Overclocking and improving the CVPPC

Improving the picture quality

A number of people have commented unfavourably on the picture quality from the CVPPC card - there are "shadows", or more correctly "ringing" on light to dark transitions in the image. The picture is also not as sharp as it could be, and the two effects get worse as your horizontal scan rate increases. In my case, at 86Khz, the effect is quite noticeable, while under 40KHz it is invisible. These ringing effects are caused by a combination of factors in the circuitry between the CSPPC and your monitor. Sometimes the monitor itself can cause these problems, especially if it is cheap and nasty, so check it first. All video cards made these days contain circuitry to minimise RF interference with other equipment in order that they will comply with the CE standard (for Europe). Other countries have their own standards. This circuitry is usually a ferrite bead in series with the red, green and blue outputs in the video card. Unfortunately sometimes the capacitance in the video cabling can interact with the inductance provided by the ferrite beads resulting in ringing in the image and/or blurry pictures.

In order to fix the problem, we need to first look at capacitance in the cabling. In the CSPPC there are actually two cables. Firstly there is the cable from the VGA connector leading to the monitor. Make sure it is good quality low capacitance (sometimes called "low loss") cable. Don't skimp on it. Get the best you can find. Also, if possible, make sure it is as short as possible. In my preliminary tests I tried several different cables, and each gave a different result. The other cable is the one supplied by Phase 5 which runs between the main board and the small daughter board which contains the VGA connector. It is simply a 10-way IDC type cable, which, unlike my previous report, contains a copper mesh shield under it's black sheath. This shield adds a fair bit of capacitance to the system, which causes a big drop in sharpness. The best way to cure this problem is to make up a new cable, making sure it's as short as possible. Shortening the cable helps to improve picture sharpness further. You will need 2x 10 pin IDC female connectors and an appropriate length of 10-way IDC ribbon cable. The connectors are simply crimped onto the cable using a crimp tool. Make sure that the polarity is right on each end (for obvious reasons!) Line up the small arrow symbol on each connector with the red stripe on the cable. Look carefully at the existing cable if you are not sure what I mean. The external sleeve provided on the Phase 5 cable is not needed, however if you want you can use a length of heatshrink tubing. If you are not confident of making the cable properly, a computer technician can do it for you after you have measured the length you need. My new cable was about 30% shorter than previously, and as it no longer had a shield, all traces of ringing and blurring were gone. Note that there may be an increase in RF interference radiated by the monitor cabling after this modification.

If, after doing the above modifications you are still getting poor results, we will need to look at the ferrite beads. These modifications will void your warranty and could cause radio and TV interference. They may also be illegal in some countries. This mod should only be attempted by an electronics technician familiar with surface mount components. We need to remove the ferrite beads from the circuit by shorting them out. The ferrite beads are located on the small daughterboard. There are three of them - one each for red, green and blue. They are very small surface-mount components, and are rectangular and black in colour. They are soldered directly to the board at each end. Do not confuse them with the three transistors nearby which have 3 solder terminals. Using a very fine soldering iron, a very steady hand, and a single strand of very fine wire, bridge out each ferrite bead. You may find it easier to remove the ferrite bead first if you wish and then bridge the gap with the wire and solder or preferably, a surface mount link. Check your work with a magnifying glass after you finish. Again, I must emphasise how delicate this work is, and I advise you only attempt it if the picture has not been improved by changing the cables.

Overclocking

It is now possible to overclock the CVPPC if you own Cybergraphx V4. You will need to update to version 4.2. Go to http://phase5.a1k.org/index.php?cgx to get the updates and note carefully the warnings and comments there. Once you have updated your files and fitted the heatsink & fan to the Permedia chip, you will need to add the new tooltype IINSTALLEDAFANONMYPERMEDIA2 to your CVisionPPC monitor in Devs:monitors. Note that you have to type IINSTALLEDAFANONMYPERMEDIA2 backwards to make it work. You will then need to add the MELTDOWN tooltype, also to the CVisionPPC monitor to specify your desired overclock speed. This is in the format MELTDOWN=n where n=clock frequency in MHz. For example to overclock to 90MHz, use MELTDOWN=90. The allowable range is 80MHz to 100MHz. The default, unoverclocked speed is 80MHz.

Experimentation will be requred to determine the highest reliable setting. An excessive setting will result in random corruption on the screen. Preliminary experiments have shown that about 92-93 MHz are reliable for both 2D and 3D functions. As usual your mileage may vary, so experiment with your values. It is also a good idea to leave the case off your Amiga during these experiments to monitor the effectiveness of your heatsink/fan. Assuming you have overclocked to about 90MHz, you should expect about a 10% speed improvement to some functions.

The above tooltypes do not improve the speed of the data path from the CSPPC to the CVPPC. To do this you will need to overclock the CSPPC. Currently, I have not fully evaluated the effects of using both methods of overclocking together other than a brief test to confirm that it will work. The CVPPC benchmarks quoted on my CSPPC page are for a CVPPC without the MELTDOWN tooltype. Remember that good cooling is vital . Even if you choose to not overclock your CVPPC, I strongly advise fitting a heatsink as the Permedia chip always runs very hot. In fact I know of one case where a voltage regulator on a CVPPC got so hot that it unsoldered itself and charred the PCB - this was on a standard, unoverclocked CVPPC in a towerised A4000!. Fortunately there are now available commercially made "chipset coolers" which are ideal for our application. One available here in Australia is the CS100 kit made by "Just Cooler" from Taiwan. This kit consists of 3 different sized self-adhesive heatsinks. The largest one has a fan incorporated but is too large to fit on the Permedia. The smallest heatsink will fit, and a small fan, available from any electronics retailer can be attached to it. Other alternatives intended for cooling chips in laptops may also be suitable.

Video Switcher/Scandoubler?

There is no provision for a video switcher/scandoubler on the CVPPC. Some people have wondered if the 10 pin video connector can be used for this as it looks similar to the scandoubler connector on other video cards. The answer is that it is not the same and damage could possibly be the result if one is used in that connector. There is (as far as I know) no switching signal provided for an external video switcher, so any video switcher added externally will have to be manually operated, though I wonder if the LCD glasses connector could provide a suitable signal if programmed appropriately. The latest Cyberstorm Flash ROM update now allows promoting various boot menu screens through the CVPPC. This update also provides some limited promotion of ECS/AGA screens through the CVPPC.