BORSCHT

A BORSCHT circuit is a subscriber line interface circuit (SLIC) that sits on the line card of a telephone exchange or central office switch. It performs seven distinct electrical functions needed to operate and supervise an analog telephone line, all within a single integrated circuit or circuit block. The acronym itself, though memorable, is purely mnemonic; the term stuck because telephony engineers needed shorthand for a function set that had grown too long to recite in full.

The seven functions are: battery feed (supplying DC voltage to the telephone handset and line), overvoltage protection (clamping transients from lightning or power line contact), ringing (generating the AC signal that makes the handset ring), signalling (detecting off-hook and on-hook conditions, DTMF tones, and pulse dialing), coding (compression of voice signal in digital systems), hybrid (the two-to-four wire hybrid that separates transmit and receive paths), and test (loopback and diagnostics for line testing). In practice, not every BORSCHT implementation performs all seven functions with equal sophistication; some functions may be integrated with other circuits on the card.

BORSCHT circuits became standard in the 1980s and 1990s as telephone carriers migrated from analog to digital switching while retaining analog subscriber lines (the final mile). A typical line card in a digital exchange might contain 8 to 24 SLIC channels, each with its own BORSCHT circuitry. The circuit must tolerate line voltages ranging from roughly 24 volts DC (on-hook) to 90 volts AC (ringing) and currents of 20 to 100 milliamps without distorting voice frequencies or introducing crosstalk.

The ringing function is particularly demanding: it must generate a 16 to 20 hertz AC signal at 40 to 130 volts RMS, capable of driving capacitive and resistive loads on lines up to several kilometers long, while the hybrid circuit must achieve 30 to 40 decibels of echo cancellation to prevent the caller from hearing their own voice delayed. Battery feed must supply at least 100 milliamps per line while maintaining regulation under varying load.

Modern BORSCHT functions are often integrated into larger mixed-signal ASICs or silicon-on-insulator chips, but the functional partitioning remains recognizable. The term is now primarily historical; as IP telephony and VoIP replace circuit-switched service, pure BORSCHT circuits have become rarer. However, telecommunications technicians and network engineers still encounter legacy POTS (plain old telephone service) systems, and understanding what a BORSCHT circuit was and did remains necessary for maintaining or troubleshooting older exchanges and line cards.