[mythtv-users] vga to component converters

[Cory Papenfuss]

 	The one technical difficulty rarely addressed in the homebrew 
transcoders is the fact that HDTV specifies a differential sync signal. 
Regular 480i still uses single-ended (i.e. 'TTL'-like) sync.  I think that 
480p or higher uses bipolar sync so that it has no net DC value.

 	Chances are pretty good that a TV will still sync on unipolar 
component in.  Remember that the sync signal is superimposed on the Y 
input.  If you get color or brightness weirdness vertically on the TV 
after it's all done, it *didn't* have a DC restore circuit.  Shouldn't 
hurt anything, but the work may be lots of work and not work out.

 	Building a bipolar sync isn't impossible, but it is more 
difficult.  To do it right you'd need a PLL circuit to sync to the 
unipolar input signal.  Since you've got access to the modeline directly, 
however, you can likely "advance" the sync pulse so the unipolar sync 
triggers the rising edge of the bipolar pulse.

 	If nothing I said makes sense, just try the HOWTOs as-is...
Report back as to whether it works or not... :)

-Cory

On Mon, 26 Sep 2005, Jason Temple wrote:

> Thanks Cory...I'm checking out the howto's now to see if this is indeed the 
> route I want to go.  I'm just looking for 720p for the time being (under the 
> assumption that if/when someday I get a tv that can take 1080i, it'll have 
> vga in), so on-the-cheap is important here.
>
> thanks for the help here!
>
> jas
>
> Cory Papenfuss wrote:
>
>> On Mon, 26 Sep 2005, Erich Boleyn wrote:
>> 
>>> 
>>> Jason Temple <jason at temple.net> wrote:
>>>
>>>>     I was wondering what most of you HD guys use to connect your front
>>>> end to your tv.  I don't have a fancy flat panel tv with vga imputs
>>>> unfortunately, but my Samsung tube can do 720p through it's component
>>>> connector.  I've seen a number of converters anywhere from $150-$300
>>>> that will take a vga signal and output component...my questions are: Are
>>>> they any good?  Are there any issues with X (Xorg supports them?  Any
>>>> issues with modelines, clocks?).  Are they all pretty much the same?
>>>> Although I'm planning to set this up with 720p, I want it to support
>>>> 1080i for future sets.  Any input would be greatly apprectiated!
>>> 
>>> 
>>> I used to use a tube TV which had Component video inputs (still have it,
>>> but going to sell it soon when I get my act together) and an Audio
>>> Authority 9A60.  The TV supported 480i/p and 1080i, though I had a
>>> bear of a time finding a working mode.
>>> 
>>> Here's what I did:
>>>  --  NVidia FX 5200 w/2 outputs, one DVI and one VGA.
>>>  --  Tube TV is an AdventTV 32" HDTV model.
>>>  --  Using NVidia proprietary drivers.
>>>  --  1280x960 32-bit color buffer.
>>>  --  One putting out 1280x960 @ 75 Hz.
>>>  --  One putting out 1280x960 interlaced using this modeline:
>>>        ModeLine "1280x960hdtv" 54.540 1280 1424 1480 1616 960 1016 1032 
>>> 1124 interlace +hsync +vsync
>>> 
>>> NOTE:  I first tried a 480p mode with no lasting success (it would glitch
>>> regularly, looked like a sync issue), and only after some experimentation
>>> got the 960 interlaced line above working stably.
>>>
>>     Component is almost exactly the same as RGB on a VGA port.  The only 
>> significant difference is that the color has been transformed into YPbPr. 
>> There are a number of schematics and howtos out there on how to build your 
>> own transcoder.  If you are too lazy to do that, but one for $100.
>>
>>     Also, resolutions on digital DV are not nearly as wishy-washy and 
>> ill-defined as for analog NTSC/PAL.
>> 
>> 480p :=  720x480   @ 60Hz frame, 60Hz field rate, 31.5kHz Horiz
>> 720p :=  1280x720  @ 60Hz frame, 60Hz field rate, 44.9kHz Horiz
>> 1080i := 1920x1080 @ 30Hz frame, 60Hz field rate, 33.75kHz Horiz
>> 
>> (e.g. http://videosystems.com/mag/video_getting_tune_dtv/)
>>
>>     One can quibble about 1080p at 24, 480p at 59.9 Hz, etc, but the above are 
>> the de-facto definitions.  Why would someone use a 1280x960 modeline to 
>> output to a HDTV?  It's not a standard size and barely a standard 
>> frequency.  The computer and/or the HDTV will be doing scaling.
>>
>>     Do yourself a favor... read some of the HOWTOs on modeline generation 
>> (e.g. the one in the mythtv wiki that someone copied from one of my 
>> previous posts).  Random tweaking of numbers in a modeline is a good way to 
>> destroy a monitor/tv.
>>
>>     In the meantime, may I suggest these as more standard modelines:
>> 
>> Modeline "480i"  14.349 720 760 824 912 480 484 492 525 interlace
>> Modeline "480p"  28.698 720 760 824 912 480 484 492 525
>> ModeLine "720p" 74.086 1280 1320 1376 1648 720 722 728 750
>> ModeLine "1080i" 74.175824 1920 1960 2008 2200 1080 1084 1094 1125 
>> interlace
>>
>>     YES they will have overscan... that's how TV works.  Reducing it in the 
>> horizontal dimension is relatively simple.  Reducing it in the vertical 
>> dimension violates the standard frequencies or resolutions or both.
>> 
>> -Cory
>>
>>  --
>> 
>> *************************************************************************
>> * Cory Papenfuss                                                        *
>> * Electrical Engineering candidate Ph.D. graduate student               *
>> * Virginia Polytechnic Institute and State University                   *
>> *************************************************************************
>
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-- 

*************************************************************************
* Cory Papenfuss                                                        *
* Electrical Engineering candidate Ph.D. graduate student               *
* Virginia Polytechnic Institute and State University                   *
*************************************************************************