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Florian Bador



Joined: 08/05/13
Posts: 26
Loc: Santa Monica, CA
How to deal with monitoring input impedance while recording to AD converter?
      #1047213 - 08/05/13 11:42 PM
Hi!

I am working on a new setup where the final MAIN MIX will end in an Avalon VT-747SP (simply 2 balanced XLR outputs)
After that, it goes directly to an Apogee Rosetta for recording.
The problem is that I would like to listen what I am recording...

It's hard for me to imagine an Avalon XLR cable going to 2 different input plugs (weird way of working IMO)
It would go to the Rosetta and a PreSonus Central Station Plus e.g. (which is passive but still... input impedance will obviously vary with the monitors, and the cable is going all the way through the remote control! and then to the monitors)

The problem if I use something active is that it will have to be really expensive to make sure it doesn't mess up my final signal, and I don't even know what that would be.
Another solution would be to route the digital recording back to the Rosetta in the other direction and monitor it from there... but that's a bit ridiculous I think.

Anyone has a PURE & better way of solving this problem?
THANKS!

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Florian Bador, my personal website.


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Hugh RobjohnsAdministrator
SOS Technical Editor


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Loc: Worcestershire
Re: How to deal with monitoring input impedance while recording to AD converter? new [Re: Florian Bador]
      #1047240 - 09/05/13 08:25 AM
Quote Florian Bador:

It's hard for me to imagine an Avalon XLR cable going to 2 different input plugs (weird way of working IMO)




This is not weird at all -- it is actually the entire reason why we use voltage-matched analogue interfaces in pro audio, instead of the original impedance-matched interfaces that our industry started with!

It is perfectly acceptable to split passively a line level signal and feed it to multiple destinations because all properly designed analogue equipment has a very low output impedance (well under 100 ohms, typically) while inputs present very much higher impedances -- typically 10k ohms or more. This arrangement ensures that the level lost through the passive split is a tiny fraction of a decibel and thus entirely negligible and certainly inaudible.

Quote:

It would go to the Rosetta and a PreSonus Central Station Plus e.g. (which is passive but still... input impedance will obviously vary with the monitors, and the cable is going all the way through the remote control! and then to the monitors)




If you vary the monitoring level then you would also usually vary the input impedance to some extent with a passive monitor controller, and in the context of a passive split that could also affect the signal level received at the other split destinations very slightly. But again, the level shifts are likely to be tiny fractions of a decibel and not significant...and if you leave the monitoring level control alone while digitising the signal then nothing will change anyway!

To hang some numbers on it, the input impednace of the Rosetta 200 is 100k ohms, while that of the Presonus Central Station can be anywhere between 2 and 5k ohms depending on the setting of the trim and volume controls. I can't find an output impedance for the Avalon, but let's assume it is 100 ohms.

Worst case level loss is through the passive split is going to be 0.43dB (if the CS is at the 2K end of the range), and 0.18dB at the 5k end.

If the Avalon output impedance is lower -- say 50 ohms -- those figures drop to 0.22 and 0.09dB, respectively!

Quote:

Another solution would be to route the digital recording back to the Rosetta in the other direction and monitor it from there... but that's a bit ridiculous I think.




Not ridiculous at all -- I'd say essential! The most quality-critical stage of your signal path is the final conversion through the Rosetta. I would therefore suggest that it was imperative that you monitor the decoded Rosetta output if you want to be reassured that your A-D conversion is of the highest possible quality with no overloads! The D-A in the Central Station is a pretty good converter and they put that facility into the box for a reason...

Personally, given your equipment, I'd use a passive split on the analogue output from the Avalon to provide an option to monitor the analogue mix signal, and also take a digital split of the Rosetta converter to provide the option to check the digital conversion.

H

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Technical Editor, Sound On Sound


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Florian Bador



Joined: 08/05/13
Posts: 26
Loc: Santa Monica, CA
Re: How to deal with monitoring input impedance while recording to AD converter? new [Re: Florian Bador]
      #1047339 - 09/05/13 07:10 PM
Thank you very much, I feel less guilty

Interesting... I will probably keep Avalon > Rosetta with a single short cable and nothing else, then route the signal back to the analog output of the Rosetta for monitoring.

After all, that is the closest thing to what people will hear!
They will not hear my analog signal from the Avalon output, but an analog conversion of the digital conversion of it

If the conversions hurt the dynamic of the signal e.g. then I won't even know it because it's the way I heard it since I stated the mix.
For that reason it is probably in fact, the best way to monitor a recording!

THANKS

--------------------
Trust Music, a new music platform! (my profile)
Florian Bador, my personal website.


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