Limitations Of Sound Interfaces

Sound cards and sound interfaces are not designed for making transfer function measurements. They merely provide a cost-effective solution since almost all computers have sound cards. We demonstrate these weaknesses given measurements made on a PreSonus Firepod sound interface. The output from channel 1 was directly connected to the line input on channel 1, and the sampling rate was $f_S=44.1$kHz.

1. Sound interfaces attenuate frequencies near DC and near half of the sampling rate $f_S/2$. Figure 3 shows the measured magnitude response of the sound interface. Ideally it would be constant for all frequencies. However, Figure 3 shows that the -3dB point for the DC blocker lies near 5Hz. The anti-aliasing filters cause the magnitude to roll off sharply such that the high frequency -3dB roll-off point is about 20.9kHz.

   Figure 3: Magnitude response of the PreSonus FirePOD sound interface when the channel 1 output is directly connected to the channel 1 line input

   

2. The same sound interface filters cause the phase response measurement to be incorrect as well. Although ideally the phase response would be 0 radians for all frequencies, Figure 4 reveals that the phase measurement is distorted in roughly the same regions where the magnitude measurement is distorted.

   Figure 4: Phase response of the PreSonus FirePOD sound interface when the channel 1 output is directly connected to the channel 1 line input

   

3. Even when a computer is programmed to pass an input signal flowing into the sound interface input out of the sound interface output as fast as possible, there is a delay or latency. This delay is typically on the order of tens of milliseconds. Figure 5 shows the impulse response measured on the PreSonus card. The latency is approximately 62ms. The latency could have been decreased some by adjusting software settings in pd. The impulse response also rings noticeably due to the anti-aliasing filters.

   Figure 5: Impulse response of the PreSonus FirePOD sound interface when the channel 1 output is directly connected to the channel 1 line input

   

   For reference, we provide the measured impulse response shown in Figure 5.