Author Topic: How to Calculate Attenuation Needed  (Read 1383 times)

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Offline kolbep

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How to Calculate Attenuation Needed
« on: April 11, 2014, 09:22:22 pm »
Hi All,
Let me chuck some dB stuff at you, and lets see what we can handle.

At a production for a High School, I am going to be Videoing, I normally grab my sound direct from the desk using a Zoom H1.
For the past 2 years I have done this for the school, I have had a hikkup with my sound

The 1st year, I had medium distortion (2 x Mono Jacks -> 1 x Mini JACK Cables from the rec out on the desk to the H1).
The 2nd year, I made up a cable that had 2 Attenuators in it (one for each channel), but I think I lowered the signal too low, because I was getting digital ticking sounds (probably from the desks internal digital fx module/usb module/etc)

The main problem with the Zoom H1 is that it uses digital attenuation, so by the time you have controlled the level with the level selection, it is too late, the Line Level signal coming into the Audio chip is TOO HOT.

So what I was thinking, is attenuating exactly to match the Maximum Output of the Desk, with the Maximum Level into the Zoom H1's Chip.

The Desk is a ZED22FX, and the specs for the REC Out are :
Rec Outputs (phono sockets) 0dBu nominal. +21dBu maximum.

I have checked the datasheet for the chip in the Zoom H1, (http://www.ti.com/lit/ds/symlink/tlv320aic3101.pdf), and it gives
AUDIO ADC
Input signal level Single-ended 0.707 VRMS

So I plug those valies into a calculator http://www.sengpielaudio.com/calculator-db-volt.htm
and it says 21dBu = 8.691117573 volts (RMS)
So, if that is the case, and I want to attenuate the signal so that even if the mixer is pushing out just below clipping, I will not distort on the Zoom H1, then I need to attenuate by 12.2929 Times.

Now I am getting lost (never really did pay attention to dB's in Technika Electrica class in high school [And that was 14 years ago!!!]

Am I working this out right, or has my train climbed off the track, and fallen down a cliff already (by the way, I know somebody that used to work for the Bloutrein in Johannesburg, and if you said something was a train wreck, he pounced on you  :box:) ???

Ok, now the next part, if my attenuators look like this (YAAY ASCII ART  :-+)

-----------------/\/\/ R1 /\/\---------------------------|2.2uF|-----------
                                              |
                                              \
                                              /
                                              \
                                              R2
                                              /
                                              \
                                              |
 -----------------------------------------------------------------------------

Then what should the ideal values for R1 and R2 be.

Thanks for all the assistance. I can never really figure out this db stuff  |O
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