Measuring Frequency Response of a PC ADC
A PC ADC is a an ADC connected to a PC that can record audio and save that audio as a WAV file. This is contrast to a hardware or chip ADC that relies on a lot more layers of hardware and software to function.
The QA40x can be used to measure the ADC's frequency response using a single chirp. One ADC channel can be measured at a time. The top-level process is as follows:
- A reference chirp is prepared and exported from the QA40x software.
- The QA40x hardware is set to frequency response mode, and a chirp is emitted.
- That chirp is captured by the ADC and saved to a WAV file.
- The WAV file is edited such that the left channel is the ADC-recorded WAV, and the right channel is a reference WAV exported from the QA40x software.
- The stereo WAV is imported, and the captured WAV ADC frequency response is displayed.
Before starting, it's important to make sure you pick an FFT size and stay in the FFT size for the duration of the project. For this effort, we'll use a 48K sample rate and 64K FFT size.
Connect the QA40x hardware in loopback. This can be balanced or unbalanced. Balanced connections will be 6 dB "louder" so keep that in mind when verifying levels for export.
In the Frequency Response Settings, specify a level of -4 dBV (or -10 dBV if balanced). What we're aiming for here is a level that does not exceed +/- 1V in amplitude. Also, make sure you check the "Use Right Channel as Reference" checkbox.
If we run a sweep, the response is flat, even at the edges. We don't see the effects of the QA40x input capacitors or digital filters because we've indicated we want to use the right channel as the reference. In other words, we're only seeing differences between the left and right channels.
To export a wav, you must be FREQ display mode, and also dBR mode. The dBR mode sets to 0 dBFS point for the export. Since we specified -4 dBV (630mVrms = +/- 889 mVp), we can specify 0 dBV as the reference:
Export the file as Chirp 64K Reference.wav
Connect the ADC to the QA40x hardware output (balanced or unbalanced). Start recording on your Computer ADC, and press CTRL+SPACE in the QA40x software application. This will emit a single chirp.
Save the captured chirp on your PC.
We now have two WAV files. The first is the Chirp 64K Reference.wav and the second is the WAV recorded by the ADC. We can use something like Audacity to combine the WAV files.
In Audacity we can load each WAV. The aim here is to get the recorded WAV in the left channel and the reference wav in the right channel.
Below we see the WAVs along side each other. The upper wav is what we recorded in the ADC, and the lower wave is the reference.
And with some manipulation, we can get them aligned fairly closely (it's not super critical) and trimmed to the same length (do not change the length of the reference waveform).
Finally, we take the two mono tracks and convert to a stereo track. And we export that merged data to a WAV, making sure to preserve the sample rate and data format (float, +/-1).
Now we load the merged WAV into the QA40x software using the File->Import->Import WAV data, making sure we're still in FREQ RESPONSE mode and that our FFT sizes are the same. The resulting response is shown:
