Differential signaling is a technique for electronically transmitting information by using two different signals. By comparison, single-ended signaling uses only one signal. Differential signaling is effective for both analog signaling and digital signaling. Some sound systems still use analog systems, but most communication protocols, such as Ethernet, Universal Serial Bus (USB), RS-422 and RS-485, use digital signaling.
The receiver reassembles the original signal by reading the difference in voltage between the two transmitted signals. This technique allows the receiver to ignore the absolute voltage of the signals, which can vary considerably as they travel between transmitter and receiver. Therefore, differential signaling is a much more reliable method of electronically transmitting information than single-ended signaling.
Differential signaling has twice the resistance to noise as single-ended signaling. This is an advantage with low-voltage electronic devices such as mobile and portable phones. These devices continue to trend toward lower voltage supplies to reduce unwanted radiation and to save power.
The high logic level in a single-ended digital system is the supply voltage (Vs). The low logic level in this system is the ground voltage, or 0 volts (V). The difference between these two voltage levels is therefore Vs – 0 V = Vs. The voltages in a pair of differential signals have the same magnitude but opposite polarity, so the voltages are +Vs and –Vs. This means the difference in voltage between the two signals is (+Vs) – (–Vs) = 2Vs, thus doubling the noise immunity of the signal.
Single-ended signals such as RS-232 have the advantage of requiring only one wire. A voltage of at least 12 volts indicates a signal, and a voltage of less than three volts indicates a lack of signal. This provides some noise immunity for a single-ended signal because natural sources of electricity rarely produce three volts.
The primary disadvantage of a single-ended signal is that it cannot function at high speed. Inductance and capacitance are electrical effects that tend to cancel out high-frequency signals, thus limiting the speed of a single-ended signal. This type of signal also requires higher voltage levels to avoid a high error rate during transmission.
Computers generally use differential signaling to minimize the effects of electromagnetic interference. These devices use direct current, which cannot screen out interference. A computer’s low-voltage signal line and high-voltage power supply line frequently share the same ground. This can cause the power line to induce a significant voltage in the signal line, causing interference.