Since the phone system was designed to transmit the human voice, a number of compromises were chosen. The human ear can respond only to frequencies in the range 20 Hz <f< 20,000 Hz. However, the frequency range of the human voice is about 30 Hz <f< 3,400 Hz. The bandwidth of the phone system was designed to cover only the human voice bandwidth. This severe limitation of the phone system is a major impediment to the speed of modems. The maximum bit rate of a modem is limited to 2400 baud. Larger effective baud rates are accomplished through data compression and encoding techniques.

For example, using QAM techniques, four data bits can be transmitted at the same time. This is the basis on which the 9600 baud rate modem is designed (4*2400=9600). Newer techniques allow 8 or 12 data bits at the same time, yielding 19,200 and 28,800 respectively. All of these compression and encoding techniques put more stringent requirements on the modem's interface with the telephone line, in particular with the transformer that is used to interface with the line.

The transformer isolates the modem from the telephone line while it carries the modem signals from the sending modem to the telephone line and then carries the return signal off of the telephone line and back to the modem.

At the same time this trend to faster communications is occurring, another trend to minimization is occurring. The minimization trend is due to the desire to put modems into laptop computers via the PCMCIA interface. This trend drives the need for the 100EL, the 2829EL, and smaller laminations. Previously, 2425EL laminations were used.

1. Phone line connection devices
   1. Modems
   2. Fax machines
   3. Security systems that attach to phone lines
   4. Sophisticated phone systems
      
2. Hi-fidelity speaker crossover devices
   
3. Audio transformers