks looking to do something like this.

A place where all you have to do is download the necessary files and then follow the directions/screenshots to get it all to work.

So we would need a few different sections...one for using X, Y, and Z programs, and one for using A, B, and C programs.

You know, the most used set-ups out there...for the pay programs, and for the free programs...then how to use console, directwire, etc.....might need to break it into multiple pages though...we'll see

BenOBS · 01-30-2007, 12:48 PM

Awesome!

Chapeau!

durwood · 01-30-2007, 01:11 PM

Quote: Originally Posted by RedGTiVR6

durwood - I saw your page that you created pertaining to the VST plug-ins. if you wouldn't mind linking to it in that page it would be great.

No problem. Done!

Quote:

I'd really like to make it a one stop shop for folks looking to do something like this.

A place where all you have to do is download the necessary files and then follow the directions/screenshots to get it all to work.

So we would need a few different sections...one for using X, Y, and Z programs, and one for using A, B, and C programs.

You know, the most used set-ups out there...for the pay programs, and for the free programs...then how to use console, directwire, etc.....might need to break it into multiple pages though...we'll see

I agree. I'm still working on the kxproject how-to. The office is setup at home and I have internet at home again! The Superbowl party i'm hosting this weekend for our Championship Bears is going to bite into my time this weekend. More delays...