Display Lag - Causes of Display Lag

Causes of Display Lag

While the pixel response time of the display is usually listed in the monitor's specifications, no manufacturers advertise the display lag of their displays, likely because the trend has been to increase display lag as manufacturers find more ways to process input at the display level before it is shown. Possible culprits are the processing overhead of HDCP, Digital Rights Management (DRM), and also DSP techniques employed to reduce the effects of ghosting – and the cause may vary depending on the model of display. Investigations have been performed by several technology-related websites, some of which are listed at the bottom of this article.

LCD, plasma, and DLP displays, unlike CRTs, have a native resolution. That is, they have a fixed grid of pixels on the screen that show the image sharpest when running at the native resolution (so nothing has to be scaled full-size which blurs the image). In order to display non-native resolutions, such displays must use video scalers, which are built into most modern monitors. As an example, a display that has a native resolution of 1600x1200 being provided a signal of 640x480 must scale width and height by 2.5x to display the image provided by the computer on the native pixels. In order to do this, while producing as few artifacts as possible, advanced signal processing is required, which can be a source of introduced latency. Interlaced video signals such as 480i and 1080i require a deinterlacing step that adds lag. Anecdotally, display lag is significantly less when displays operate in native resolutions for a given LCD screen and in a progressive scanning mode. External devices have also been shown to reduce overall latency by providing faster image-space resizing algorithms than those present in the LCD screen.

Many LCDs also use a technology called "overdrive" which buffers several frames ahead and processes the image to reduce blurring and streaks left by ghosting. The effect is that everything is displayed on the screen several frames after it was transmitted by the video source.