Small DC Offset on 301 audio Inputs

When using delays with lots of feedback, I noticed there was a tendency for the signal to gravitate upwards, away from the 0 line when using any of the four audio inputs. I assumed there must be a DC bias of some kind.

So to find it, I tried using a sine osc unit and getting audio input 1 to control the V/Oct of that unit. This gave me a shift pitch up by around 39 cents. So I reckon that is around 32mV (1000mV/12 * 0.39). A fixed HP filter at 0Hz after the inpit, solves the problem but i wondered if this could be built in digitally to the 4 audio inputs. Input 3 and 4 had slightly lower offsets.

Is anyone else getting this? As I guess it could be my case.

I just set up the patch you described (sine osc with IN 1/2/3/4 connected to v/oct in) and can confirm a DC offset of 34-35 cents for each input with nothing connected.

I personally like that all of the inputs are DC coupled. Perhaps there could be an auto-calibration routine in the Admin -> System Settings that you could run with nothing plugged into any of the jacks, and it could zero out the DC bias on the inputs?

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I had thought the audio ins weren’t dc coupled but clearly I was wrong. The routine sounds like a good one, if it is possible to implement. On the other hand having to use a high pass filter is not the worst thing.

Oddly, I don’t get a dc offset on the abcd inputs.

The small DC offset on the IN1-4 inputs is due to the additional analog anti-aliasing filtering on those inputs.

Thanks Brian, good to know it isn’t a fault. :slight_smile:

How difficult would it be to add in either a digital calibration option or auto filter, maybe add it to admin pages etc, in a future release?

How is this better than just putting the HPF in front of your delay network?

Just from a workflow perspective, I would want to have this set once and hidden. It’s not a major thing of course but when just sampling even it is good to know you have an exact a replication of the audio presented as possible. With that in mind is an offset a more accurate choice than a hp filter? Or is the hp filter at 13.75hz so shallow and low as to be having a zero audible effect?

Unfortunately, auto-calibration is complicated by the fact that the DC offset of the anti-aliasing filter is probably temperature-dependent.

You can set the cutoff of your Fixed HPF as low as as you like.

Honestly, I think you are optimizing something that is not worth optimizing. This DC offset is tiny and inaudible :wink: You only came across this problem when you were playing with high feedback delay networks in which case you would still need the HPF. Incorporating DC-blocking HPFs in a delay network is not something to be hidden and forgotten but rather should be part of the patch design.

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Ok, fair enough. :slight_smile:

Would it be worth adding a switch to enable the built in delays to feature a dc blocking filter in their feedback path then? Maybe you don’t need this if the signal is high passed before entering the delay though. :face_with_monocle: