Most digital music devices rely on high-speed DAC converters to create the analog waveforms required to produce high-quality sound. In this lab you will create a very simple sound-generation system that illustrates this application of the DAC. Your goal is to play your favorite song. For the first step, you will interface a 74HC595 serial in/parallel out shift register to the SPI port. Please refer to the 74HC595 data sheets for the synchronous serial protocol. The second step is to create a DAC from the 8-bit digital output of the 74HC595. You are free to design your DAC with a precision anywhere from 5 to 8 bits. You will convert the binary bits (digital) to an analog output current using a simple resistor network. The third step is to convert the DAC analog output to speaker current using an audio amplifier, such as the MC34119. It doesn’t matter what range the DAC is, as long as there is an approximately linear relationship between the digital data and the speaker current. To do this you will have to run the analog circuit in its linear range. Be careful not to saturate the analog circuit. The performance score is based not on loudness, but on sound quality. On the other hand, sound quality will be a function of the number of DAC bits, the linearity of the analog circuit, and the periodic output rate. If an analog signal is noisy, you can add filter capacitors. It is important to add a 0.1 μF bypass capacitor on the power connection of the 74HC595 to prevent output glitches during serial input transmissions. The fourth step is to design a low-level device driver for the DAC. A single 8-bit SPI frame is all that is required to set the DAC output. The fifth step is to design a data structure to store the sound waveform. You are free to design your own format, as long as it uses a formal data structure (i.e., struct). Compressed data occupies less storage, but requires runtime calculation. On the other hand, a complete list of points will be simpler to process, but requires more storage than is available on the microcontroller. The sixth step is to organize the music software into a device driver. Although you will be playing only one song, the song data itself will be stored in the main program, and the device driver will perform all the I/O and interrupts to make it happen. You will need public functions Rewind, Play, and Stop, which perform operations as does a cassette tape player. The Play function has an input parameter that defines the song to play. A background thread implemented with output compare will fetch data from your music structure and send them to the DAC. The last step is to write a main program that inputs from three binary switches and performs the three public functions.
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