Tips T0017

1. Drivers Explained:
When you install a new piece of hardware into your PC, you need some software that will allow programs to use this hardware. This software is called a driver.

Arrangement

The following diagram shows how these components are arranged within the PC.

The driver provides the connection between Windows and the hardware and a program may use the hardware by asking Windows to perform the functions it requires. For example, if the program was a video capture application it would ask Windows to start capturing. Windows would then use the functions within the driver to setup the video capture card and to start it capturing. By using this method of drivers, the Windows programs do not need to know anything about the actual hardware in order to use it.

Driver Types - Video For Windows

There are many different types of driver in use in Windows, but we will just concern ourselves with the drivers used for video capture. The original drivers used for video capture were the Video for Windows (VfW) drivers. VfW is a system that was added to Windows 3 many years ago to support most of the features needed for dealing with video under Windows. The VfW system supports different compression formats and different capture devices. Any program that uses VfW has a number of Windows functions that it can use, for example:

• Start capture
• Stop capture
• Grab frame
• Configure capture

(there are many more, but this is the basic idea).

This system has done well, but around the time of Windows 95 it was realised that VfW had some shortcomings. All of VfW file handling is based on 32 bit numbers, so is limited to a maximum file size of 4GB for a captured AVI (see The 4GB File Limit article). Also, some of the more complex capture devices (eg. TV tuners or web cams) didn't have support in VfW (there was no VfW function to change the channel on a TV tuner, for example). The solution to this was WDM.

Driver Types - WDM

WDM stands for Windows Driver Model, and is an all-encompassing driver format. WDM is designed to replace VfW by providing a much more flexible way of dealing with drivers. Rather than having specific functions (like "Start capture" and "Grab frame" in VfW), WDM drivers can be extended by supporting standard functions as well as adding their own. This allows WDM to support a standard set of functions, as well as adding functions specific to the particular capture hardware.

WDM is supported by Windows 95, 98, ME, NT4, 2000 and XP and it is hoped that only a single WDM driver needs to be written for a particular piece of hardware and it will work under all the above versions of Windows. This should end the situations whereby different versions of drivers were available for different versions of Windows.

All this extra flexibility comes at a price, and WDM drivers can be substantially more complex to develop than VfW drivers. Unfortunately, this leads to some problems.

WDM vs VfW

As Windows 95, 98, ME, NT and 2000 all supported both WDM and VfW, some hardware manufacturers didn't see any need to develop the more complex WDM drivers when the simpler VfW drivers would still work. This leads to some problems:

Software that uses DirectX to access the hardware, cannot use VfW drivers. DirectX uses WDM exclusively and although it tries to "wrap" VfW drivers for use in DirectX, this is not always successful.
VfW capture drivers are limited to a maximum file size of 4GB
This issue directly affects EditStudio, as it uses DirectX and does not use VfW at all. EditStudio relies on the video capture drivers being WDM in order to work properly.

The End Of VfW

Rather than continue to support the older VfW along with WDM, Microsoft has decided to stop support of VfW drivers with Windows XP onwards. This means that any hardware that still relies on VfW drivers will not work under XP. This change has been at least 6 years coming, so the hardware