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apok · Posted - Sun Mar 30, 2003 5:13 am

Hi

Greenhorn question - sorry.

I'm getting distortion in live recording that sounds and looks like clipping even though the peaks are well below 0dB. In spectral view it shows as a clutch of regular bright vertical lines and sounds - well, as bad as clipping always does.

I'm using an Egosys U24 USB soundcard into a laptop. Recording from a normal amplified source is no problem at all, and has produced some very nice results. But when recording live (Shure PG81 via a basic mic preamp) I can't stop the clipping, even if I turn the record levels way down low.

Using the same mic/preamp setup into a standard tape deck doesn't produce any distortion at all, but I have to set the record level a bit lower than normal.

There's no physical signal level control on the soundcard, so I'm wondering if the signal from the preamp is overloading the soundcard and degrading the signal, so that any subsequent software control is useless. Is this possible? Any other ideas?

Thanks for your advice!
Adam

SteveG · Location: United Kingdom

apok · Posted - Sun Mar 30, 2003 8:14 am

Steve

Many thanks for replying so fast. Here are the numbers:

U24:
Input level: -10dBV unbalance: -10dBV nominal, +6.5dBV max
Output level: -10dBV unbalance: -10dBV nominal, +3.0dBV max
Input impedence: Unbalanced Mode 10K
Output impedence: Unbalanced Mode 100

The output of the preamp is 3.2V max./47kOhm.

Adam

SteveG · Location: United Kingdom

The answer is Hmm.... :)

Your mic preamp can provide an output of about +2dB max. You'd think that this was going to be okay, but if the gain of the USB port is set to a nominal -10dBv, and not adjustable, then anything above 0dB is going to make the record level clip - this will occur at 0dB. I suspect that what the +6.5dB max figure might mean is that there is no danger of damage ocurring below this level!

So if you run your mic pre so that it provides the nominal 3.2v max that it is capable of, then this will overload the input by 2dB, and since this is the clipping threshold, it's going to sound dreadful.

So my question is - if you record a few dB quieter (further away?) does this still happen? I reckon that you are going to need some sort of attenuator between the pre and soundcard, at any rate.

apok · Posted - Sun Mar 30, 2003 12:57 pm

Aha! That must be it.

Yes, if I put the mic much further away the clipping disappears, but the sound is of course horribly thin and weedy.

So an attenuator (or better preamp) it is then.

My thanks for your advice. What a great thing this forum is!
Adam