Confirming The Quality Of One’s Tools

When experimenting with a new audio path I like to take measurements. Long ago, in an age of techno-pre-history known as the late 1980’s,  I craved what were then an emerging class of computerized test instruments, like the Audio Precision System One. Happily, today such costs are unwarranted given the current class of programmable smart devices. I’ve been very happy with Audio Tool For Android running on my Nexus 7 tablet.

If I am to trust the measurements that Audio Tool allows me to make I need to start by confirming the validity of it’s measure using a known reference signal. I was also making use of the Zoom H2 Handy Portable Stereo Recorder, so I decided to record the output of the sweep generator in Audio Tool to the H2. Then I tool the resulting WAV file into Adobe Audition to see what resulted.

The physical connection between the tablet and the Zoom recorder was a simple 3.5mm analog cable.

This is a screen capture from Adobe Audition set to display frequency vs time. The vertical axis runs 0 Hz to 22 KHz, consistent with the use of a 44.1 KHz sample rate to make this test recording.

The left half of the presentation shows a sweep tone that starts at 100 Hz and moves up to 20 KHz over 20 seconds. The fact that it’s displayed as a straight line up to the right indicates that the signal path had no significant impact on the signal.

The right side of the display is a white noise burst. White noise by definition contains equal energy at all frequencies. Thus it should, and does in this case, fill the entire available audio channel.

This is about as clean a signal path as possible with the tools that I have at hand. It’s certainly good enough to establish a reference against which to compare commonly available audio codecs. I should be able to trust measurements taken using the same gear, but with a pair of soft phones inserted into the signal path.