I was interested in using sound cards to plot pairs of signals in XY mode,1 and so measured the performance of a couple of popular USB sound interfaces.

Basic set up

I fed a 1V peak-to-peak sine wave into the input of the units, then grabbed a 10s sample with SoX,2 and looked at the statistics:

$ sox -d foo.wav trim 0 10 stat						
Input File     : 'default' (coreaudio)					
Channels       : 2							
Sample Rate    : 48000							
Precision      : 32-bit							
Sample Encoding: 32-bit Signed Integer PCM				
In:0.00% 00:00:10.01 [00:00:00.00] Out:480k  [    |    ]        Clip:0
Samples read:            960000						
Length (seconds):     10.000000						
Scaled by:         2147483647.0						
Maximum amplitude:     0.383392						
Minimum amplitude:    -0.383575						
Midline amplitude:    -0.000092						
Mean    norm:          0.243714						
Mean    amplitude:    -0.000053						
RMS     amplitude:     0.270708						
Maximum delta:         0.001953						
Minimum delta:         0.000000						
Mean    delta:         0.000744						
RMS     delta:         0.000835						
Rough   frequency:           23						
Volume adjustment:        2.607						

From this I extracted the ‘RMS amplitude’ at various frequencies.

I also used the sum of the ‘Maximum amplitude’ and ‘Minimum amplitude’ to get some measure of the interface’s DC-offset. This isn’t perfect, and is likely be noisy because we are using extremal values.

Also, our logging window is not synchronized to the incoming signal so we are unlikely to see an integral number of cycles. The incomplete cycle will have a non-zero DC component, and at low frequencies this might be a significant contribution.

If there are \( n \) cycles the worst-case fractional contribution will be \( 2 / \pi \). At 20Hz, \( n = 200 \) which implies a fractional noise level of about 0.16%.

I made no attempt to measure the phase changes at different frequencies.


It is simplest to just see the frequency responses plotted on a log-log scale:

Behringer UCA202

This sells for about £23 and seems well-regarded by Amazon reviewers. It offers a stereo input on a couple of phono plugs, and stereo outputs on phono plugs, headphones and optical.

Internally it is based around the Burr-Brown/TI PCM2902.3

The data above were collected at a sampling rate of 48kHz.

Key results:

Other experiments show the UCA-202 inverts the signal. They also suggest that things behave oddly at the high-frequncy end—I don’t understand this and so can’t explain it succinctly.

Generic CM6202 interface

These sell for about £7 and offer 5.1 analogue outputs and stereo inputs on 3.5mm jack sockets, and optical input and outputs.

Internally, the main chaip is a C-Media CM62064

The data above were collected at a sampling rate of 44.1kHz.

Key results:

Other experiments show that this CM6206 interface is not compatible with an iPhone or iPad using the Apple’s Lightning to USB cable because it draws too much power.