Input lag

In video games, input lag is either the delay between the television or monitor receiving a signal and it being displayed on the screen (see display lag below), or the delay between pressing a button and seeing the game react.

In electronic hardware development, input lag is the delay between an electronic input signal being generated (pressing a button as above) and processed (IO ports have been read, and memory updated to reflect the state of the input). In this field, the phenomena detailed below, are referred to as Output Lag. More formally, the terms 'Input Latency' and 'Output Latency' are also used. Due to gamers being largely unaware of the phenomena of input lag at this low level, the terms input lag and output lag became confused over time, and the above description, where input and output lag are combined into a singular phenomena known as input lag, has become popular.

Overall, the correct terminology is clear - A delay between a physical input occurring (e.g., I/O pin voltage change) and it being processed electronically (I/O pins read by processor and memory registers updated to reflect the state of the pins) is input lag, and a delay between an electronic output being sent (e.g., memory register set to reflect the desired state of an output) and it being processed into a physically observable phenomena (e.g., memory register read and I/O pin voltage modified accordingly), is output lag. In short, Input Lag occurs on input devices, Output Lag occurs on output devices.

Display lag

Display lag is a phenomenon associated with some types of liquid crystal displays (LCDs), and nearly all types of high-definition televisions (HDTVs). It refers to latency, or lag measured by the difference between the time there is a signal input, and the time it takes the input to display on the screen. This lag time has been measured as high as 68 ms, or the equivalent of 3-4 frames on a 60 Hz display. Display lag is not to be confused with pixel response time. Currently the majority of manufacturers do not include any specification or information about display latency on the screens they produce.

Analog vs digital technology

For older analog cathode ray tube (CRT) technology, display lag is extremely low, due to the nature of the technology, which does not have the ability to store image data before display. The picture signal is minimally processed internally, simply for demodulation from a radio-frequency (RF) carrier wave (for televisions), and then splitting into separate signals for the red, green, and blue electron guns, and for timing of the vertical and horizontal sync. Image adjustments typically involved reshaping the signal waveform but without storage, so the image is written to the screen as fast as it is received, with only nanoseconds of delay for the signal to traverse the wiring inside the device from input to the screen.