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AD converter calibration
Old 26th August 2007
  #1
Gear Addict
 

Question AD converter calibration

Hi
My preamp clip at +27dbU. Who can tell me why I should calibrate my ADC to adjust 0dbfs to +27dbU ? Will I have a greater dynamic range ? It's now at +15dbU. What's wrong if i go on at +15 ?

thanks
Old 26th August 2007
  #2
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A/D Calibration

From my understanding you want your A/D converter clip 3-4 dB before your mic pre. It is like a balancing act to get the full dynamic range capable from your mic pre and A/D without introducing too much noise from the electronics themselves. Benchmark media discusses this concept in the manual of their AD2K+...Benchmark Media -- PDF Directory

Last edited by VBAcoustic; 26th August 2007 at 08:09 PM.. Reason: mistake
Old 26th August 2007
  #3
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Quote:
Originally Posted by VBAcoustic View Post
From my understanding you want your mic preamp to clip 3-4 dB before your AD converter. It is like a balancing act to get the full dynamic range capable from your mic pre and A/D without introducing too much noise from the electronics themselves. Benchmark media discusses this concept in the manual of their AD2K+...Benchmark Media -- PDF Directory
Eeh. What they write is exactly the opposite, see page 9. It makes sense, save some headroom in the mic preamp before going to full digital scale. The top few dB close to clipping in analog equipment probably has increased distortion (unless you are after that distortion as an effect of course).

Gunnar
Old 26th August 2007
  #4
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I always set my A/D to clip before the preamp does. The Lynx Aurora A/D is set so that the top peak in the music being recorded is as close as possible to zero dB to maximize the encoded S/N ratio. The preamps I use seem to have another 12dB more of headroom above that.
Old 26th August 2007
  #5
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My mistake

Thanks for the correction. I did completely misread this info. of course you would want the A/D to to clip prior, thus providing the headroom in the microphone gain stage.
Old 27th August 2007
  #6
The trade off is more THD at the analog stage and more dynamic range to tape vs. less THD and less dynamic range to tape. Given that most pro A/D converters yield a usable 110+ db of range, I say don't push your analog gear so hard

The long answer is that most vintage / vintage re-issue gear was going to tape, often calibrated to 0VU = +4dbu - now people usually used 0VU as a starting point, and pushed a little harder, but +18VU was serious clipping even on a studer 827 - which (given the previous calivration) would be equivalent to +22dbu. So I'd say that calibrating your meter to +18dbu = 0dbFS is a reasonable medium. Calibrating to +22 or +24 becomes more reasonable to me if you're someone who shoots for -12 or -18 when tracking.

So, nothing is wrong if you continue at +15, unless you then also shoot for like -18 in that environment at which point you're loosing a slight to modest amount of dynamic range, which (in 24 bit with decent converters) also ISN'T REALLY A BIG DEAL for most types of music being recorded today.
Old 27th August 2007
  #7
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Bob Olhsson's Avatar
 

At least 10 dB. of analog headroom is always a good idea.
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