CRTCs vs. outputs [LWN.net]

CRTCs vs. outputs

Posted Jan 23, 2007 13:25 UTC (Tue) by daenzer (subscriber, #7050)
Parent article: LCA: Updates on the X Window System

There's some confusion between CRTCs and outputs. A CRTC scans out the pixel data from memory and converts it into a serial signal. Several outputs can pick up this signal, process it, and send it to an output device. But they can't display anything the CRTC didn't scan out in the first place, so the number of independent viewports is essentially limited by the number of CRTCs, not outputs. Most current popular GPUs (such as that in Keith's laptop) have two CRTCs.

Nice article though.

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Does this explain TV-Out configuration?

Posted Jan 23, 2007 13:44 UTC (Tue) by rankincj (guest, #4865) [Link] (2 responses)

Is this why the TV-Out instructions on my old G400 MAX and Radeon 9200 video cards say that the monitor will be driven at the same frequency as the TV, and so anyone intending to use a PAL TV should be sure that the monitor is happy running at 50 Hz? This is one restriction I would really like to avoid!

Does this explain TV-Out configuration?

Posted Jan 23, 2007 14:06 UTC (Tue) by daenzer (subscriber, #7050) [Link]

Possibly, although some TV encoders can convert more or less arbitrary input signals.

Does this explain TV-Out configuration?

Posted Jan 23, 2007 15:58 UTC (Tue) by jzbiciak (guest, #5246) [Link]

Does the TV Out display the same thing as the monitor? If so, then there's probably only one display generation pipeline that feeds to multiple physical outputs. My interpretation of the CRTC abstraction is that it also allows a single display controller to control multiple heads, each potentially displaying something different.

Even then it seems like you might get into restrictions on refresh rates or pixel clocks between the two outputs if there is only one pixel clock source, or if there are synchronization requirements between the two pipelines during refresh. I haven't looked closely at modern display hardware, though, so I really couldn't say.