It is a simple device that emulates a dictaphone, and stores the voice digitally. The Analog circuitry in this project contains a microphone pre-amplifier, an anti-aliasing filter, a postfilter and a power-amplifier.
The heart of the Digital Voice Recorder (DVR) is an Atmel AT90S8535 microcontroller. It is a powerful RISC 8-bit microcontroller, with 8kB internal FLASH program memory, 512 bytes EEPROM and 512 bytes of SRAM. It’s RISC CPU core can execute almost 1 instruction per cycle. The core is complemented with a rich set of peripherals, including I/O ports, timers, PWM, A/D converters, etc. Details can be found at Atmel’s Website.
A 16 Megabit data-storage FLASH chip (AT45D161) is attached to the MCU through a serial interface (SPI). This stores the sampled data from the MCU, and also sends back stored data to the MCU. Four pushbutton switches and four LEDs form the user interface. These are connected to the MCU’s I/O ports. The audio is picked up by a condenser microphone, and amplified by an op-amp based amplifier. It is filtered and passed to the MCU’s A/D converter, which samples the audio at a rate of 8kHz. The 10-bit samples are scaled down to 8 bits before storage. During playback, the samples stored in the FLASH chip are sent to the MCU’s PWM module, which produces a 15kHz PWM signal whose pulsewidth is proportional to the amplitude of the sample. This is filtered and amplified, and is passed to a loudspeaker.
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