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  #1  
Old 07-23-2013, 01:29 PM
the.engineer the.engineer is offline
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Default Audio level alignment across HD I/O & Omni

Bon jovi everybody

This thread is about an alignment discrepancy between the Avid units, your thoughts would be greatly appreciated!

I've recently calibrated the the analogue ins and outs on my HD I/O with thanks to this thread: http://duc.avid.com/showthread.php?t=340270 and in doing so I learnt a ton about reference levels, digital alignment scales and this brilliant resource: http://www.ips.org.uk/faq/index.php?...es_from_HR.jpg that basically ended up with me sticking a volt meter on nearly every output I had.

Turns out that it's really difficult to find out which scale Avid uses to calibrate their equipment. I eventually found that the HD I/O has a maximum input/output level of +22dBu, so they calibrated it as follows 0VU = -18dBFS.

Great! fantastic! Wait a minute... doesn't the Avid Omni have a maximum output of +24dBu? And aren't all the inputs calibrated to -20dBFS? Actually, the output from the volt meter (having a 0dBFS sine wave through it) was reading 13.6V! That's over +25dBu!

I know it's only 2dB difference and in the grand scheme of things its not a huge issue but I thought at least the products in the HD range would have been calibrated to the same levels, surely? Now, if I calibrate the HD I/O to -20dbFS I'll be giving it headroom that it just doesn't have!

Keep rocking
JW
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Old 07-26-2013, 02:04 AM
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DetroitT DetroitT is offline
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Default Re: Audio level alignment across HD I/O & Omni

plus the Omni software cal is too coarse for me.
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Old 07-29-2013, 11:38 AM
the.engineer the.engineer is offline
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Default Re: Audio level alignment across HD I/O & Omni

Yes, I found that too. I could calibrate the outputs to -18dBFS but not the inputs. No good really.
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Old 08-02-2013, 12:21 PM
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O.G. Killa O.G. Killa is offline
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Default Re: Audio level alignment across HD I/O & Omni

Quote:
Originally Posted by the.engineer View Post
Great! fantastic! Wait a minute... doesn't the Avid Omni have a maximum output of +24dBu? And aren't all the inputs calibrated to -20dBFS? Actually, the output from the volt meter (having a 0dBFS sine wave through it) was reading 13.6V! That's over +25dBu!

I know it's only 2dB difference and in the grand scheme of things its not a huge issue but I thought at least the products in the HD range would have been calibrated to the same levels, surely? Now, if I calibrate the HD I/O to -20dbFS I'll be giving it headroom that it just doesn't have!
Interesting observation but it doesn't really tell anyone anything. If you ran a sine wave at 0dBFS and it came out to 13.6V then the unit isn't calibrated to -20dBFS or -18dBFS.

You see, the thing is, when you calibrate to -18dBFS (+4dBu), the loudest the system can technically output is 22dBu, even if the converters can handle putting out even more. If you calibrate to -20dBFS (+4dBu), the loudest you can put out is 24dBu even if the converters can go louder without distorting.

That is why you align everything. Ideally you want your DAW to start clipping before your converters do, that way you can see that you are clipping. If you are clipping your converters BEFORE your daw clips, the clip indicators won't go off and you won't know you are clipping unless you zoom all the way in on the waveform to see the rectified tops of the waveforms.

The other important point is you want the same voltage to carry throughout all your gear. If your "0" point is +4dBu on one device and 0dBu on another, then you will instinctively turn down the output of the +4 or turn up the output of the 0 to get the metering to look the same, but you are ultimately changing the voltage.

Most professional electronic gear is setup at 1.228V (rms)=+4dBu=0VU. Now you can make that equal to whatever you want in your daw based on how dynamic you want your music to be. Do you only do metal or heavily compressed Rock music? Then you don't need to calibrate to -20 if you don't want to. But whatever you calibrate to, that is what puts out the 1.228V so it aligns with 0VU throughout the rest of your studio.

If you need headroom because you are doing something that is very dynamic like orchestral or trailer music... then calibrate +4dBu=-20dBFS or even -22 or -24dBFS. The only thing you have to check in that instance is that you don't hit the upper limit of the converter. At -24dBFS calibration, your interface outputs/inputs 1.228V (rms) when it is at -24dBFS. Output a sine wave at -4dBFS, -2dBFS and 0dBFS and make sure the voltage increases with each and make sure the sine doesn't start turing into a square wave. If there is no voltage difference between -2dB and 0dBFS then you are past the physical capabilities of the converters and you should recalibrate down by 2dB... but it could be that the converters can handle it no problem.

Also just to make sure it is clear, even if two devices have different max outputs, (+22dBu and +24dBu) they can still be calibrated to the same ref level. They can still both be calibrated to -18dBFS=+4dBu or -20dBFS=+4dBu from the factory. What you have to do is check to see if -18dBFS or -20dBFS outputs 1.228V (rms). If both output 1.228 at the same internal dBFS level, then they are calibrated the same. If one device outputs 1.228V when it is set at -20dBFS and the other outputs 1.228V when it is set to -18dBFS, THEN they are not calibrated the same. It really has nothing to do with the max dBu output of the device.
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  #5  
Old 08-03-2013, 02:18 AM
Jack Ruston UK Jack Ruston UK is offline
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Default Re: Audio level alignment across HD I/O & Omni

The HDIO is freely adjustable over a normal range of values, but the Omni is fixed at the input (-20dbFS) and adjustable with trims at the output. The trims are coarse when you're matching voltages, but don't forget, we're still talking about 0.1dB steps! I did ask whether it'd be possible to recalibrate the inputs internally, or perhaps via a firmware update, but it seems that at the moment that's not an option. So the fact remains, unless you calibrate everything in your system around the 0dBvu=-20dBfs of those Omni ins, you can't have them all the same. It's a bit of an oversight IMO, especially since all Avid/Digi interfaces have come calibrated at -18dBfs in the past...So a lot of us have sessions interfacing with analogue gear at that level. I think the main thing to understand about the Omni is that it's not really designed to operate as 'pairs' of ins and outs (for hardware inserts or whatever). You can do it on a couple of channels, but it's not like the HDIO's in that respect. It has other advantages. You can of course use the analogue trims to calibrate the outputs.

J
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