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Issue #202 May 2007
The Wittness Camera
Build a Self-Recording Surveillance Camera
Grand Prize Atmel AVR Design Contest 2006
by Alberto Ricci Bitti
Start | Solid-State Recording | Full Interaction | Complete Picture | Basic Instinct | Filled To Capacity | Speech Preparation | Circuit Implementation | Concept To Prototype | Picture Inspection |Design Evolution | Sources & PDF
FULL INTERACTION
Having a file system in place opens up numerous possibilities in
the world of 8-bit microcontrollers. Once you try it, it’s hard to go
back. The ability to access huge data files is exploited with the camera’s
interactive setup (see Figure 2).
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| Figure 2—I used an IR remote for system set up. The camera responds interactively via a voice-prompt menu system. The huge capacity of the flash disk is exploited for storing voice samples. The PWM reconstructs the audio waveforms. |
During the set-up process, you control the camera with an IR control (an off-the-shelf universal remote), and the camera responds with voice prompts. The prompts guide you through the editing of all the operating parameters (e.g., date, recording mode, picture resolution, and the number of frames to take for each trigger). The voice menu block is the software module that runs the user interface. It decodes the RC5-encoded pulse train from the IR receiver and interprets the key presses for editing the settings from the ATmega32’s nonvolatile memory. During this process, it invokes the speech synthesizer module, which provides the primitives needed to compose and play the messages.
The speech synthesizer takes advantage of the existing mass-storage capabilities by using the file system in Read mode. The flash memory disk must be prepared in advance with a special set of files with audio samples for each word or status message to be pronounced (e.g., the file “15.PCM” stores the waveform samples for the word “fifteen”). To reconstruct the audio signal, simply open the file and send the samples to the ATmega32’s PWM feature.
Sample compression is unnecessary. Years ago, devoting more than 1 MB of flash memory to this system would have been regarded as extravagant opulence. Today, this figure represents about 0.1% of the flash memory disk capacity! Actually, voice menus are cheaper than an LCD module, and far more effective, because it’s expected that the camera will be concealed or placed out of the way.
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