3 Channel video buffer



Introduction

Having been in need of a video buffer for two projects[1][2] in a row, I thought it would be nice to have a small circuit board with all the required components.

So I designed a simple circuit based on Texas Instruments's THS7314, a 3-Channel SDTV video amplifier/filter with a 6-dB gain. With 3 channels, the circuit can be used to buffer not only composite (CVBS) video, but also S-Video (Y/C), component (YPBPR) and RGB.

The capacitors required for AC coupling are present in this design, but DC coupling is also possible as solder points are available for both coupling types.

Here are a few possible applications of this circuit:



[1]: C64 Composite video output and LCD TV
[2]: Ajout d'une sortie Composite à une console APF TV Fun

goto top Up


Schematic

goto top Up


Example: APF TV Fun Composite output mod

As detailed in my APF TV Fun console repair article, I used this circuit to add a composite output to this old console. The CVBS signal is captured at the input of the RF modulator and is then amplified by this circuit before being sent to the monitor.


goto top Up


Example: NES-2 composite output mod

Nintendo did not think necessary to have a composite video output on the NES2 console. Also, there appears to be an interference problem on the PCB causing vertical stripes (see comparative pictures below) across the screen.

I modified a NES2 console to capture the video signal directly at the PPU output pin. To reduce interference, I cut the connecting to the PCB and used coaxial cable for wiring.



Left: Picture before the mod (through RF output). Right: Picture after the mod (Composite signal). The vertical stripes are much less pronounced. In fact, they are as faint as they were with the original NES.

Before (RF output)

Before (RF output)

After (Composite output)

After (Composite output)


goto top Up


Example: SNES component (Y/Pb/Pr) output

Test setup

Test setup

It is possible to add an Y/Pb/Pr (aka. Component video) to an SNES console. While the Pb/Pr[1] signals are not available on the AV multi-out connector, they are accessible internally from the video encoder chip. Here is an example modification I did on one of my consoles.

Note that there are several SNES revisions and that this mod is not always possible. Here is the (very incomplete) informaton I have: (There are long forum threads with more information)
Here is where I got the different signals from: Each video component (Y, Pb and Pr) are taken on the BA6592F pins and routed to an AC input on my video buffer board. The corresponding AC outputs (and GND) are then sent to individual RCA connectors of the colors usual for component video.

Here are a few pictures of the mod:
Signals on the BA6592F

Signals on the BA6592F

5V source

5V source

Gnd

Gnd

Video buffer

Video buffer



And here is the nice finished result. I say nice, but I'll admit the opening for the RCA jacks is a bit rough... it's up to you to do better!



An now here is a comparison between composite, s-video and YPbPr formats. On my Panasonic CT-27D10 set, s-video quality is very close to YPbPr quality. But by looking carefully, we can see that the picture is slightly sharper when using YPbPr. On an LCD monitor the difference would probably be more evident.

Composite

Composite

S-Video

S-Video

Y/Pb/Pr

Y/Pb/Pr


Composite

Composite

S-Video

S-Video

Y/Pb/Pr

Y/Pb/Pr


Composite

Composite

S-Video

S-Video

Y/Pb/Pr

Y/Pb/Pr



One should know that the 240P video the SNES generates is not supported by many LCD TVs. If you get a message like "not supported" on your screen, don't blame your work too quickly. Try again with the old CRT TV in the basement first...

I only got an image on a Panasonic PT-47WX52CF projection TV and a Panasonic CT-27D10 CRT TV. My LCD displays were disappointing... But this does not really matter. With CRT monitors, video is displayed in real time (i.e. No annoying filter in your way trying to "improve" the image at the cost of several frames of lag) and are therefore the best choice for non-HD consoles anyway.
Dynex DX-15L150A11

Dynex DX-15L150A11

RCA RLED1945A-E

RCA RLED1945A-E


Simon from Australia who told me about the possibility of this mod was getting an image that was a bit too bright. After he added 75ohm resistors on the input of each signal line, the picture was perfect. In my case, this did not seem necessary. This difference is probably due to the fact that he has a different SNES version...
The mod

The mod

YPbPr vs. SCART RGB

YPbPr vs. SCART RGB

Without 75Ohm resistors

Without 75Ohm resistors


[1]The Y signal is actually available for S-Video.

goto top Up


Disclaimer

I cannot be held responsible for any damages that could occur to you or your equipment while following the procedures present on this page. Also, I GIVE ABSOLUTELY NO WARRANTY on the correctness and usability of the informations on this page. Please note, however, that the procedures above have worked in my case without any damages or problems.

Now you cannot say that I did not warn you :)

goto top Up


Trademarks used in this site are the property of their respectives owners.
Copyright © 2002-2017, Raphaël Assénat
Website coded withWebsite coded with vimLast update: August 23, 2016 (Tuesday)