This example project implements acoustic range, finite infinite response (FIR) filters using the LPC4350 demo board and the ARM CMSIS DSP library. A sound resource of a science fiction “zap gun” plays out the headphone jack of the demo board. This sound sample is great for demonstration since it has many low-, mid-, and high-frequency components along the acoustical band. You play the sound resource by touching the capacitive touch buttons on the demo board as explained by the menu shown on the demo board’s LCD. It is best to listen to the sound with a headphone or ear buds. Each of the four buttons plays the same sound resource
1) Raw – The unprocessed sound plays according to its format (sample rate of 44.1 kHz, 16-bit sample and mono sound).
2) Low Pass – filtered through a low pass, Butterworth filter with a cutoff of 5 kHz, similar to the bandwidth of a common analog telephone. When played, notice that high pitch components are removed similar
to the sound heard over a telephone.
3) High Pass – filtered through a high pass, Butterworth filter with a low frequency cutoff of 8 kHz. Notice that the sound is lower in volume since only the upper harmonic components are played.
4) Backward Mask – The sound resource sample plays in reverse order. The “Zap!” is now a “Zoup!” The digital filters were designed with a favorite public domain tool, WinFilter (http://www.winfilter.20m.com/)
accompanying this example. With exposure of this free DSP design tool via NXP, I hope its author expands its features.
Steve Sabram — a pioneer of mobile platform application development — has a track record covering capital equipment, robotics, mobile systems architecture, embedded systems, instrumentation, low power DSP and smart Internet client devices.