X marks the spot at 32000
Audio cards at the 32000 sample rate were analyzed for a common X crisscross
pattern. A linear sine wave sweep is the signal displayed in the
spectrogram images below. Signal generation and collection were in full
duplex loopback mode with both the
DAC sample rates set to 32000.
This gallery is part of the Full Duplex DAQ comparison
Six of the ten audio devices exhibit the distinct symmetrical X mark and one
device has X-like crosshatching. Why is this artifact so common?
The spot is dead center located at half the
Nyquist frequency (8000 Hz)
and halfway into the time duration of the linear sweep. There is nothing
special about the position of the spot other than it is exactly in the
middle. This geometry can have a number of
The positive slope line of the X looks like a frequency inverted (folded)
version of the main spectrogram linear sweep fundamental. Most of these
audio devices have good quality ADC and DAC anti-alias filters yet for some
reason the fundamental is being inverted and mixed in. This artifact is
probably related to the 48000 / 32000 = 1.5 decimation ratio.
It is interesting that this phenomena is present in so many audio devices made
by different manufacturers.