Question regarding transcoders, colorspace, and black crush - AVS Forum
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post #1 of 27 Old 06-02-2006, 06:43 AM - Thread Starter
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When converting Y,Pr,Pb to VGA PC monitors by way of >$100 transcoders (Vdigi VD-Z3/X2VGA2, etc), I have noticed unacceptable black crushing in 480P (the problem is even worse with HD). I want to fix this without clipping whites as well.

I have searched a lot on what would cause this, but only found vauge answers regarding colorspace and proper syncing.

I realize Y,Pr,Pb and VGA have 2 different colorspace formats, and SD and HD content are different still- could a lack of colorspace conversion on cheaper transcoders cause blacks to be crushed?

I think I also read that syncing has something to do with black/white levels.
Could someone please tell me exactly what causes black crush, or point me to some additional information on the subject?
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post #2 of 27 Old 06-02-2006, 08:00 AM
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Problems with bt.601/bt.709 will only affect one or the other, not both, so that isn't the problem.

I guess the units are lacking when it comes to "stretching" the greyscale from 16-235 to 0-255.

I take it you've recalibrated your display for the different points that black and white lie at, right? If not, do so.
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post #3 of 27 Old 06-05-2006, 09:27 AM - Thread Starter
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I have a theory that the devices I'm using do no Y,Pr,Pb to RGB colorspace conversion whatsoever.
The transcoders I've tried were: Vdigi VD-Z3, X2VGA2, and one I built myself.
I've also tried a cheap video processor (Viewsonic N6).

The homemade transcoder is little more than a LM1881 sync seperator and some amplifiers. I don't have schematics or additional info on the Vdigi or X2VGA2, but I'm guessing they do no colorspace conversion either.

However, if you look at National Semiconductor's description of their LMH1251 - YPbPr to RGBHV Converter, it states:
Quote:


The color space conversion from YPBPR to RGB in the LMH1251 is realized with a very high precision fully analog dematrixer that provides superior chroma accuracy that is less than 2.5% of amplitude & 1.5° of phase error on a vectorscope. It is equipped with a smart video detection circuit which automatically senses SDTV and HDTV video formats and applies the appropriate color space conversion.

Anybody have additional Y,Pr,Pb to RGB info, or know of any devices that use the LMH1251?
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post #4 of 27 Old 06-11-2006, 03:33 PM
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Hey Chad N.:
I've found a transcoder that might use the LMH1251 chip.
It's built by a company named Mayflash. Check their description of the transcoder (the first one on the products page); it's just like the description National has for the chip.
(sorry, I can't post any URL's yet so you'll have to use google).
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post #5 of 27 Old 06-11-2006, 04:38 PM
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Quote:
Originally Posted by hrlslcbr View Post

Hey Chad N.:
I've found a transcoder that might use the LMH1251 chip.
It's built by a company named Mayflash. Check their description of the transcoder (the first one on the products page); it's just like the description National has for the chip.
(sorry, I can't post any URL's yet so you'll have to use google).

This one?

http://www.mayflash.com/vgabox/vga001/vga001.htm

Color accuracy evangelist and CalMAN insider
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post #6 of 27 Old 06-11-2006, 05:08 PM
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Yep

EDIT: I think we'll only know if it uses the LMH1251 chip if someone buys one and opens it. Here's the answer I got from Mayflash:
Quote:
Hi There,

Sorry, your question belongs to our technical secret. We can not answer you, but we can promise this item is a very good one. If you want, pls have a try. Thanks!

Best Regards

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post #7 of 27 Old 11-10-2006, 09:57 AM
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Quote:
Originally Posted by Bear5k View Post

This one?

http://www.mayflash.com/vgabox/vga001/vga001.htm

Received the Mayflash VGA001 transcoder today from playasia and getting curious results. Source device is a Humax HDCI2000 high definition satellite receiver and I want to hook up my Barco BG801 to it. The Barco only has VGA input hence the transcoder. Connected via the transcoder and the Humax set to 1080i the picture certainly doesn't look HD. The Barco indicates only 28 kHz line frequency which is way too low for 1080i.

To investigate, I connected a Dell 2405FPW panel to the component outputs of the Humax. Beautiful 1920x1080 HD picture. Then, I hooked up the Mayflash transcoder again and connected it's VGA output to the Dell's VGA input. The Dell panel detects a 480x640 signal Out of curiosity, I set the Humax to 720p output, and the Dell detects 832x624. Ok, let's try 576p: the Dell says 800x600.

So it seems to downscale. A quote from the product description:

Quote:


The device can convert YPbPr signal to the monitor with low consumption,

This piece of Asian English should probably translate to "The device will convert YPbPr signal to the monitor with low resolution"

Quote:
Originally Posted by hrlslcbr View Post

I think we'll only know if it uses the LMH1251 chip if someone buys one and opens it.

I am very tempted to open it and see what chips are in there. Unfortunately there are no screws and I can not risk to damage the box because I will almost certainly return it to playasia.
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post #8 of 27 Old 12-03-2006, 03:00 PM
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Thanks for sharing this with us. I was going to buy one, but now I certainly won't . Does it have any dip switches that may improve the quality?
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post #9 of 27 Old 12-09-2006, 01:38 PM
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It does not have any dip switches.

Anyways I finally opened up the case. The Mayflash uses the LMH1251. Other chips on the PCB include the LH6739MQ triple video buffer as described in the LMH1251 datasheet, and an 8-pin chip marked "ZG04". I could not find any information about this device, but it seems to be an inverter since the hsync and vsync outputs of the LMH1251 are routed through it. Curiously though, only hsync is negative at the VGA output as observed with an oscilloscope. Vsync remains positive as output by the LMH1251.

There are unused solderpads on the PCB next to the component/VGA passthrough input switch. There is room for two more switches and two resistors. Traces from these unused pads go to pins 22 and 23 of the LMH1251 involving SD/HD mode switching. I noticed that with the missing components both these pins are left floating. Normally this means a logical "1", but not with this device. Pins 22 and 23 both measured 0 volts. According to table 2 in the datasheet, this puts the device in SD conversion scheme and expects a bi-level sync on the component input as opposed to tri-level sync in the HD conversion scheme. So I connected pin 23 via a 100 ohms resistor to +5 V to put the device in "auto" mode. I also spotted some bad solder joints which I repaired.

After these modifications/repairs the Dell 2405FPW still showed 480x640 picture with it's VGA input connected to the Mayflash being fed 1080i, but the Barco CRT projector displayed a decent picture, especially when I calibrated it on the new videomode it had discovered. It still detected 28 kHz line frequency, but that is actually correct with European 1080i50 sources. I apologize for incorrect observations in my previous post.

The BBC was running a simulcast on their HD and SD channels and the difference was convincing when flipping between the two. I did not notice any crushing of the blacks. Isn't this a NTSC thing?

I have no idea why the Dell 2405FPW would detect the odd 480x640 size. Maybe it is confused by the different hsync/vsync polarities that the Mayflash outputs.

To sum up, I think the Mayflash VGA001 component to VGA transcoder suffers from QA problems but otherwise works as advertised. Once I have located my digital camera I will post a picture of the PCB.
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post #10 of 27 Old 12-09-2006, 04:11 PM
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Thanks for all this information. I think I will buy one after all if I just need to solder some resistors .
By the way, look at the seventh page on the datasheet. It explains something about using an inverter for the horizontal sync (didn't understand it though... ).
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post #11 of 27 Old 12-29-2006, 05:57 PM - Thread Starter
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I bought the VGA001 and tried it out. Right out of the box it handled 480P, 720P, 1080i.
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post #12 of 27 Old 12-30-2006, 07:13 AM
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I would like to seek clarification on the output display of the Mayflash VGA001.

If I were to input a 720p signal, would it output a resolution of 1280 x 720 @60Hz on say, my CRT monitor? Meaning, input resolution = output resolution? Or would it crop/resize the image and run at some 4:3 resolution like 832 x 624?

Same with 1080p, would it output 1920 x 1080 @60Hz instead of maybe 1600 x 1200?

I don't have experience with electronics, so I can't open the thing up to modify it if needed. Returning it would be troublesome.

The LMH1251 seems to have a problem outputing 1080p at fast frame rates eg 60Hz without an external circuit.

Does the VGA001 suffer from this?
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post #13 of 27 Old 12-30-2006, 09:24 AM
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This device is a transcoder; it doesn't scale the video at all. It just converts from YPbPr to RGB (used by most PC monitors), so yes; if the input is 1280x720 @ 60 Hz you'll get 1280x720 @ 60 Hz on your PC monitor or projector.

Chad N.: do you notice any problems with it (black crushing, etc), or any of the problems koef had?
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post #14 of 27 Old 12-30-2006, 09:43 AM
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Quote:
Originally Posted by hrlslcbr View Post

This device is a transcoder; it doesn't scale the video at all. It just converts from YPbPr to RGB (used by most PC monitors), so yes; if the input is 1280x720 @ 60 Hz you'll get 1280x720 @ 60 Hz on your PC monitor or projector.

Chad N.: do you notice any problems with it (black crushing, etc), or any of the problems koef had?

Yes, thats what transcoders are supposed to do.

But after seeing koef's experience, especially with the projector and LCD makes me wonder if the input = output and if my CRT monitor would face the same problem as his LCD.

The fact that he had to modify the transcoder to have proper image quality does not assure me of the item quality. Is this a problem with older batches since Chad.N doesn't experience this problem?
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post #15 of 27 Old 12-30-2006, 10:40 AM
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I believe it was the Dell monitor that had problems with the negative sync that the transcoder outputs, because the box doesn't have a scaler at all, so it just can't output any other resolution than the input resolution.
Maybe Chad N. could try it on a CRT and share some pics

Anyway, I'm going to order it from playasia next week if I have time.
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post #16 of 27 Old 12-31-2006, 03:20 PM - Thread Starter
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I opened the VGA001 up and noticed the same thing koef did - there are pads for 2 surface mount resistors and 2 surface mount switches.
I think his monitor was the problem though.

I put an 0-scope on the green output, and found a very clean signal (with even the sync on green stripped properly) - something the Vdigi does not do. However, in VGA passthrough mode, the transcoder seems to lower contrast a bit so whites are somewhat greyer. On the o-scope the VGA passthrough signal is about 83% of the amplitude of VGA straight from the PC's video card.

I tested the transcoder on a crt with Xbox media center, and switched resolutions between 480P, 720P, and 1080i. All 3 resolutions displayed correctly (although my crt stretches 16:9 to 4:3). I noticed the picture is identical to the Vdigi unit and a X2VGA2 - I do still get crushed blacks - but I'm almost convinced it is not the transcoders fault. I think it depends on the monitor.

Reason being is, I have tested a home-made transcoder, the Vdigi, X2VGA2, VGA001, a Viewsonic N6 video processor, and even modded my Xbox to output in VGA mode. ALL gave me crushed blacks with my LCD and CRT.

Another thing that backs up the "monitor crushes blacks" theory: People complain that the Xbox 360 washes the image out when the VGA cable is used vs component out. After looking at several screenshots of component vs VGA, it appears Microsoft either increased the gamma or changed the black level on VGA out, so when played on a PC monitor, the 360's blacks are not crushed.
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post #17 of 27 Old 12-31-2006, 10:06 PM
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Still getting crushed blacks? Apparently the Xbox 360 VGA cable outputs video levels (16-235) and a normal PC monitor would expect 0-255.
So, is this transcoder good enough, or is it just the same quality found on comparable (<$100) transcoders?
Thanks for your comments.
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post #18 of 27 Old 01-01-2007, 06:10 AM
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Chad N, when you opened the VGA001, did you see an external circuit like the following connected to the LMH1251? It's to correct some glitch in 1080p60:

http://www.national.com/an/AN/AN-1493.pdf

I don't want to get this, then end up having problems with the PS3's 1080p60 output in games.

Perhaps you can test another source like a DVD-player or the video card's TV-out to see if crushed blacks are inherent to the VGA001?
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post #19 of 27 Old 01-01-2007, 08:31 AM
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Heres a link to a very good transcoder that I have been using for some time with my Toshiba XA1 .

http://www.crescendo-systems.com/rev_transcoder.html

You can read a thread on the transcoder at http://1080p.dk
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post #20 of 27 Old 01-01-2007, 10:39 AM
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Reviews on the latest transcoder with adjustable gamma can be found here ....

http://www.curtpalme.com/forum/viewtopic.php?t=2511
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post #21 of 27 Old 01-04-2007, 09:40 AM - Thread Starter
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hrlslcbr, yes I am getting crushed blacks. Also, I cannot tell any difference between it and the Vdigi's visual quality, but then again I'm switching between the 2 and not doing a literal side-by-side comparison. I don't have a nice enough camera to take comparison pics.

mtkc, no, the VGA001 doesn't have this circuit...kind of disapointing, but I believe the transcoder came out before National released this note.

dominical2, can you buy the adjustable gamma version of the transcoder you posted yet?
It seems adjustable gamma is THE way to go.
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post #22 of 27 Old 01-04-2007, 10:40 AM
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Quote:
Originally Posted by Chad N. View Post

hrlslcbr, yes I am getting crushed blacks. Also, I cannot tell any difference between it and the Vdigi's visual quality, but then again I'm switching between the 2 and not doing a literal side-by-side comparison. I don't have a nice enough camera to take comparison pics.

I thought the chip would be as good as national says it is.

Quote:
Originally Posted by Chad N. View Post

It seems adjustable gamma is THE way to go.

Yep, nice advantage over many 'cheap' transcoders. It's a little expensive though...
One thing I don't like is that it auto detects what input is active and you can't switch from VGA pass through and YPbPr like with the VGA001 without opening the case.
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post #23 of 27 Old 01-05-2007, 09:33 AM
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You can easily wire in a few small switches if you would like to bypass the jumpers .
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post #24 of 27 Old 01-25-2007, 04:04 PM - Thread Starter
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I've dug out an old CRT monitor and found something interesting. The monitor has a manual analog brightness adjustment. A transcoder displayed on my newer CRT and LCD monitors shows black crush. Upping the brightness on the newer CRT just washes out the entire image.
However, increasing the brightness on the old CRT allows me to eliminate black crush without washing out the image.
I don't know much about how monitors handle the black level, but it seems my newer monitors set the black level automatically, and this cannot be changed. With the old monitor the user has to manually set the black level.
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post #25 of 27 Old 04-11-2007, 05:00 PM
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Koef are there any updates on this?

I have a Mayflash transcoder on my 19" monitor, it works for my xbox outputing 480p, but my monitor will constantly blink and I hear a "click" noise from the monitor.

Does anyone else have this problem?

It's a Proview 19" widescreen monitor.

I've also tried the transcoder on my Samsung 215TW 21" widescreen monitor but it doesn't show a picture at all.
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post #26 of 27 Old 04-26-2007, 11:00 PM
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Maviryk: I get similar issues with the mayflash.My monitor flickers and clicks but no image appear on the screen (xbox 480p/720p).Interestingly I had the same problems with a home made xbox vga cable (which I tried with two different sync stripper chips) with the compaq V90.The mayflash works fine with 2 other monitors, an old shamrock 14" crt and a Chimei 19" LCD.
Also my homemade PS2 VGA cable with sync stripper works fine on the Compaq as does my dreamcast VGA box.This leads me to believe that there is something non-standard about the xbox sync output, which leads to some monitors not being able to handle it.
To test this theory I tried bypassing the sync inverter chip on the mayflash to see if it would fix things but it didn't help (didn't affect things on my working monitors, either).I might try setting auto mode like koef did earlier in the thread, but I don't see how this could be affect the issue I am having.

Also if it is just a problem with the xbox sync output, testing with another console should work fine.
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post #27 of 27 Old 05-27-2007, 12:54 AM
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Just got the mayflash working on my compaq monitor.I had to connect pin 5 of the vga output connector to ground, because it wasn't connected on the mayflash.For some reason my monitor needs this ground connected despite the fact that the 3 R,G,B grounds and the sync ground were all connected.Well, that was a easy fix in the end, and I am quite impressed with the quality of the output.
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