MythTV hw question

[Dave Johnson]

Derek Atkins writes:
> Some VGA outs can only get up to 1080i and can't get the throughput
> of 1080p, especially since I'm asking for nearly 995328000 B/s (unless
> I've miscalulated 1920 x 1080 x 4 x 120...

That's the first issue, since these boards use main memory for the
video you'll need high bandwidth RAM to feed the chipset. 1080p is
only about 8MB so size isn't an issue bandwidth may be.

The second issue is the max dot clock the chipset can drive out the VGA
port.

My CN400 shows: (II) VIA(0): Clock range:  20.00 to 230.00 MHz

That clock must include not just the visible picture but the sync and
blanking after each line needed by the actual display device on the
other end.

Some searching found these lines for 60hz:

Modeline "1920x1080i60"  74.25  1920 1976 2008 2200  1080 1083 1085 1125 +Hsync +Vsync Interlace
Modeline "1920x1080p60" 148.50  1920 1976 2008 2200  1080 1083 1085 1125 +Hsync +Vsync

148Mhz clock is needed for 1080p at 60hz, 1080p at 120Hz would require a
297Mhz clock which is beyond the VIA chipset's limit of 230Mhz.

There is also good info here:

http://www.linuxis.us/linux/media/howto/linux-htpc/video_card_configuration.html



I plugged in the 1080p at 60Hz line and got this:

(II) VIA(0): ViaValidMode: Validating 1920x1080 (148500)
(II) VIA(0): ViaModePrimaryVGAValid
(II) VIA(0): Required bandwidth is not available. (497664000 > 461000000)

The clock was ok, but memory bandwidth was insufficent.  This system
has a PC2700 DIMM, a PC3200 DIMM would probably be enough and DDR2
would be fine too.   The X driver actually measures the memory
bandwidth on startup.




-- 
Dave