Author Topic: Why is my VGA signal better than hdmi?  (Read 2373 times)

Afkpuz

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Why is my VGA signal better than hdmi?
« on: July 17, 2008, 01:34:38 am »
I've tried this on two different TVs now and the vga signal is clearly better than the hdmi at the same resolution.  Using hdmi gives a very grainy image.  It should be noted that I am using a dvi-hdmi adapter and an hdmi cable to run to the tv.  Could that be the culprit?

pigdog

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Re: Why is my VGA signal better than hdmi?
« Reply #1 on: July 17, 2008, 01:39:43 am »
Hi,

What kind of video card are you using?

Are you using the Olevia 232T 32" LCD HDTV, if so the DVI/HDMI adapter to TV via HDMI cable should be O.K. unless you bought a really cheap/bad cable.

Thanks.
« Last Edit: July 17, 2008, 01:46:26 am by pigdog »

Afkpuz

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Re: Why is my VGA signal better than hdmi?
« Reply #2 on: July 17, 2008, 02:01:31 am »
It's very possible that I bought a bad cable.  I will try with a diferrent hdmi cable.

pigdog

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Re: Why is my VGA signal better than hdmi?
« Reply #3 on: July 17, 2008, 03:12:40 pm »
Hi,

If you find that your cabling is o.k. then you'll have to concentrate on your video card and xorg.conf file.

Did you set the AVWizard for 720p?  That's your TV's native resolution.

On my TV native resolution is 1080p and was set to that in the AVWizard. 

When the system booted up it ran in 1080p showing the Kubuntu splash screen but at a point in boot up (loading xorg) would switch to default 640x480 until I got my xorg.conf file sorted out.

colinjones

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Re: Why is my VGA signal better than hdmi?
« Reply #4 on: July 17, 2008, 03:35:53 pm »
HDMI and DVI are signal compatible - so that converter is essentially just a passive thru, so that in itself isn't the issue.

Generally it is highly unlikely that a "bad" cable would give you a grainy picture. It is serial digital stream, so like any other it is either going to work or not, or perhaps produce chunks and areas where the picture breaks up or tears just like digital TV when the signal is poor. But a uniform degrading of the picture is not likely to come from the cable - VGA sure, not HDMI/DVI.

I suppose the video chipset could be systematically corrupting a bit or two for a pattern of pixels that might give that effect. But changing the card should sort that out. Driver? Seems unlikely - this is nvidia?

The native resolution suggestion is an interesting one - likely that the TV is using very different systems to scale a digital image than an analogue one. Digital images are often much more difficult to scale well(cheaply!) - definitely a good idea to find the real native resolution of the panel (read the specs very carefully because they will often state 1080, but when you read the detailed specs that just means it will accept a 1080 frame and scale it down to the native 1280x768 panel resolution). Make sure you are matching the panel to your xorg. nvidia-settings has some useful data on scaling and native sizes..

Zaerc

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Re: Why is my VGA signal better than hdmi?
« Reply #5 on: July 17, 2008, 09:44:43 pm »
Can you post a couple of pictures so we could see the difference?
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NicCOConnor

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Re: Why is my VGA signal better than hdmi?
« Reply #6 on: July 17, 2008, 10:24:02 pm »
This may be a rediculous question but by any chance is the cable over 50-60 feet.

When an HDMI cable length gets too long the signal loses strength. This results in picture distortion, sparklies (where single pixels drop out of the picture), or no picture display at all, only sound capabilities. If any of these problems occur than the cable length is too long, and the signal needs to be adjusted by additional devices.

Nic

Afkpuz

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Re: Why is my VGA signal better than hdmi?
« Reply #7 on: July 17, 2008, 10:29:26 pm »
No, the cable is not over 50 feet.  It was cheap, as I bought it on newegg (around $15).  I'm taking screenschots as I type this.  I'll post them shortly

Afkpuz

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Re: Why is my VGA signal better than hdmi?
« Reply #8 on: July 17, 2008, 11:18:51 pm »
Well, I was able to mess with my xorg.conf file and fixed the problem.  Thanks for all the help!

agentsmith

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Re: Why is my VGA signal better than hdmi?
« Reply #9 on: July 21, 2008, 09:59:36 pm »
Afkpuz, what exactly did you have to do in the xorg.conf file to resolve the issue?  Would be interested to know for future reference and benefit of all.

Thanks,
@gentSmith
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Seeker

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Re: Why is my VGA signal better than hdmi?
« Reply #10 on: July 22, 2008, 01:32:29 pm »
From a purely AV point of view, it sound like a pixelmapping issue (imagine a picture with 1919x1079 stretched on a 1080p screen).
HDMI/DVI-D signals do not degrade, they simply stop working.

Try fiddling with the settings on the TV (turn up the sharpness) and se what you get.

From a LinuxMCE point off view, I am shoeless, but perhaps someone can help based on the above suggestions.

hari

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Re: Why is my VGA signal better than hdmi?
« Reply #11 on: July 23, 2008, 12:43:53 am »
HDMI/DVI-D signals do not degrade, they simply stop working.
that's not completely correct. You can have single pixel errors with DVI/HDMI.

best regards,
Hari

ps: I have a Spatz HDMIMAX for sale with a very good Monitor S1 10m cable.
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Afkpuz

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Re: Why is my VGA signal better than hdmi?
« Reply #12 on: July 23, 2008, 10:27:46 pm »
What I did is manually went into my xorg.conf and set my monitor resolution and refresh rate, etc.  then, I just used the vga input without messing with hdmi.  I know it's a sacrifice, but it works.