DragonOSD+ black border contrast mod

Post by **Daniel Wee** » Sat 03 Apr 03 2010 2:15 am

Note that these contrast mods are work in progress and some parts may not be complete. Before you perform the mods, talk to me. In the meantime, I am working on an add-on solution which may prove to be easier to use for most users.

In particular, some of these mods could cause sync to be lost when the menu selection bar appears on certain portions of the screen due to the lack of voltage clamping. Again, I hope to have this addressed soon.

(Note: The diode reversal modification is being deprecated. Simply REMOVE THE DIODE altogether. Much easier to do.)

I have noticed that with some equipment, the text border doesn't appear as black as I would like it to be. This thread deals with various mods that I am trying out in order to improve this situation. The first modification that should be done is to reverse diode D1 as indicated in the image below:-

: DragonOSD profile 014c.JPG (107.52 KiB) Viewed 33056 times

The changes may not be very noticeable but with some types of equipment, this could show some improvement. Since the board uses high temperature solder, you should use an iron set to around 350 degrees C for this job. Heat up one leg while lifting that end of the diode gently. It doesn't matter if it doesn't come off completely on the first go, just so long as it comes off a bit. Then turn the board around and do the same for the other leg. Heat it up while lifting it gently with a pair of tweezers. Repeat with leg 1 and 2 alternatively until the diode comes off. Then turn it around and solder it back one side at a time. The side of the diode without the "bar" should be towards the edge of the board and should be the opposite orientation of the adjacent diode (also marked A2).

After doing this modification, your video may look a little brighter.

Voltage-level modification - 1k resistor

The second part of the modification involves a small 1/8-watt 1kohm 5% resistor (brown-black-red-gold). This needs to be soldered from the video input to the 3.3V supply. The suggested method of doing this is as follows:-

: DOSD+ Contrast 006a.JPG (89.52 KiB) Viewed 33015 times

: DOSD+ Contrast 006b.JPG (101.51 KiB) Viewed 33015 times

If you find the blacks too black, you can use a 1.5kohm resistor instead of the 1kohm resistor.

The following photos show the effects of the modification:-

: No modification.; before-mod.JPG (55.15 KiB) Viewed 33014 times

: With 1k-resistor modification. Note that this photo was taken slightly later in the day but the blacker text borders are evident.; 1k-mod-applied.JPG (66.63 KiB) Viewed 33014 times

Base pull-down resistor modification - stacked 10k 0603 resistor

Now, after performing the above modification, you may find that the black border is too thick and you want to pull it back just a tad to keep things nice and clean. In order to do this, you will need to make another minor modification - soldering an additional 10k 0603 SMD resistor over one of the onboard resistor. This means that the new resistor will be stacked over the existing one as follows:-

: Before stacking; DOSD+ Contrast 008a.JPG (84.67 KiB) Viewed 32978 times

: After stacking; DOSD+ Contrast 009a.JPG (112.69 KiB) Viewed 32978 times

The impact of this change is as follows:-

: Before modification; 4k7-base.JPG (60.52 KiB) Viewed 32978 times

: After modification; 3.2k-base.JPG (61.93 KiB) Viewed 32978 times

Daniel

Post by **Daniel Wee** » Mon 05 Apr 05 2010 10:38 am

In examining several different types of cameras, it appears as if there are two major categories of problems that can arise. Sometimes these appear on their own and sometimes both appear together. These two issues and their resolutions are as follows:-

Cameras with different video signal amplitudes

Some of the cameras tested exhibited non-CVBS levels. They are either too large or too small. This can result in problems with contrast, over-saturation (or under-saturation), and or white smearing. Where the amplitude is too high, this is actually quite easy to resolve. In the case of my Chinese made CMOS camera with this problem, the sync amplitude was 440mV whereas the correct levels should be around 260mV to 280mV.

: Non-CVBS compliant signal from Chinese-made CMOS camera. Signal was nearly twice mandated by the standard.; non-CVBS.JPG (26.56 KiB) Viewed 33015 times

To resolve this, I simply added a 220ohm resistor across the camera video output and ground. This brought the amplitude levels much closer to the CVBS levels. The actual value of this resistor may vary somewhat so some experimentation might be required.

: The same camera after placing a 220ohm resistor across the video input (usually yellow) line and ground. Voltage levels now approximate CVBS levels.; 220ohm-CBVS.JPG (39.31 KiB) Viewed 33015 times

Lack of AC coupling

The general rule here is that video sources will AC couple their output and not their inputs. In this way, the industry ensures that the various interconnecting video equipment will not have circulating DC currents. However, coupling capacitors tend to be large and some miniature camera manufacturers have put in inadequate or no coupling. This will result in over-saturation of colours, or faded colours, for example.

: Non-AC-coupled signal from Chinese-made CMOS camera.; no-AC-coupling.JPG (43.62 KiB) Viewed 33015 times

To resolve this, insert a large electrolytic capacitor into the video line between the camera and the video terminating device (the OSD in this case.) The effect of this will be much better dynamic range in the video image. In my tests I used a 1000uF electrolytic capacitor (which was rather large) but I think a 470uF or even a 100uF may suffice in many circumstances. The trade off is size - the smaller ones may not work as well as the larger one but you have to experiment to see how small a value you can get away with. Since the video signal is usually just 1V p-p, the voltage rating of the electrolytics can be as small as 5V or even 3V. The negative side of the capacitor goes to the video terminating device (the OSD in this case.)

: Same camera but AC-coupled with a 1000uF electrolytic capacitor.; 1000uF-coupling.JPG (39.45 KiB) Viewed 33015 times

Conclusion

In some cases it may be necessary to apply one or both of these modifications in order to bring the camera signal into CVBS compliancy. Where both modifications are required, the resistor will go across the camera before AC coupling to the terminating device. (In fact it will work either ways but it seems preferable to do it as suggested here.) It can be hard to identify exactly which of the two modifications are needed if you are not experienced or if you do not have the tools to measure the signals. When in doubt, try one at a time, and then both together. This should not be too difficult as they are fairly easy to implement.

: Example of a case where the signal was non-CVBS complaint, AND not AC coupled. Your video may look a little different though, depending on your camera type.; no-AC-no-CVBS.JPG (41.28 KiB) Viewed 33015 times

: If both mods are applied, it should look like the following.; both-mods.jpg (24.06 KiB) Viewed 33012 times

Daniel

Post by **Daniel Wee** » Mon 05 Apr 05 2010 11:21 pm

Note that there is another trace that needs to be cut and a jumper made which has not yet been documented. Doing just the 75ohm mod without this additional change could result in loss of sync under certain conditions. It is not recommended to do this mod before that other complementary modification is published.

To reduce aliasing and get a more pleasant looking border after performing the above modifications, you will need to make another change. This change requires a 75ohm (0603) SMD resistor. The actual value can be as high as 100ohms so you have some room to experiment to see what works best. First, cut the trace as indicated in the following photo:-

: DragonOSD profile 014f.JPG (131.9 KiB) Viewed 32987 times

Use a sharp Xacto to cut the trace, taking care not to scratch the surrounding board. In particular, be sure not to scratch the enamel coating off the ground-plane as this could lead to a short-circuit.

Place a 75ohm (0603) across the pads as indicated below and solder it into place:-

: DOSD+ Contrast 011a.JPG (102.74 KiB) Viewed 32987 times

This completes the mod. If you have a DMM with continuity function, check that neither ends of the 75ohm resistor is shorted to the adjacent pads or to ground. The effect of this change is as follows:-

: 75ohm-impact.jpg (117.92 KiB) Viewed 32953 times

Daniel

Post by **Daniel Wee** » Wed 07 Apr 07 2010 2:06 pm

Note: All the previous modifications, including this post, are now being deprecated in favour a an improved solution.

The cumulative effect of the recommended changes:-

1. add the 1k resistor for improved contrast
2. stack the 10k 0603 resistor for less black smearing
3. remove the diode indicated above
4. insert 75ohm collector resistor

is as follows:-

: Cumulative.JPG (75.2 KiB) Viewed 32953 times

Daniel

Post by **Daniel Wee** » Sun 09 May 09 2010 10:55 pm

This modification will isolate the sync-detector from the overlay output, potentially resolving some sync issues experienced with certain camera models. It involves cutting a single PCB track, removing a diode, and soldering a jumper wire. The jumper wire I used is an enamel-coated one. It may look like a bare wire in the photos, but it is in fact an enamel-coated insulated wire. Do NOT use a bare wire for this jumper as it may risk short-circuiting with some other exposed part. The wire is also known as magnet wire, commonly found in transformers, inductors, motor windings, and such. You may use other suitably small wire.

Some notes about soldering. Before you solder the wire - be sure to tin the wire ends, as well as the jumper points, with a clean, small-tipped soldering iron, and leaded (low-temp) solder. Using the right tool is critical to getting the job done properly and easily. Using the wrong tools may result in board damage. So make sure you have a proper iron (25W to 40W), the right kind of solder. PCBs and components can easily be damaged by heat so if try not to keep the iron on the board for more than 2-seconds at a time. If your solder isn't sticking - holding it longer isn't the right way of making it stick. More likely, your iron tip is dirty or oxidized. Clean it, dip it in soldering paste/flux, make sure it is all shiny and clean, and the solder should stick.

The following three photos indicate the steps needed for this mod:-

: 1-Sync mod 005.JPG (130.81 KiB) Viewed 32793 times

: 2-Sync mod 006.JPG (131.11 KiB) Viewed 32793 times

: 3-Sync mod 009.JPG (117.57 KiB) Viewed 32793 times

After completing the modification, and before you apply power to the board, use a continuity meter to check that the jumper is not shorted to ground. Keep the removed diode somewhere safe as it may be needed if you decide to improve the contrast of the board through another modification later on.

Daniel

Post by **Daniel Wee** » Mon 21 Jun 21 2010 9:08 pm

This modification supercedes all earlier modifications and ONLY APPLIES TO V1 boards.

To perform this modification, you will need:-
1. A fine tipped soldering iron (30W or better). The needs to clean and shiny.
2. A pair of long nosed tweezers
3. Low temperature solder
4. A 220ohm, 1/8-watt, 5% resistor
5. A 10k, 0603, 5% resistor
6. An NXP BAS32L diode
7. A sharp blade
8. Some enamel coated magnet wire

The needed components look like the following:-

: DOSD+ Contrast mod 024a.JPG (57.25 KiB) Viewed 32582 times

Using the blade, you will need to cut three traces (verify with a continuity meter that the cut was good) on the board as indicated by the following photo:-