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percy 31st May 2006 03:43 PM

SNR/Dynamic range worsens in low frequency range.
What is the reason for SNR and thereby dynamic range to degrade in the low frequency range ?

I was just testing a soundcard using RMAA and the charts show that the snr and dynamic range gradually degrade in the low frequency range. It starts off at -108db at 20Khz and by 100hz it is down to -93db, -90db @ 30hz and -87db @ 20hz.

What does this generally indicate ? Is this something to do with the "digital" part of the system (jitter, etc..) or "analog" (psu decoupling, filtering, bad caps, etc..) ?

jackinnj 31st May 2006 06:24 PM

you pick up noise from the mains -- not just 60hZ and its harmonics, but the intermodulation products -- to reduce the problem you must be careful about routing wires and cables.

percy 31st May 2006 08:55 PM

the cable is just a short (6") shielded stereo patch cord going from line-out to line-in, and its not too close to the mains cable.

Why do I faintly remember that I observed the same thing when I was testing an old sony discman which was running off batteries. There was a sudden drop in SNR/DR below ~1Khz.

jcx 1st June 2006 12:41 PM

Check freq response, high pass filters in dac and adc multiply and cut signal at low freq

Op amps can have 1/f noise corners higher than 1 KHz, fet and cmos inputs usually have higher 1/f than bipolar

bipolar op amp input current noise can have a higher 1/f - too high coupling/feedback impedance can make input current noise prominent

High freq noise can alias down to audio band, so noise at ~ n*sampling frequency translates to low freq noise, oversamping/delta-sigma adcs move this aliasing up to their oversampling freq

On board soundcards can be pretty ugly

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