May 2006, Issue 190

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

LINE INTERFACE

The key component in an apparatus connected to a telephone line is the data access arrangement (DAA). The DAA in the TAM-TAM was derived from the application note “Low Cost Telephone Line Interface (DAA, FXO).” Two optocouplers were added to the design from the Midcom application note. One is in the line to the transformer. It detects the short interruption of the line current that occurs when a caller hangs up the phone. This prevents the TAM-TAM from recording silence. The other optocoupler detects when the local phone is taken off hook. This process also stops the recording process. (You don’t want to record someone who hasn’t been forewarned!)

Another change from the system described in the application note is the addition of an AC bypass of the solid-state switch and the current sense optocoupler. The telephone company sends the caller ID information while the phone is still on hook. Therefore, an AC path through the transformer must be created even when current isn’t flowing through the DAA.

The DAA also includes a two- to four-wire converter. This block converts the balanced telephone line into the unbalanced ADC input and unbalanced DAC output. The ADC input is used for recording messages from the telephone line. The telephone line (playback of messages to the caller) and the audio amplifier (local playback of recorded messages) share the DAC. Two analog switches direct answering machine prompts to the phone line and local playback to the speaker.

A wall plug delivers 5 V, which a linear regulator reduces to the 3.3 V needed by the different ICs. The 5 V is used for the audio amplifier to reduce the strain on the regulator and increase the available audio power.