Terminology

Index

Source Device

Definition: A device that is providing an audio/video stream.1

Examples: Video game console, Blu-Ray player, set top box, computer, etc.

Transport Interface

Definition: The technology used to transport an audio/video stream from a source device to a sink device.

Examples: HDMI, DisplayPort, DVI, composite video, RCA audio, component video, VGA, etc.

Additional notes: This website focuses on consumer electronics technologies with raster scanning (or similar) video transport interfaces.

Sink Device

Definition: A device with audio/video input. In the case of HDMI, this is a device with an HDMI input.2

Examples: TV, sound bar, home theatre receiver, monitor, HDMI switch, etc.

Video Latency

Definition: The amount of time between the video signal entering the sink device‘s input to the actual presentation to the user on a display.3 Not to be confused with input lag, which includes half of a video frame’s transport time. Video latency is relevant to audio/video synchronization (lip sync).

Example animation: The video latency of the sink device is 19.6 ms because the presentation of each line of video is delayed by 19.6 ms, relative to the time it was transported by the source device.
Example animation: The video latency of the sink device is 20 ms because the presentation of each line of video is delayed by 20 ms, relative to the time it was transported by the source device.

Unlike input lag, a display’s video latency may be identical at different refresh rates. Video latency will be consistent for all pixels in a frame of video if the display presents each pixel a fixed time after receiving that pixel’s data. Some displays will delay different pixels by different amounts when their native refresh rate is different than the video signal refresh rate.

Video latency is measured at the center of a display, 4, 5 but care must be taken to ensure you are not accidentally measuring input lag since some measurement tools are not designed to measure video latency at the center of a display without additional calculations on the reported value.

Expected range: 0 ms and higher

Additional notes:

This value may be reported by HDMI sink devices via EDID information or via CEC messages for the HDMI Auto Lip Sync feature.6 Beginning with HDMI 2.1 it is mandatory that devices report this information.7

Chaining sink devices: There are many scenarios when a number of sink devices are chained together, such when as a receiver, switch, splitter, or audio/video processor are used with a display. In this case, each of these sinks may add audio or video latency.

Advanced topics: Advanced Topics in Video Latency and Audio/Video Synchronization

Audio Latency

Definition: The amount of time between the audio signal entering the sink device‘s input to the actual presentation to the user on speakers.8

Some latency is inherent to embedding an audio signal into an HDMI video stream, which is why the spec allows for +/-2ms of audio/video synchronization error.9

Expected range: 0 ms and higher

Additional notes:

This value may be reported by HDMI devices, such as TVs, sound bars, receivers, etc., via EDID information or via CEC messages for the HDMI Auto Lip Sync feature.10 Beginning with HDMI 2.1 it is mandatory that devices report this information.11

Chaining sink devices: There are many scenarios when a number of sink devices are chained together, such when as a receiver, switch, splitter, or audio/video processor are used with a display. In this case, each of these sinks may add audio or video latency.

Lip Sync Error

Also known as:

  • AV-sync error (Audio/Video Synchronization Error)

Definition: The time difference between when a synchronized audio and video signal are presented as sound and light, relative to the video signal. Not to be confused with Lip Sync Delay, which is a delay that can be added to audio to correct lip sync error.

Expected range: Negative and positive values where a negative value means the video leads the audio and a positive value means the audio leads the video.12,13

Calculation: Lip sync error can be calculated by subtracting audio latency from video latency.

Examples:

These lip sync errors are ordered in terms of best to worst:

  • 0 ms (best)
  • -30 ms (ok)
  • +30 ms (worst)

For more information on why, read Acceptable Audio Latency and Lip Sync Error.

Additional notes: The HDMI specification requires that a source device “shall transmit audio and video data streams over HDMI with no more than ±2 ms of audio delay relative to the video”14

Additional resources: https://en.wikipedia.org/wiki/Audio-to-video_synchronization

Advanced topics: Advanced Topics in Video Latency and Audio/Video Synchronization

Lip Sync Delay

Also known as:

  • AV-sync delay
  • Audio delay (a manual user setting in a receiver, for example)

Definition: A time delay added to an audio signal to bring lip sync error to zero. This is not a measurement, but instead is a setting on a device that can be adjusted manually by the user or automatically through a feature such as HDMI Auto Lip Sync. Because it is a delay on the audio signal, it can only be used to correct a negative lip sync error.

Expected range: 0 ms and higher

HDMI Auto Lip Sync

Definition: An HDMI feature introduced in the June 2006 HDMI Specification 1.3.15 This feature was improved in HDMI Specification 2.0 to enable dynamically adapting to changes in latency of different display modes or signal types.16 Support for this feature became mandatory in HDMI 2.1.17

This feature is implemented by an HDMI device by making use of audio and video latency information that is shared through EDID and CEC messages.18

A detailed description of how this feature functions, including numerous examples, is found in Section 10.6 “Auto Lipsync Correction Feature” onward of the HDMI Specification 2.0.

Input Lag

Definition: Video latency plus half of the time it takes to transport a frame or field of video at a given refresh rate.

Example animation: The input lag of the sink device is 28 ms because the presentation of each line of video is delayed by 19.6 ms, relative to the time it was transported by the source device, and half of the frame transport time is 8.0 ms.
Example animation: The input lag of the sink device is 28 ms because the presentation of each line of video is delayed by 19.6 ms, relative to the time it was transported by the source device, and half of the frame transport time is 8.0 ms.

Limitations:

This measurement cannot be directly used for calculating lip sync error or assessing audio/video synchronization; video latency must be used instead.

The frame transport time included in this measurement only partially reveals the benefit of a higher or variable refresh rate for interactive systems.

Expected ranges:

24 Hz: 20 ms and higher
25 Hz: 20 ms and higher
30 Hz: 16 ms and higher
50 Hz: 10 ms and higher
60 Hz: 8 ms and higher
120 Hz: 4 ms and higher
144 Hz: 3 ms and higher
240 Hz: 2 ms and higher
360 Hz: 1 ms and higher

Note: These expected ranges do not apply to video modes with larger blanking intervals, such as HDMI Quick Frame Transport.

Additional notes:

Before 2012, this term was instead used to describe video latency. See The History of Audio/Video Latency & Input Lag for more details on this.

Input lag has many different meanings that extend beyond video latency.19 The definition provided on this website is intended for reviews of consumer electronics sink devices, such as TVs.

Other Definitions

Terminology such “video latency”, “input lag”, “display latency”, etc. are often used to describe many different things. The definitions provided on this website are the most common definitions of these terms, as used for review of home entertainment / consumer electronics sink devices and for the purpose of audio-video synchronization. The term “latency” or “lag” is not specific to these contexts, so it is reasonable that others use these terms to describe other things in different contexts.

Unfortunately, a discrepancy of definitions and introduction of new terms is common even within the specific context that this website focuses on, such as the HDMI® Licensing Administrator’s definition of the term “display latency”20, which is different than the term “video latency” as defined by the HDMI Specification.21

Further Reading

Last updated on October 10th, 2021.


  1. HDMI Specification 2.0, Section 4.2.1 “Terms Incorporated From HDMI 1.4b (Informative)”
  2. HDMI Specification 2.0, Section 4.2.1 “Terms Incorporated From HDMI 1.4b (Informative)”
  3. HDMI Specification 2.0, Section 10.6.1 “EDID Latency Info”
  4. IDMS v1.1, 10.3 Video Latency, “Description”
  5. Murideo 8K Seven Generator User Manual, “ARM AV LATENCY”
  6. HDMI Specification 2.0, Section 10.6.1 “EDID Latency Info”, Section 10.7 “Supporting Dynamic Latency Changes: Dynamic Auto Lipsync”
  7. CNET: HDMI ARC and eARC: Audio Return Channel for beginners “eARC and HDMI 2.1”
  8. HDMI Specification 2.0, Section 10.6.1 “EDID Latency Info”
  9. HDMI Specification 2.0, Section 10.5 “Relative Audio / Video Latency”
  10. HDMI Specification 2.0, Section 10.6.1 “EDID Latency Info”, Section 10.7 “Supporting Dynamic Latency Changes: Dynamic Auto Lipsync”
  11. CNET: HDMI ARC and eARC: Audio Return Channel for beginners “eARC and HDMI 2.1”
  12. https://en.wikipedia.org/wiki/Audio-to-video_synchronization
  13. Harkwood Sync-One2 v2 User Manual “What do all the numbers mean?”
  14. HDMI Specification 2.0, Section 10.5 “Relative Audio / Video Latency”
  15. HDMI Specification 1.3, Section 8.9 “Auto Lipsync Correction Feature”
  16. HDMI Specification 2.0, Section 10.7 “Supporting Dynamic Latency Changes: Dynamic Auto Lipsync”
  17. CNET: HDMI ARC and eARC: Audio Return Channel for beginners “eARC and HDMI 2.1”
  18. HDMI Specification 2.0 Section 10.6 “Auto Lipsync Correction Feature” and Section 10.7 “Supporting Dynamic Latency Changes: Dynamic Auto Lipsync”
  19. Wikipedia, “Input lag”
  20. HDMI® Quick Frame Transport (QFT)
  21. HDMI Specification 2.0, Section 10.6.1 “EDID Latency Info”