CurtPalme.com Home Theater Forum

[gjaky]

While the guys a re talking about the difference between port3 and port5 on a barco I got the idea to look at my NEC PG xtra's signal path.
The neck board is based on VPA13 chip which is a 130MHz video amplifier -this should be fine, the pre driver stage is on the VIDEO OUT board, it consists of pre driver IC (LM1201) which has about 200MHz bandwidth and the additional transistors have bandwidth varying from 450MHz-2GHz -this is also fine.
Now the interesting part: the RGBHV input is going directly to the GAIN CTL board and the reception IC is a CXA1209P which only has 100MHz bandwith -and The NEC specs tell also 100MHz bandwidth for the whole machine! However the CXA1299P IC is pin-to-pin compatible with the CXA1209P but has 150MHz bandwidth compared to 100MHz, of course the whole bandwidth is not only depending on the component specs but board layout and signal routing as well, yet I think this would worth a try since 1080i-96Hz needs about 150MHz bandwidth so every additional MHz over the factory specs are welcome. The only problem is obtaining the CXA1299P ic, it seems the CXA1209 is a common type, not like the 1299, but maybe not imposible. So anyone who is interested in this experiment look out for a CXA1299P

[CasetheCorvetteman]

Ill be interested to see what you can achieve with this

[macgyver655]

Don't you know that's a Sony part... and Sony parts don't exist in the real world.... And what's it doing in your NEC?????

This place has stock: http://www.bdent.com/CXA1299P-p/cxa1299p.htm

But frankly, China has lots and much cheaper: http://www.alibaba.com/showroom/cxa1299p.html

Of course you could also smash open some older Sony DVD players (possibly some other brands also) as many used this chip.

[CasetheCorvetteman]

Will it yield an improvement?

[gjaky]

[CasetheCorvetteman]

Yes, there is abit more to the XG than just more bandwidth!!

[dvh99]

would be nice if there was a way to get the ad835 (250mhz) to work in the nec.

[gjaky]

Update: As a good reliable cource of CXA1299P is still out of sight I looked for substitution, so far I find the CXA1779P ic (150MHz) the most suitable (yes, also a sony part, but seems to be more common, it is on sale right now on ebay: 3$ each, in fact I found mine in the "drawer")
Unfortunately this IC has much more pins, and contrast-brightness controls are operating at different levels, So I had desig a little converter PCB which would fit in the place of the old CXA. Now I have to fabricate the board, I'll keep you informed.

[CasetheCorvetteman]

Sounds good mate

[CIR Engineering]

The good thing is that even when many of these older chips are rated at say 100 MHz, they often perform well at much higher rates. However, if you can get a higher bandwidth IC you may see an improvement. Just look out for new problems such as ringing, streaking, and smearing. If there is a peaking circuit working with this IC you will probably need to adjust the capacitance it uses. When adjusting peaking, use the highest frequency you plan to run with the projector OR the most important frequency to you. These projectors were designed for such a huge range of frequencies the peaking circuit is often not optimized for HDTV. We have the advantage now of using frequencies closer together so you may get some improvement there as well.

Good luck and I am interested in what you find.

craigr

[gjaky]

[CIR Engineering]

A peaking circuit is used to add "pre-emphasis" to the video signal. Pre-emphais is kind of like increasing the sharpness where sharpness in this case is artifically amplifying the frequency of the video signal.

This is also refered to as "preshoot" and "overshoot." If you look at a white window on an oscilloscope, when you add peaking you will see that the point of transistion between black to white has a peak (amplified response). Here is a photo of extra peaking as shown on an oscilloscope.



What this looks like on the screen of a projector is that the transistion from black to white will show a shadow or slight ghosting of the black onto the white. You can observe this on almost any analog TV (or even many digital TVs) by putting up a white window (or sharpness test pattern) and increasing the sharpness. The left side of the white window will get highlighted and will "ring."

Every component (each IC, trace, etc) in the video chain will cause some attenuation to the amplitude of the frequency response. Peaking is used to add extra amplitude to help reduce the attenutaion of the amplitude. Too much peaking results in ringing, but just the right amopunt of peaking will help preserve the original frequency response of the video signal for when it is passed to the next component in the chain. This makes the image on screen appear sharper without negative side effects.

You may not even have a peaking circuit on the IC you are changing, I have not looked. But it is a comon place to have a peaking circuit in the video chain so it's worth checking into. Also, you may discover that the peaking circuit is already optimized even after changing the IC.

Good luck!

craigr

[gjaky]

Thank you Craig for your clear explanation, I've seen that, but I thought the overshoot was due impedance mismatch caused by my scope probe.
Also you made me thinking about lower bandwidth ICs could go well at higher freqencies. I made some measurement on the actual application of the CXA1299P and found out the preamp is used as a buffer and level controller rather than real preamp, the gain factor is typically less than 6dB. I figured out the bandwidth is meant to be the -3dB point related to the maximal gain factor, also we can assume that the roll off of the amplifier is -20dB/decade as other video preamp datasheets suggests.
I made a little chart. I marked the 6dB line as a guide, in reality the circuit is not driven that hard, more like in the 4-5dB range, but be pessimistic, as you can see the crossing point is around 250MHz.
I made an other graph using with a 150MHz BW predriver, in this case the crossing frequency is over 350MHz.
Now It seems to me not this CXA IC is the weakest point of the chain.

[CIR Engineering]

[gjaky]

Now I did some realy basic tests.
Resolution pattern in 1080p, the horizontal lines are a touch tighter, than the vertical lines, so the focus is better than the bandwidth -so still not a bad idea to increase the bandwidth.
I found some ringing as well, present on all colors. I Ran through the schematics, and found that the video amp has dedicated pin for peaking circuit, it consist of two RC circuit in parallel, any hint how to adjust them toward the good direction

[CIR Engineering]

Usually decreasing capacitance will reduce overshoot and ringing.

Increasing capacitance will increase overshoot and increase sharpness.

It is also worth stepping back a few steps from the screen to see if you can still see the ringing. You might be able to see it right at the screen or on the tube face, but with real images it may be ok.

craigr

[CIR Engineering]

Those photos are with the new IC installed right?

Can you compare the old IC with the new one and take a photo of your 1:1 and sharpness pattern?

craigr

[gjaky]

[CIR Engineering]

Try looking at a 30 IRE window from your seating position.

craigr

[gjaky]