![]() ![]() Residual noiseĭespite the name, this is not a noise test in the sense of “random” noise. The reason is that in some cases the “mic” input may have a low impedance that will affect low-frequency response in a way that doesn’t reflect realistic usage. If you are using a device that has both “mic” and “line” inputs, this should be done with both sets of inputs if possible. With a program that easily measures and plots frequency response – like FuzzMeasure Pro – set the output level to -10 dB and run a frequency sweep at each available sample rate. 100 mV) is probably more representative of typical usage anyway. Remember that these gadgets are typically running from a single 5 V supply. The reason for running at -20 dB is that sometimes distortion levels drop markedly at lower signal levels. This is also consistent with the AES17 standard for measuring digital equipment. To keep things simple, I don’t usually present the 0 dBV result. As noted, not all ADCs can be driven to 0 dB FS, hence the -1 dB test. In tests 4-6 below, tests are run at 0 dB, -1 dB and -20 dB. Basically, by making 0 dBV equal to 0 dB FS for both input and output, everything becomes easier to explain. The purpose of this step is to ensure that following steps are easy to deal with. This is only noticeable at low signal levels, but turning it off now may save some of your remaining hair! I’ve done it a number of times and not damaged anything so far, but still… Also, I’ve discovered that the phantom power supply can add noise to the signal on some interfaces. For one thing, it’s probably not a good idead to connect line-level outputs to inputs with 48V on them. If using a mic input, be sure to turn phantom power off.It’s not critical for the purposes of this “simple” test protocol. In that, just adjust the gain controls to get is as close as it can be, and make a note of it. Some interfaces have digitally controlled gain, and can’t be set to exactly 1.0 V for full scale.In this case, make a note of this fact – it’s most academic but should still be noted. In that case, set the signal generator to e.g 90% and the ADC level also to 90% of full scale. Some ADCs can’t be driven to full scale output i.e.Set the input gain control so that the signal is just below full-scale, typically 0.99 of full scale, or -0.1 dB. Usually this will be the “line” input although sometimes it can be the “mic” input – it does depend on the specifics of the interface, but this is the input that you would use if making line-level measurements. (Depending on the program, it may be 0 dB or 100%.) With a multimeter, set the output gain control so that the voltage measured at the soundcard output reads 1.0 VRMS. Since most of these units have balanced inputs and outputs, the voltage is measured across the balanced line, not from one output to ground.Ĭonnect a loopback cable from the output to the appropriate input. Level calibrationįirst, set the digital signal generator to output a 1 kHz sine wave at full scale. These are Mac programs but there are Windows equivalents. The measurement programs that I am using are FuzzMeasure Pro and the Electroacoustics Toolbox. I’d appreciate any comments or suggestions. Here’s what I’ve come up with, with some explanation and reasoning provided where appropriate. Doesn’t require access to any additional expensive equipment.The idea is that I can characterize any such device in a way that: So, I decided to put together a measurement protocol for these devices. Sadly, I don’t have a spare $20k laying about for a proper test rig. But I also have another goal in mind, which is to use one of them as the basis of a “cheap and cheerful” measurement rig for electronic components, which is a much more demanding task. I’m in the process of testing a number of audio interfaces aka “sound cards.” Partly this is because I’m reviewing a couple of units for the purpose of making room and loudspeaker measurements.
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