Andy Cytomic's thoughts on sample accuracy, relevant in Renoise?

[Name_User]

I don’t care much about fidelity, I just want my kicks to hit hard, this is why I left Ableton Live.

My questions is this: What built in filters in Renoise introduce phase issues, if any? I suppose I could go and test myself.

Is there anything to watch out for when doing paralel processing? Any known issues?

Also, let’s say we compose in Renoise on a netbook at 44.1khz and then mix down on a more powerful home PC at 88.2khz. Is there anything that gets affected by this switching back and forth?

I know that in FL Studio this was a problem. The built in Granulizer sounded different depending on sampling rate and interpolation used. The “lower quality” had more harmonics for bell sounds for example, which altered the way I placed notes in the composition.

http://www.kvraudio.com/forum/viewtopic.php?t=363217&start=120

I’m glad I’m not the only one that worries about sample accuracy!

There are issues with Live 8’s lack of alignment of automation and song position information to plugins which I want fixed as much as everyone else here, but there is another issue which is not very well understood, and more general that applies to all DAW users.

There is something called a “two path polyphase IIR filter” as outlined in the paper “The Design of Arbitrary-Band Multi-Path Polyphase IIR Filters” by Dr Artur Krukowski and Dr Izzet Kale, which is commonly used for oversampling. It’s used in Live 8’s EQ 8 in HQ mode and it’s used by loads of people in the audio industry since it’s low cpu and has a very steep filter rolloff. So what’s the down side? In DSP there is always a tradeoff, it’s this case it’s the non-linear phase response. This will delay your incoming signal by somewhere from 2.x samples to 4.x samples (depending on the steepness of the design) at DC that with frequency in a non-linear way.

Ok, so what does that mean? This means if one of these polyphase IIR oversampling filters is in your signal path that you have a fractional (sub-sample) delay unit processing your entire signal, and the fraction of a sample it delays you signal increases with frequency, so there is absolutely no way you can compensate for this, other than to run ALL your other tracks through the same filter to delay them by the same amount. But hang on, it’s not that easy, since if you chain three plugins, each which uses their own design of an IIR polyphase oversampling filter, then you need to reconstruct the exact same “clean” version by passing all your other audio tracks through exactly the same total chain, but without doing any processing on the signal. That is just to align two tracks together. Have different plugin chains on different tracks and it becomes impossible to correct for.

I repeat, this issue is not the sole problem of Live 8’s oversampling as used in the EQ8 HQ mode, it’s everywhere and it’s an issue in all DAWS - but many developers keep quiet about using it since the polyphase IIR solution is cheaper in cpu and does a really good job if you don’t care about your phase going out of whack with other tracks. You basically have to verify this yourself by putting a single sample spike through a plugin and see what it looks like once it’s processed since developers can’t even declare the delay to a host since each frequency has it’s own fractional delay and hosts only take a single integer as the delay to compensate (which is fine, that should be enough!)

There is a solution, and that is to use linear phase FIR oversampling filters. Ok so what’s the tradeoff for this fix? Slightly more cpu usage and more latency, but this time the latency is constant and can be corrected for by the phase so the crack of your transients all stay put and don’t turn into limp plops when they phase with other parts of your track.

So in summary the polyphase IIR oversamplers do a really good job for the amount of cpu they use, and they introduce minimal latency into the signal path, but the latency they do introduce is non-linear and can’t be corrected for easily, and so can cause phase issues. The FIR alternative uses more cpu and latency, but the latency is constant and can be corrected for. So either way it’s a bummer.

A much better solution is to run your entire project at 88.2 khz or 96 khz, then you don’t need to oversample in individual plugins, and all your EQ shapes are very analog like in shape and phase, and your aliasing is reduced across the board so all your mixes are clearer and tighter. If the oversampling can be bypassed in your plugins (which I allow in mine by not all developers support) then there is no extra latency introduced so nothing needs correcting for. The cost here is increased CPU usage across the board as well, with every single plugin taking twice as much cpu.

[kazakore]

@Name User

Also, let’s say we compose in Renoise on a netbook at 44.1khz and then mix down on a more powerful home PC at 88.2khz. Is there anything that gets affected by this switching back and forth?

The Cabinet Simulator sounds different at different sample rates. I believe it was stated the IR used for the different models was taken and stored at only one sample rate (44.1kHz??) and thus there are differences (and different level of CPU use??) at the different sample rates.

Nearly all software digital filters will give you some non-linear phase changes! In fact all hardware analogue ones will too!!

From your quote: “it’s everywhere and it’s an issue in all DAWS”

Although that was specifically talking about issues with oversampling filters the same carries to ones for more general use. Different models are worse than others though, and the names don’t always quite tie in with measured responses either. The Multi-Band Send had a big issue with its Mid band when first released but I believe that at least has been fixed, at least up to a reasonable extent.

Linear phase filters use a lot more CPU and add a lot more latency. This is why plugins that use them are generally targeted at the Mastering markets, rather than as production tools. But computers are getting faster… Maybe one day Renoise should offer a “HQ” switch for all Filters and EQ but I’m not sure I’d say that time is quite here yet…

Also; at the end of the day your ears are a far more important tool for this job that your eyes! You can filter sounds (especially simple waveforms) so that they look almost unrecognisable on a scope but the differences are indistinguishable to the ears. As you do note in your post transients suffer far worse than a repeated waveform but it’s still best not to get too hung up on the technicalities and I would suggest some ABX testing so you can see (or rather hear) how much difference it really makes.

(EDIT: Changed software/hardware to digital/analogue when talking about filters in the second paragraph.)

[2_daze_j (Jonathan)]

Slightly off topic: I’ve been testing/demo’ing the open beta of Cytomic’s new Drop Filter, and I thought it was great when I first dl’d it, but I am in no rush to buy, and will most likely pass on it. Sound quality is all very subjective I suppose… For some people, they are worrying about removing every last bit of phase, while other people are looking for the dirtiest, oldest break in a record store… I guess I am with the dudes who are diving in the crates because I really like, “horrible sounding,” stuff… A lot of people are running after the new top of the line reverb vst, and I am not… They sound outright plastic to my ears… My favorite reverb… is the horribly useless one in the Nomad Factory Liquid Bundle… No 2Caudio in my studio!! That is for sure… I think I like the Valhalla Room… But just between you and me… That is not such a, “clean, great, pristine,” sound…

So anyway… I have actually been comparing The Drop vs PSP Nitro v1 vs Renoise’s Native Filters and what not… The Drop has a really cool self oscillating feature, and a huge CPU hit on my quad core… There are low frequency things that are outright obvious, but in the mid range… I think the difference is negligible…

So just a couple of last things…

1. You probably don’t want to switch your sample rates, “mid project.” Choose one and go with it. From the very start, to the very end… Switching it up is bad news!! I can’t recall where I heard that, but I am pretty sure that was advice I got… ( That is not unique to Renoise, just in general, its not good to jump around with sampling rates during a project )

2. I am not aware of any parallel processing issues…

3. I am one of the people who believes that they, “can hear something odd, and less than desirable in Ableton’s audio engine, or however Ableton Renders.” There is something, “off color, and muddy.” imho… ymmv

[kazakore]

@2 daze j

So anyway… I have actually been comparing The Drop vs PSP Nitro v1 vs Renoise’s Native Filters and what not…

Would you care to expand on that? Maybe not in this thread though…

Specifically Nitro Vs Renoise native mainly. I bought Nitro years ago, on a No Brainer deal right at the end of V1’s life. Although I recently tried to register it on my laptop and couldn’t get the code to work and just keep forgetting it. Been wondering how worth my time it is pestering PSP for a solution…

[2_daze_j (Jonathan)]

@kazakore

Would you care to expand on that? Maybe not in this thread though…
Specifically Nitro Vs Renoise native mainly. I bought Nitro years ago, on a No Brainer deal right at the end of V1’s life. Although I recently tried to register it on my laptop and couldn’t get the code to work and just keep forgetting it. Been wondering how worth my time it is pestering PSP for a solution…

Sorry for the late response, I became very busy, in the last several hours. If I knew how to make benchmark tests, and measurements, than I think I would go for a whole new thread.

Nitro vs Renoise Native

But I do not know how to analyze all that. – I use spectrum analyzers sometimes, to help me when I mix sub bass

I also bought PSP Nitro on No brainer. I don’t think Nitro’s filter models far surpass anything in Renoise. Nitro has the wet/dry knob, and some cool routing. but the actual filters, I just don’t see where they, “blow Renoise away.” Nitro 2, might be a different story. But my hot button price to upgrade is $29. Even with todays discount email, and making the price $61, I feel like… “this filter is not that worth it.”

I would try to elaborate more, but you would be waiting forever… We are having the Baseball World Series right now. In short, yes you should pester PSP and get your codes to authorize. But, If you are looking for a great new filter, try Cytomic… Its cool, it still does not, “far surpass anything that is already in Renoise.”

But I don’t want to start a flame war, with anybody who might be interested in the Drop. Even me… I am kinda interested in the Drop… but I haven’t heard much more than a self oscillating feature that is pretty cool… meh… maybe when the full 1.0 is ready, and is available to demo. Its hard to tell in the beta, what is there, and what is not…

Cheers