I’m using a Numark “Stereo IO” USB sound card, which has RCA inputs and outputs (suggesting it’s meant for consumer-audio line levels). With both the Line and Phono inputs, recording levels in Audacity are low (about -14dB peak). Tests with a calibrated signal generator at 440Hz show clipping at 3.3V P-P for the Line input and 20mV P-P for the Phono input; i.e. for Line In, 3.3V = 0dB in Audacity (Mac version, under OS/X 10.6.8)
If Line Out for consumer audio equipment is typically 0.32V, then I’m wasting most of the sound card’s 16-bit range.
Is this typical of USB sound cards? Is it typical of the Numark Stereo IO? Do I add a preamp, or look for a different sound card?
I don’t know how it’s wasting anything. That’s 20dB headroom and 76 dB noise floor. That’s broadcast levels. We would kill to have more people record like that. Most people try to record live performances jammed right up against zero which causes no end of problems. Clipping is fatal.
Where is the show coming from?
Phono Inputs have very serious frequency tilts, so measuring those is an adventure.
http://en.wikipedia.org/wiki/RIAA_curve
Koz
After I posted, it occurred to me that using 10% of the card’s range (.32V max vs 3.3V full-scale) amounts to 3.2 bits (=log2(10)) of headroom, which doesn’t seem so bad.
For what it’s worth, Numark tech support is unable to tell me the device’s full-scale input level. I’m not sure they even understand the question.