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Let's assume I have a static computer desktop on my external display that is attached by HDMI (or equivalently, it could be attached by DVI, as DVI and HDMI are compatible). The 60Hz picture doesn't change at all but stays static instead.

Is HDMI transferring the full picture 60 times per second digitally? Or is there some kind of buffer on the display device, so that HDMI transfers only the modifications to the picture?

Does the answer change when DisplayPort is used instead of HDMI/DVI?

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  • \$\begingroup\$ It mainly transmits the differences \$\endgroup\$
    – D.A.S.
    Commented Apr 27, 2018 at 16:16
  • \$\begingroup\$ This improvement is really a pure bandwidth issue. 18Gbps gives a comfortable pipe for two high bit-rate 1080p streams with audio or 14 bit color on 4k \$\endgroup\$
    – D.A.S.
    Commented Apr 27, 2018 at 16:19
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    \$\begingroup\$ @TonyStewartEEsince1975 Do you have a source for your first statement? \$\endgroup\$
    – user39382
    Commented Apr 27, 2018 at 16:31
  • \$\begingroup\$ My observations of content controlled screen errors due to excessive bit error rate from CM noise on my ASUS MOBO tells me the clustered pixels that group in error when the content shows the errors as differential so the content must be as well. Puttiong my finger on the tiny CM chokes seems to improve error rate. But I have no control over group delay dynamic EQ \$\endgroup\$
    – D.A.S.
    Commented Apr 27, 2018 at 16:38
  • \$\begingroup\$ @TonyStewartEEsince1975 Unfortunately, that's not correct. HDMI errors are often a bit content-dependent -- certain data patterns can trigger errors more often than others, which is probably what you're seeing. \$\endgroup\$
    – user39382
    Commented Apr 27, 2018 at 16:42

2 Answers 2

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Is HDMI transferring the full picture 60 times per second digitally?

Yes. In a lot of ways, DVI and HDMI look like a lightly digitized version of an analog VGA signal -- the entire image is transferred for every frame, and they even have horizontal and vertical blanking periods. (In HDMI, these blanking periods are used to transfer audio data.)

Or is there some kind of buffer on the display device…

Most LCD displays will have an internal buffer which is used when displaying a signal at a different resolution than the panel (i.e, scaling), or on televisions which perform frame interpolation or other image processing.

But no, there is no inter-frame compression currently used in digital display technologies. Some newer revisions of HDMI and DisplayPort use DSC (Display Stream Compression) at very high resolutions, but even that is an intra-frame compression technique -- each frame can still be decoded on its own.

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  • \$\begingroup\$ I'm accepting this instead of the answer with more upvotes, as this explains that there is intra-frame compression in newer revisions and also explains the reason LCDs have an internal buffer. \$\endgroup\$
    – juhist
    Commented Apr 27, 2018 at 19:13
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At least at lower resolutions and in the absence of "content protection", HDMI bears a surprisingly strong resemblance to analog video used with CRTs. A device to convert analog video to HDMI can convert voltages to pixel values and send them real time without buffering. Likewise a device to display HDMI on a CRT (again, absent content protection) could convert the pixel values to analog values and use them to modulate the electron guns in real time as they are received. If anyone were still making CRT-based monitors with HDMI inputs, this would allow video games using them to provide somewhat faster responsiveness than LCD panels.

Slightly confusing the issue, however, many LCD minitors will continue to show an HDMI frame (the last one received) until they get the next one, or until they time out. It may be thus possible for a monitor to continue show a static screen while no HDMI data was being transmitted. On the other hand, functioning devices which generate HDMI will send it out continuously, without regard for whether any of the data differs from the previous frame. Among other things, HDMI was designed at a time when CRTs were still in use, and CRTs definitely need to be sent the image continuously, 60 or so times/second (or 30, with interlacing), in order to display it continuously and without flicker.

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