May 2006, Issue 190

ARM-Based Modern Answering Machine
Philips ARM Design Contest 2005 First Prize

USER INTERFACE

I don’t like poorly designed user interfaces, so the TAM-TAM doesn’t have configuration buttons or a display. Think about it. Typically, you must configure such a product only once. Whereas a manufacturer has to deal with a microphone and its associated circuitry (hardware an end user won’t use more than once or twice), you can configure the TAM-TAM with a PC by editing and saving a simple text file.

Like all configuration files, the TAM-TAM’s configuration file (tam.txt) must be stored in the root directory. You may download a complete configuration file from the Circuit Cellar FTP site. You can use any text editor to modify the sample file. You can develop a simple front end in Visual Basic that will create the tam.txt file, generate the sound files, and verify that all the files referenced in the configuration file exist in the flash memory card’s root directory.

The other aspect of the configuration process is the creation of WAV files (e.g., the answering machine’s greeting message). WAV files are also recorded on a PC. The simplest recording application on a Windows-based PC is the Sound Recorder program, which has a limited number of default profiles. The TAM-TAM uses one of them. Standard rates range from 8 to 44.1 kHz. There are 8- and 16-bit formats for one to two channels.

Because the goal is to play and record files on a telephone line, there’s no point in using a rate higher than 8 kHz. The phone lines are limited to frequencies below 4 kHz. Phone companies carry voice signals at 8,000 samples per second. They also use nonlinear coding, which provides roughly the same dynamic range as a 13-bit linear coding would provide.

Finally, note that stereo isn’t played on phone lines, so mono is good enough. I selected 8 kHz with 16 bits and one channel. The Sound Recorder program shows this as:

8.000 kHz, 16 bit, mono 15 kb/sec

You’d think that someone at Microsoft would realize that 16 bits are equal to 2 bytes. If you’re sending 2 bytes 8,000 times per second, then your recording rate is 16 KBps, not 15 KBps! Despite this small glitch, there’s no need to worry. The format is correct.

You can perform more complex editing with more sophisticated tools. Adding music is a popular option. Doing it digitally is better than playing an old tape on a boom box near the microphone of your twentieth-century answering machine!