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DIGITAL FILTERS MADE EASY

Part 2: The Rest of the Story
by Dan Cross-Cole

Start ý All the Pieces ý Using the DSP/FFT System ý Sources and PDF

ALL THE PIECES

In the sampling circuitry, the audio input should be fed through a capacitor to block any DC component from the signal source (see Figure 1). The DSP board provides the necessary isolation, but other inputs may not.

Figure 1ýThe DSP/FFT system uses the DSP board to provide different audio band-pass filters. The amplifier shifts the filtered AC signals to the 0- to 5-VDC range. The ADC board digitizes the audio and sends the data to the PIA board, which plugs into the ISA bus of the docking station. The laptop then uses the FFT program to display the frequency components.

Besides multiplying the signal by 10, the amplifier adds 2.5 V to the signal, allowing an AC signal of 0.5 V (peak to peak) to just span the 0- to 5-V range of the analog-to-digital converter (ADC) (see Figure 2).

Figure 2ýThe amplifier adds 2.5 V to the AC signal from the DSP board and provides a voltage gain of 10. The output ranges between 0 and 5 VDC.

The ADC board uses a commercial 1-MHz oscillator module (see Figure 3). A 25-wire ribbon cable connects the ADC to the peripheral interface adapter (PIA) circuit. Seasoned readers will recognize this circuit from my book, 26 Hardware Projects for the Home Computer.

Figure 3ýThe ADC conversion board is connected to the DB-25 connector on the PIA board by a ribbon cable.

The PIA board is designed for an ISA bus (see Figure 4). The PIA chip provides eight input bits, eight output bits, one read/write signal, and seven control bits. For this application, you only need the eight input (data) bits and three control bits. These bits are controlled by the QuickBASIC FFT program.

(Click to enlarge)

Figure 4ýThe DB-25 connector allows ribbon cables to connect to different devices (in this case, the ADC board).

The FFT program takes samples, calculates the Fourier Coefficients, and then displays only the constants up to about 1.5 kHz (see Listing 1). Note that the program is designed to interface to the hardwired ports on the PIA board. If you choose to set up the ports differently on the PIA board, youýll need to change the program.

The program also does a host of other useful things. It sets up the PIA automatically, but you can manually change the settings. To set up the FFT routine, type "F", hit the enter key, and you should see the display appear. Fluent programmers will discover that only a fraction of the total data available is displayed.

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