1080i YPrPb-->VGA Transcoder

[peebee]

I'm interested in building or buying an HDTV YPrPb-->VGA transcoder, similar to the one from Key Digital, as sold at

http://www.copperbox.com/lite/popinfo.php?lc_code=KD-CTCA2-OB&uneek=681850089

for $229.

But that price seems incredible to me. Isn't this just three analog adders/subtractors? Seems to me that they should be able to sell something like this for maybe $30 or $50. Am I just too used to the prices for NTSC stuff? Is 30MHz bandwidth circuitry that much more difficult to design and expensive to manufacture? Or is it still such a new, small market that they can charge that much above cost? Could I whip up something equivalent at home using 3 opamps for 10% of the price?

 

[peebee]

By the way I'm hoping for answers like either:

"Yeah, they're crazy, you could do this yourself no sweat"

or

"Peebee, you obviously have no concept of the extreme difficulty and expense of working with HD signals. Stick to your day job, you silly 60Hz power engineer you!"

 

[nbucska]

Hi peebee:
I think the price is not too bad -- in electronics the retail price is usually about 4-5 times of cost -- but
to build ONE you pay single prices -- if you can get
the parts -- with shipping.

You spend time on designing -- and it is not trivial
to get reasonable quality.

Unless you have HF experience, I think it is too difficult
project.

You need instruments to adjust and debug.

If you value your time, it is not worth to build.


<nbucska@pcperipherals.com>

 

[peebee]

Thanks, nb. At least this thing is cheaper than the VIBE mini tube array.

 

[nbucska]

yes, and it may even do something....

<nbucska@pcperipherals.com>

 

[peebee]

Well, let me get a little closer to the real problem.

I have two YPbPr HD sources. My TV has one YPbPr input, so to switch between them now means uplugging the three RCA's from one source and plugging in the three from the other source. I'm looking for the poor man's HD component video switch. (I also have a VGA input, which led to my original question, but $229 seems steep for what I'm trying to accomplish here).

Idea #1: What if I just use 3 Y-splitter cables on the input? Am I at any risk of damaging the TV if both inputs are accidentally simultaneously applied? I'm like 99% sure that this would be OK, but the TV just cost too much to risk burning one up testing this idea . . . . Estimated cost, $20.

Idea #2: One source (the cable box) has a switched AC outlet. What if I use that to drive relays via a wall-wart transformer to ensure only one source was ever connected? Electrically, this should be a snap to design, and cheap, but I have little experience designing RF circuits. Any suggestions on appropriate relay type (eg, reed)? Relay configurations (SPST vs DPDT vs SP3T)? Can all 3 shields be bonded from each source (or all 9 input & output shields), and I just switch the center conductors, or should I keep the shields separated and switch them too? I see that shielded relays are available, but I'm not familiar with them -- do they make any sense to use, and if so, should I bond the cable shield to the relay shield? Estimated cost, $40 to $60, including RCA jacks, but not counting a spare wall-wart transformer.

Am I just going to end up with a 5MHz low-pass filter?

Am I just a tightwad?

 

[IRstuff]

This might help:

http://www.keohi.com/keohihdtv/learnabout/rgb_vs_yprpb.html

TTFN

 

[peebee]

Cool site.

I did some more poking around and found this site:

http://ei.cs.vt.edu/~

http://wwwbtb/book/chap11/video.html

, which says
Y = .3R + .59G + .11B [not unweighted as I'd assumed]
Pb = .493(B-Y)
Pr = .877(R-Y)

If I'm doing my math right,
R = (Pr/.877) + Y
B = (Pb/.493) + Y
G = Y - .58Pr - .38Pb

Now, that still doesn't sound so hard. Nothing three buffers and a few voltage dividers and an output stage wouldn't take care of. Unfortunately, I also found out that 40MHz is the target, not 30, but it still doesn't sound so bad. It occurs to me I might need to play with the sync pulse, that might make things a little uglier, but it doesn't seem like anything a strategically placed one-shot wouldn't take care of (maybe I just dropped the response down to 4MHz now, though).

 

[nbucska]

And synchronous horizontal/vertical sync. pulses plus
color sync. burst...

Not impossible but not 60Hz, either... IEven if you calculate with minimum wages, you will still waste more than $229.- worth of time !




<nbucska@pcperipherals.com>

 

[IRstuff]

nbucska makes a valid point. As I recall from my brief perusal of the site cited, Y has the composite sync, while the chroma signals do not. Likewise, in RGB, only one, usually G, or none, has the composite sync. This means that you need to include a sync stripper that generates V and H syncs as required.

TTFN

 

[peebee]

So, any thoughts on the Y-splitters or relays? (&quot;Idea 1&quot; & &quot;Idea 2&quot; above)

 

[IRstuff]

Neither will work unless you strip the sync. Standard RGB is actuall RGBHV, where composite sync is separated into vertical and horizontal syncs. You cannot get that from the three analog signals without a sync stripper.

Additionally, the CTCA2 supports both pos and neg HV syncs, sync on G and composite sync on H. This requires more circuitry. Additionally, CTCA2 supports auto format.

TTFN

 

[joshuadavenport]

Has anyone got anywhere on this? I've been looking to build a cheap off the shelf components YPrPb to RGBHV transcoder. I've read about the lt1339 chips, the LM1881 chips, and all sorts of others. Anyone sucessfully build a transcoder for cheap? And if so, how so?

 

[VEBill]

Go to your local game shop (the ones that sells games for game consoles) and purchase an Audio-Video switch box (or two). The switch will have three RCA jacks (yellow, red, white), as well as S-video I/O, and these three circuits are typically perfectly fine even for video bandwidth signals.

Assuming that you want to also switch the audio, then you'll need another switch box.

Eventually, every home theatre ends up with too many sources and not enough inputs.

These switch box are about US$20 or less.

 

[twey]

peebee:

The formulas you shown for the Y/Pb/Pr are for the old NTSC/PAL standard (525/625). It does not match with HDTV (1080/720) standard. Here is the one for HD:

Y' = 0.2126R'+ 0.7152G'+ 0.0722B'
Pb = [(0.5/1-0.0722)]/(B'-Y')
Pr = [(0.5/1-0.2126)]/(R'-Y')

And, all three signals has embedded tri-level syncs. However, the syncs in Pb and Pr are below the signal level which will not cause problem. Y' has negative sync at -300mV that should be easy to strip off by clamping and level shifting. Syncs also can be extract easily by using the same way, or by a class C AMP.

Now you need to separate H sync and V sync from the c-sync extracted from Y'(so call composite sync). The V sync is embedded in H syncs at vertical retrace time with serration pluses. It should be able to extract from with a low pass filter. You may want to re-shape it with a Schmitt trigger. You can use composite sync for H sync. You do need to increase the amplitude to TTL level for VGA.

Bandwidth, the 1080i has the dot rate at 74.25 MHz which make it a 74.25/2 = 37.125 Mhz signal (think one-on-one-off). You may want to have three time of that (111 MHz)on your OP for maintain good signal quality.

The video signals should be impedence controlled, therefore the Y-cable would work. You will see low signal level and reflection. You will need good quality switchs (mechanical or electronical). Since you already building converter, you can have two isolated inputs to do so.

At this point it looks like a $15 material cost project.

Twey

 

[peebee]

Thanks, twey, very interesting & informative post.

This has become much lower priority for me right now, though, as it's taking way way longer for HD-DVD and HD-Tivo to come out than I ever would have imagined.

Plus, if that stuff all goes HDCP, I'm screwed. Oh well.