Originally modems came as external boxes, and it was not until the late eighties - with the advent of DSPs and developments in CMOS technology - that the concept of embedding modems into application-specific products began to appear. All of a sudden, modems began to find themselves integrated into all manner of terminals that needed to transfer digital data using the telephone network. These included fax, point-of-sale, credit-card, and alarm systems, as well as personal computers. The cost, size and power-consumption advantages of embedding modems into equipment remain the major reasons why embedded modems are so important today.Despite the trend towards broadband networking, the dial-up telephone network is still the ubiquitous means of communication on the planet. Today, embedded modems are finding their way into many new applications; this is driven predominantly by the cost reduction of devices and the transition of adopting the software modem (soft modem). With the evolution of processors increasing in capability and reducing in price, modem developers have taken advantage of this by further integrating the modem function into the processor.The soft modem can be implemented on many of today's popular processor and controller architectures. The only additional components required are those for analogue/digital conversion and interfacing to the telephone network. This idea is well tried and proven, having been adopted by notebook computers over the last decade. Although there are still markets for the dedicated hard modem, the soft-modem approach is growing considerably due to significant cost advantages.For the soft-modem solution, the main external components are the analogue front end and DDA (direct-access arrangement). It is the DAA that remains the most challenging aspect. Most countries in the world require an assurance that equipment connected to their dial-up network will do no harm to it, and also that the equipment itself is safe, especialally against high-voltage events. Each country has its own set of requirements that the equipment manufacturer has to get approval for before being allowed to sell and deploy. As a consequence, the challenge for an embedded modem solution is to be able to meet these distinct requirements with a single bill of materials. The DAA fundamentally provides the necessary electrical high-voltage protection; controls the on and off of the circuit; sets the amount of current taken from the dial-up network; controls return loss to provide maximum power transfer into the network; minimises the amount of echo returned; and, when off hook, provides the ringing tone to the device. In addition, newer functionality such as line in use and caller-ID are expected. The modern-day DAA is the answer to these challenges. Integration of the DAA analogue circuitry into a device such as the Teridian 73M1x22 allows a system to control various DAA functions such as line-matching impedance and loop-current control by software. This, with the sophisticated and accurate circuitry built into the device, provides reliable mechanisms to deliver line-monitoring capabilities such as determining if a line is in use before making a call, or if during a call the handset is picked up.As previously highlighted, a major requirement is to prevent high-voltage surges from being passed from the equipment to the dial-up network and vice-versa. There have been many ways to achieve this: electrical isolation; wet or dry transformer, optical coupling and - recently - capacitive coupling. Teridian's method to provide electrical isolation uses a standard low-cost pulse transformer that has a low-impedance true differential transmission path that enhances common-mode noise and EMI performance. This, coupled with the ability to provide power to the line-side circuitry, offers high performance independent of the dial-up network abilities in order to allow sufficient power to keep the DAA operational. Thanks to such innovations, embedded modems will continue to enter applications that they have previously been excluded from.
