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 Forum index » Clavia Nord Modular » NM Classic (NM1 or G1)
6db LPF unstable?
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Kassen
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PostPosted: Sat Mar 22, 2008 7:29 am    Post subject: 6db LPF unstable? Reply with quote  Mark this post and the followings unread

I've started to suspect the NM's 6db/oct LPF has a tendency to react to it's input dropping to a steady zero by (sometimes?) going unstable and outputting either a positive very small value or some sort of noise... I need to to return to zero as soon as the resolution gives instead, at least for CV applications.

Does anybody know more about this? The list archives seem kinda hard to search.

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blue hell
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PostPosted: Sun Mar 23, 2008 4:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

I don't think I ever heard anyone about this, so I guess you are the expert now Laughing
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also .. could someone please turn down the thermostat a bit.
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Kassen
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PostPosted: Sun Mar 23, 2008 5:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
I don't think I ever heard anyone about this, so I guess you are the expert now Laughing


Ah, ok, well, expect my paper soon, it's being delayed while I set up a agency specializing in advice about this issue :¬p.

More to the point, it's not so easy to make sure this is happening at all. For one thing all the modules that are good at detecting these things are a 4th as fast as the issue (if it's noise) and measuring whether a LSB has rounding issues leads to problems where the method of detection may have those as well.

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blue hell
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PostPosted: Sun Mar 23, 2008 5:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Kassen wrote:
More to the point, it's not so easy


I agree, when you go into things like this you always seem to run in a couple of seemingly arbitrary design choices made in the module design.

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also .. could someone please turn down the thermostat a bit.
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Kassen
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PostPosted: Mon Mar 24, 2008 1:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:

I agree, when you go into things like this you always seem to run in a couple of seemingly arbitrary design choices made in the module design.


Yeah. I don't even care all that much about stability as such, I just want to filter CV signals and be sure they will/can return to zero at some predictable time.

In the next days I'll try to create and post a patch that demonstrates this behavior.

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PostPosted: Sun Apr 06, 2008 5:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

A bit late, but: some filter topologies don't like too short word lengths (low resolution) in the internal filter states, which can lead to the filter getting into an unstable region when you feed it very low frequencies. Not saying that this is the case here, but you may want to test that with some very low frequency signals at a high amplitude that you cut off suddenly - then listen for noises.

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Kassen
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PostPosted: Sun Apr 06, 2008 6:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

DrJustice wrote:
A bit late, but: some filter topologies don't like too short word lengths (low resolution) in the internal filter states, which can lead to the filter getting into an unstable region when you feed it very low frequencies. Not saying that this is the case here, but you may want to test that with some very low frequency signals at a high amplitude that you cut off suddenly - then listen for noises.

DJ
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Sounds like that might be it. I'm definitely feeding it very low frequencies at high amplitudes, some even below 1Hz as I'm using it to interpolate CV signals.... I think those might qualify :¬)

I didn't think word-length for the quotients would be it, that sounds more like something for very old samplers (where I can hear it and sometimes abuse it), not for a fairly high-res thing like the NM. Still, even a small amount of instability would do it as I'm trying to detect whether a CV has returned to zero yet, so even a LSB would muck it up.

And thanks!

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PostPosted: Sun Apr 06, 2008 7:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

It's not the coefficients that are critical, it's the delay elements. I assume that those are 24 bits in the NM, and depending on filter topology that can be too little. (Again I speculate since I know nothing about what happens inside the NM)

I forgot to mention an important criteria (sorry!), and that is for this instability to occur, the cutoff needs to be low, leading to very low frequencies recirculating inside the (IIR) filter. I'm sure you can see where that leads; the recirculation over very many sample periods (ref. low f) means that numerical errors will build up, and if the filter states end up too far from what they "should be" - then the filter can blow up. 1Hz would indeed be considered a low frequency in this case.

Edit: Dang, that was a clumsy formulation - I guess I should rather be asleep now that write stuff like that... Embarassed

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Kassen
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PostPosted: Mon Apr 07, 2008 5:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Nah, I got it. The cutoff is indeed very low and indeed increasing the cut-off did seem to bring back stability, I'm fairly sure it's NOT "blowing up" though.

Do note I said it's the 6db filter, I kinda suspect a rounding error.

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