Version 1 of my microphone with
proximity detection used ultrasonic sensing and analog electronics. The
system worked, but the sensing was finicky and the analog electronics
limited flexibility in how the system could be expanded.
In Version 2, I wanted to compare ultrasonic sensing to
capacitive sensing for robustness in proximity detection. Because ultrasonic
sensing is directional, using multiple sensors allows for greater possibilities in
detecting tilt and placing where large objects, such as people, are in relation
to the microphone.
Second, I wanted to move the project from an analog electronics environment
to a digital signal processing environment. By controlling the logic
and function with a DSP chip, it is only a matter of software to change the function of the
proximity detection system from adjusting gain to adjusting bass response and equalization. Taking this idea further, it is now possible to map the ranging
of the ultrasonic sensors to various musical effects. Through software mapping,
the microphone promises to be a new type of music controller.
These aren't speakers but are the ultrasonic sensors. What you are seeing are 6 pairs of Devantech SRF10 ultrasonic rangefinders outlining the circumference of the microphone. Each pair consists of one transmitter, one receiver, and a microcontroller. The rangefinders are slaves on an I2C bus.
An Analog Devices ADSP2181 DSP development board is the I2C host and processes the microphone audio. The DSP chip calculates and overall distance
reading the distance measurements of each of the pairs of rangefinders and
applying a moving average filter.
Here's a video of my officemate Victor testing
the prototype. The microphone is fixed to a microphone stand while Victor
moves toward it and away from it.
The left channel of the microphone is not being altered by the rangefinding system. You can see its real-time recorded signal being recorded on the top 1/2 of the desktop computer screen in the video.
The right channel gain is being altered by the rangefinding system and the DSP chip. You can see its real-time recorded signal on the lower 1/2 of the desktop computer screen in the video.
The laptop computer to the left of the desktop computer is showing a graph. The left bar of the graph shows the current distance Victor is from the
microphone as calculated by the rangefinding system.
The right bar of the graph shows the amount of amplification being added to
the right channel as a consequence of this calculation. The gain is
being adjusted as the square of the distance.
Here are the resulting wave forms from the demonstration video above. My prototype is the
lower waveform while the conventional microphone signal is the upper waveform.
The upper waveform (without rangefinding correction) clearly shows a dramatic increase and decrease in the recorded
signal strength as a function of the distance Victor was from the microphone.
The lower waveform (with rangefinding correction) shows a very strong
signal when Victor is far from the microphone, and the signal actually drops slightly when Victor is close. This means the system is over-correcting for
the distance. Tweaking the adjustment of gain vs. distance is now a trivial matter
as it happens through a calculation in the DSP chip's software.
Besides seeing this visually, you can hear this aurally. Listen to the difference between
the two microphones:
Demo audio with my prototype.
Demo audio with conventional microphone.
I have already designed a new mount for the ultrasonic sensors so they
can sense proximity more successfully throughout the 3 axes. With this
new mount and the DSP environment, it is now possible to create a new
type of music controller which a musician can manipulate both through
sound and proximity. I would like to then integrate this into the
modular sound-blocks I am developing.