WDM Video Capture Driver
for Bt848 / Bt849 / Bt878 & Bt879 Chipsets.

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A bit of the Story/History maybe?

This driver was written because my video capture card (a VHX) was not supported anymore by Aimslabs (In fact, i suspect that this company has broken). At that time, I was investigating about video compression, colorspace conversion , etc, and I realized that more than 10 to 30% of the CPU time was lost converting from the RGB color space to the YV12 planar color space that most video compressors use. But my driver was working bad at that mode (in all modes I have to say, later I will explain why) , I decided to investigate on the chipset capabilities, I did get the datasheet, and I found that the chip was able, not only just to capture at full PAL size (768x576) : It was able to capture at full framerate, individual fields at full size. But that wasn't working with the drivers of the Card. Also the card can adjust Gamma (only RGB formats), adjust Sharpness ( Using the filters that it has incorporated inside the chipset, and support nearly every analog video standard. All this was implemented digitally (all process is digital inside the chip, the video signal is digitalized as the first step, better than most TV sets you can buy right now in the market). I felt that to be unable to use such a good chip was a shame, so I started to look after information about how to write a device driver. I have selected Windows as the OS, 'cause at this time, there is a good (I won't say perfect 'cause some things are missing, such as direct DMA to userspace) driver for Linux (bttv). Also, there is a lot more very good applications for video editing under Windows. While I was looking for that info about how to write that driver, I discovered that Microsoft was changing the driver model from the VxD model that is supported under Win95, to the new WDM driver model, that supports from all the user applications the tuning of TV channels through a normalized API, selecting video sources, audio modes, etc. and that driver would work on all the new Windows OS, as Win98SE, WinME, Win2000 and WinXP. Due to that cause, I decided to write a WDM driver. This was the start of the nightmare!!!! I downloaded the Win98DDK: Inside it ... Incredibly, a (very buggy, very alpha) driver for Bt848 based boards driver. I took that driver as the starting point. Soon the driver was rewritten from scratch, to clean it, implement all the missing features I wanted it to have, support all the things the chipset supports, and make it stable and reliable. I think the driver has reached this condition. Then I downloaded the Win2000DDK... The driver wasn't anymore bundled with that DDK.... Guess! I will explain some things I have found while I was writing this Driver:

  1. I have discovered that the original driver found in the Win98DDK was based (maybe written) on the original driver written by Brooktree. So , from now on, I will acknowledge as the original authors of the original code (NOT the driver you have now) to Brooktree (then Rockwell, now Conexant).
  2. The WDM model is still (obviously) under development. I have found plenty of bugs (non documented behaviour, things that don't work as they should) in the interfases. They say that this should work in this way (Microsoft) , but it does not . Some of the limitations of the driver are due this.
  3. They don't tell you, but when you update DirectX, you are updating the WDM layer (at least, the Video part) also.
  4. The WDM to VFW (Video for windows) wrapper assumes some stupid things about capture and preview channels (for example, keeps previewing (overlay mode) at full frame size, and capturing at a quarter size instead of setting both channels to the same size). This leads to a preview degradation, not to mention that Bt8xx does not support capture and preview (based on overlay) at the same time at full frame size. So, I had to implement a sort of priority channel capture, that ensures that the capture channel has priority over preview...
  5. I had to implement variable framerate at the driver level. Microsoft documentation says that the WDM layer should handle it automatically, but it does not. It simply tells the app. that everything is fine, but gets the maximum framerate from the driver... Guess what will happen with that little 2 fps AVI you were going to digitize...
  6. NetMeeting needs to have a VideoInfo with AnalogStandard set to None this one is the one it uses .... Well ...

Read the source code if you want to learn more! This is really boring stuff!

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