Computer soundcards cannot distinguish between Multiple DC input signals. To overcome this problem, a signal containing several amplitude-modulated carriers can be constructed in hardware and de-constructed through software filtering. The objective of this project was to design, construct, and implement a hardware solution to collect multiple analog DC input signals and combine them for output as a modulated signal (Figure 1).
Each input from the Radio Drum is filtered using a 2nd Order Butterworth filter (Figure 2). Tunable frequency generators provide a carrier frequency for each channel. The carrier frequencies were selected at 6.97, 7.7, 8.52, 9.41, 12.09, 13.36, 14.77, and 16.33kHz to avoid aliasing and harmonic interference (Figure 5).
Each carrier is then amplitude modulated by multiplying with one of the time-varying input signals (Figure 3). The modulated carriers are then combined into one signal through a summing circuit (Figure 4). The output signal is buffered before leaving the board. A summation of the un-modulated carrier frequencies is accomplished through the same technique and is also buffered. These two signals are then sent to the output port.
Test points are available to examine the signal of each modulated and un-modulated carrier before the summing circuitry. This allows for straight-forward sampling of each channel, independent of the others. Using software, the output is separated into its eight components. These modulated carriers are then multiplied with the respective un-modulated carrier to product the original input signal. The inputs from each channel are then interpreted through software to give a representation of the transmitter's position relative to the receiving surface.