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Comments
This thread is not meant to offend Drambo or any other apps.
@jolico : I think you should make it a practice of reducing input levels when you see aliasing to make sure that it isn’t just clipping that that the plug-ins weren’t designed to handle well.
Yes. However, with saturating effects the saturation and the aliasing both decrease when you reduce the volume. It wouldn’t always be sensible to decrease the volume when testing for aliasing. They should be tested at the upper limit of their intended input volume.
It is unusual to see harmonics produced in the bass with a simple three band EQ because designing an EQ that does not saturate is trivial and beepstreet certainly knows how to do it.
I suspect this signal clipped very slightly in the first 100 ms of the sweep.
I agree.
Do sine sweeps on everything, because some presets and even default settings can cause aliasing just by dropping them into an empty FX chain.
But this is the same sine sweep at the same level as all the other tests.
Yes.
Also, an EQ can cause clipping without boosting the level because it causes a phase shift, which changes the shape pf the waveform. Doing that does not change the RMS power level of the perceived volume level but it can push one part of the wave down and raise up a spike somewhere else.
The most extreme example of this is when you filter a square wave through a matching pair of boost/cut filters. I will make a video because this is quite interesting to see.
And dropping it again to avoid aliasing.
Basically I’m discovering that I must take extra steps with some apps.
In this EQ app, I’m boosting to a crazy +64db without compensating.
Guess which EQ I would prefer to quickly throw into my mix.
Oh oh! Looks like at least one dev needs to read up on how to do biquads with low noise!
That makes me wonder.... so we have established the aliasing police. Should we also check plugs like compressors (see how much undesirable harmonic distortion they produce), EQs (how much noise they produce, especially using high-pass filters which are often problematic), and alike? Could be fun!
That parametric EQ is great for mixing too, not just sound design.
Please post your more accurate tests if you don’t mind sharing.
We are using the apple vDSP biquad with 32 bit floating point coefficients. It can be noisy when the cutoff frequency is low and you boost +64 db. But it’s also very efficient and the noise level is fine for most normal use.
I wonder what the sine sweep itself looks like if you just boost the gain +64 db. I just ran some test signals myself and they got noisy on the spectrogram when I boosted +48 db even without applying any EQ.
That’s a cool trick. Thanks.
That explains it. For biquads, 32 bit floating point precision is typically not enough. Especially for high Q filters, or filters with a low frequency parameter (as in the example above), there will be considerable noise. Unless you go the fixed point or state space route, you'll need to use 64 bit precision. State space is much better though, it can provide the same noise floor at 32 bits as standard biquads can provide using 64 bits. There's ample of literature on how to do this.
Is there an advantage to making antialiasing optional?
I don’t see any extra CPU usage when switching it on, so why not keep it on permanently and hide the option?
👍
We have state variable filters already implemented but for that parametric eq app I wanted it to be efficient and use apple vDSP. The SNR of the 32 bit floats is totally reasonable for normally useful filter settings on that plugin. It doesn’t allow you to set cutoff freqs below 20 Hz and the highest filter q is not very high so it limits your ability to get into trouble. If you need 180dB SNR on a filter with cutoff frequency of 5hz you can buy fabfilter pro q 3. But for everyday use that vDSP biquad is fine.
I agree 100%, it is my “go to” EQ, even though I didn’t understand half of that technical talk about biceps and abdominals.