Pro 2 AD Conversion - why high pass at 30hz ?

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andrew gissing

Sophomore
Jan 11, 2011
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Perth Western Australia
www.stylusra.com.au
Hi,

Reading the pro-2 manual I came across this:

...the remote stage box contains a 12dB/Octave 30Hz filter. It is recommended that this is used at all times for optimum A/D performance. However it may be bypassed....
(page 70 op manual).

I'm puzzled by low frequencies causing an issue in an AD converter. I understand aliasing and nyquist but that's at the high end.

And I do employ high pass filters to clean up my mix... but the comment in the manual is directed at AD converter performance. This I don't understand.

Can anyone comment on this ?

Andrew
 
Re: Pro 2 AD Conversion - why high pass at 30hz ?

A DC term at the input to an A/D converter would cause earlier clipping for one polarity than the other, thus lower "clean" dynamic range than with a HPF inserted. I would expect most A/D to be HPF to block DC while perhaps much lower than 30 Hz, likewise I would expect most signal sources to be HPF and not put out DC.

Agreed the advice sounds a little funny.. not ha ha funny, but why? funny. 30Hz seems a practical corner for a HPF, but why would they ever need DC response?

JR
 
Re: Pro 2 AD Conversion - why high pass at 30hz ?

That was my first thought as well, but 30 hz is pretty high as a 3dB downpoint for DC offset removal. I could be wrong, but I thought usually the cutoff was much much lower.... like 1 Hz. I am also under the impression that the DL series I/O modules are transformer or electronically balanced (as well as almost ALL outputs you might find yourself plugging into a snake on a professional stage) and won't pass DC current anyway.... can anyone confirm or expand on this belief this for me?

My other guess would be trying to reduce LFO style modulation.

I am really interested to know the reason!
 
Last edited:
Mike Brown said:
I am also under the impression that the DL series I/O modules are transformer or electronically balanced (as well as almost ALL outputs you might find yourself plugging into a snake on a professional stage) and won't pass DC current anyway.
Click to expand...

Does being balanced preclude passing DC? I truly don't know, and would e interested to find out.
 
Re: Pro 2 AD Conversion - why high pass at 30hz ?

We talked about this a while back in the thread about DriveRack DSP measurements. The blocking ahead of the ADC could easily be as high as 30Hz. This could either be due to transformer selection or size of DC block caps in an op amp model. In audio gear there seems that there almost always will be DC block presented in some sort. High end pre-amplifiers will be as low as 0.1Hz. Low end stuff I have measured all the way up to 60-100Hz with some pull down (-3dB at 20Hz is the spec they shoot for it seems.)
 
Re: Pro 2 AD Conversion - why high pass at 30hz ?

Hello Gents,

I thought I should get involved in this thread as there were some direct questions being asked of us. So here goes with an answer......

The 30Hz filter is placed pre-A/D converter and is basically there to filter out sub-harmonic frequencies that have little or no musical relevance. The A/D converters we use in our DL431 stage boxes will convert signals down to 4Hz and during the development of the XL8 we discovered that these frequencies eat up A/D converter headroom even though they are not actually 'heard'. We found that the analogue 30Hz filter increased the usable headroom in the A/D converter and removed low frequency signals picked up by microphones on stage through vibration and physical transmission through the diaphragm of the microphones etc. Of course the 30Hz filter can be bypassed at any time on the DL431 splitters as there are obviously some signals that would extend below the 30Hz range.

The A/D conversion stage in the other stage boxes (DL251, DL351 etc) are specified as 20Hz-20Khz processes and do not have a switchable HPF on the analogue side of the A/D converters.

I hope this answers the original question. If there's anything else I can help with just let me know.

Thanks,
 
Re: Pro 2 AD Conversion - why high pass at 30hz ?

Thanks for the clarification while that sounds similar to the same language that raised the confusion.

I will gather from your repeat that there is nothing special going on with the conversion, just a 30 Hz HPF immediately before the A/D stage, which will scrub off any infrasonic energy.

It is not unusual to HPF even higher than that for live sound, and 4 Hz is little too low (IMO) for the full bandwidth position... but opinions vary.

JR
 
Re: Pro 2 AD Conversion - why high pass at 30hz ?

Hi John,

Apologies if I wasn't clear in my reply. You are correct the 30Hz filter is only scrubbing out the infrasonic energy.

There are the standard HPF/LPF filters on the channel strip. But these are part of the DSP of course and would not effect the A/D conversion.

Hope this helps,
 
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