dsPIC Audio Filter (Part 1) by Leon Heller G1HSM

I designed this simple unit a couple of years ago, when the Microchip dsPICs first became available. The dsPICs are described as digital signal controllers by Microchip, that is, micro-controllers with additional digital signal processing capability, as opposed to pure digital signal processors (DSPs) like those produced by Texas Instruments and Analog Devices. Unlike conventional DSPs, the dsPICs are quite easy to use from a hardware point of view, as many of them are available in user-friendly DIL packages.

The article is in two parts, a brief introduction to DSP and a description of the hardware, to be followed by a description of software development.

DSP applications like this are conceptually quite simple, the audio input is sampled using an analogue to digital converter (ADC), which converts the signal into numerical values. The numerical data are then manipulated by the DSP in some way, such as applying a filtering algorithm, and then converted back into audio using a digital to analogue converter (DAC). Sampling has to be done at a frequency that is at least twice that of the bandwidth of the signal, and it is usual to  filter the signal before the analogue to digital conversion to avoid ‘aliasing’, that is, signals outside the required band getting converted and mixed up with the required signals. Once this has happened they cannot be removed. For many applications such as this, the ADC,  DAC, filters and pre-amplifier are often combined in a single device called a codec (coder-decoder).

The schematic should be fairly self-explanatory to anyone who has some micro-controller experience. I used the 40 pin DIL version of the dsPIC30F4013 which is a 5V device, with the Si3000 audio codec. The codec is designed for speech systems, and is easily interfaced to the dsPIC. It contains a headphone amplifier as well as the pre-amplifier. I’ve provided a microphone input (an electret microphone works very well) and outputs to stereo headphones as well as  line input and output.

Although I’m not using them in this application, I’ve included a potentiometer which may be read using an ADC input, an LED and a push-button. These may be used for functions like menu selection, digital volume control, and debugging.
The PCB is single-sided, and I made the prototype at home. The Si3000 is only available in a SOIC (surface mount) package, and is therefore mounted on the underside of the board. The grey connector is for the Microchip ICD 2 programmer/debugger. This is quite an expensive unit, and I could provide programmed chips to anyone who wishes to build one of these units. I had a small batch of the boards made in the Far East, they weren’t very expensive.

Software development will be covered in Part 2.

Leon Heller, G1HSM June 2007.

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