1、視頻會議通過TCP / IP協議在個人電腦上的實現by John F. McGowan, Ph.D.Desktop Video Expert CenterApril 24, 1997總體簡介目前，視頻會議系統通過TCP / IP協議的局域網和廣域網在個人電腦上大量的安裝和配置仍具有許多挑戰。除了視頻會議應用程序之外，還有視頻會議系統的使用和在個人計算機之內頻繁地修改如下五個子系統等挑戰。· 視頻顯示· 視頻采集· 音頻輸出· 音頻輸入· TCP/IP網絡（包括網卡和TCP/IP的軟件）每個子系統因本身就是較為復雜的。 因此，在視頻會議系統的設施

2、和配置期間，問題可能會發生系統的任何地方。 此外，程序又必須在用戶端安裝，而在大多數個人計算機上，視頻會議系統硬件和軟件并不是標準的。通常來講，不同的個人計算機上的軟硬件導致在每臺個人計算機安裝的硬件和軟件變化極大。即使一個經驗豐富的安裝高手在一臺特殊用戶的個人計算機上也可能遇到陌生的情況。所有版本的Windows- Windows3.1, Windows 95,Windows的即將發布的“Memphis”,以及WindowsNT都是建立在一個有處理系統部件和vendor-supplied設備驅動程序的復雜的系統之上的。一些設備驅動程序實際上是通過硬件設備實現,如視頻顯示卡,視頻采集板、聲卡、

3、網卡等。其他的設備驅動程序與硬件設備驅動性能有很多共同之處,但除TCP / IP協議的實施和具有系統特征的軟件之外。設備驅動程序是非常強大的,因為在所有版本的Windows系統中他們在接入硬體和軟件上都處在一個特權水平上。即使是在Windows 95,Windows 3.1中,系統仍允許多個應用程序占用操作系統的內存或其他的應用程序的內存。設備驅動操作的特權級別就意味著驅動比一個應用程序造成更為嚴重的損害。例如，難以言喻的奇怪的系統崩潰以及以及設備間沖突事故。此外，大多數的系統配置和安裝問題都和驅動有關。盡管在此領域中將會遇到很多的問題不可能被預期,本文將會給出通過TCP / IP的網絡實現的

4、個人電腦上的視頻會議系統的有關整個安裝和配置問題的一個概述。視頻顯示 視頻會議系統的重點在于個人計算機的視頻顯示：視頻適配器和視頻驅動程序。視頻顯示驅動程序可以包含細微的錯誤，這將導致與應用程序發生沖突其中也包括與視頻會議的應用程序發生沖突，造成包括一般保護錯誤和在屏幕上更新等問題在內的許多問題。一般來說，要確保視頻卡具有的是最新驅動程序。大部分主流的視頻卡和視頻芯片供應商都會在其網站和FTP站點提供相應產品的驅動程序。對于部分用戶有可能使用特殊顯示卡驅動而不是一般的視頻芯片驅動程序的情況，用戶應向芯片制造商獲取相應的驅動程序。例如，對于鉆石系列的多媒體視頻卡，無論是鉆石顯卡還是S3顯卡，其供

5、應商在生產鉆石系列芯片時都提供了相應的驅動程序。在鉆石系列中，同S3相比大大增強了鉆石顯卡的驅動程序。一個錯誤的視頻顯示驅動程序在個人電腦上引起問題的情況通常是非常不明顯的。視頻會議所采用的數字視頻標準是工作在YCrCb顏色空間而不是視頻卡和顯示器所采用的RGB色彩空間。例等。最新一代的視頻芯片提供硬件色彩空間轉換的功能。該芯片提供了將視頻從YCrCb顏色空間（亮度，顏色的差異）轉換為RGB的顏色空間的視頻硬件解碼，這大大提高了視頻的播放速率。此外現在許多廠商的芯片都提供了硬件加速的圖像縮放功能。安裝具有這些具有先進的功能的視頻卡的需要或愿望可能將會出現。 在Windows95系統下，彩色空間

6、的轉換和圖像的縮放功能是通過DirectDraw來訪問API的。DirectDraw 的HAL（硬件抽象層）提供了實際設備的驅動程序。而在系統下，標準的圖形設備接口（GDI）并不提供硬件色彩空間轉換或對圖像縮放功能的系統下視頻卡的標準設備驅動程序是GDI設備的驅動程序。著名芯片制造廠商英特爾公司定義了一個名叫DCI的標準，即顯示控制接口的標準，支持諸如硬件色彩空間轉換和圖像縮放等高級功能。在Windows95系統下，視頻卡必須安裝正確的DirectDraw設備驅動程序以用來支持訪問硬件來實現視頻色彩空間轉換和圖像系統下，一個DCI的驅動程序必須被安裝以用來支持這些功能的實現。應用程序使用的GD

7、I調用不能訪問一般的硬件加速功能。因此，視頻會議系統的應用程序必須適當的支持DirectDraw或DCI驅動。 視頻會議系統應用程序通常含有一個文檔，自述文件，等用來列出已知的視頻顯示硬件和驅動程序的沖突或問題。 視頻顯示應處理的問題 · 重新啟動.· 檢查IRQ或其他資源沖突.· 用顯卡檢查視頻會議系統的錯誤記錄文檔.· 確保有正確或者最新的顯卡適配器驅動.· 用Windows系統的控制面板重裝顯卡驅動.· 使用顯卡安裝程序重新安裝顯卡驅動.· 下，通過編輯SYSTEM.INI中手動安裝驅動程序（請備份 原始System.

8、ini） · 安裝新的或不同的顯卡.視頻采集視頻會議系統需要一個視頻采集系統。一個視頻采集系統包括一臺攝像機，通常是NTSC或PAL復合視頻相機，要么是ISA總線或PCI總線采集卡。通常，這些都不是預先安裝在個人電腦上的。因此，對于個人電腦上的視頻會議系統的安裝，幾乎總是要涉及一個視頻采集卡和攝像頭的安裝。通常，視頻采集硬件提供了視頻會議應用程序。安裝一個視頻采集卡需要打開電腦機箱并將視頻卡插入電腦中現有的ISA或PCI插槽。插口可能會出現問題，在那里的視頻卡雖然是插在插槽中，但有可能會出現卡和卡槽接觸不完善或折斷的狀況。在這種情況下，軟件將報告無法找到的視頻采集卡或可能會發生其他狀

9、況。 視頻采集卡通常是復合視頻輸入或復合視頻輸入加上S-Video輸入。相機必須和此輸入相連接。在某些情況下，即使是有電纜的接入也會出現不完美的電氣連接出現。在這種情況下，安裝程序必須處理的等待電纜的連接，直到一個工作建立為止。一些ISA視頻采集卡要求在其芯片上通過跳線來選擇IRQ的中斷，雖然這種情況變得越來越少見但仍有出現。其他ISA視頻采集卡僅通過軟件進行設置就可以。而兼容即插即用的ISA和PCI視頻采集卡有時會遇到資源沖突，這是由于即插即型卡設計實現的缺陷所造成的。 視頻采集是通過VFW驅動程序來進行處理的。在Windows 3.1和Windows for Workgroups。而在Wi

10、ndows95系統中提供了一個帶有視頻壓縮功能的32位的VFW版本，此版本和VFW1.1e版還有一些未知的差異和聯系，Windows95系統中所提供的這個版本的VFW已經具有視頻采集的功能了。基于Windows 95 OEM服務版本2（OSR2）的ActiveMovie1.0，可以在Windows95的早期版本中安裝，但它不提供任何對視頻拍攝的支持。視頻采集卡的軟件安裝過程中應安裝VFW視頻采集驅動程序。此驅動是有如下行定義的：在 Microsoft Windows SYSTEM.INI 文件中進行驅動選擇。視頻顯示應處理的問題 · 重新啟動.· 檢查IRQ或者其他資源是否

11、沖突.· 檢查視頻采集攝像頭和網線連接.若有電氣連接故障微調網線.· 檢查視頻采集卡是否在主板上插裝好.· 用顯卡檢查視頻會議系統的錯誤記錄文檔.· 確保有正確或者最新的顯卡適配器驅動.· 用Windows系統的控制面板重裝顯卡驅動.· 使用顯卡安裝程序重新安裝顯卡驅動.· 下，通過編輯SYSTEM.INI中手動安裝驅動程序（請備份 原始System.ini） · 安裝新的或不同的顯卡.視頻會議的應用程序視頻會議系統的客戶端安裝視頻會議系統的第一步都來自于應用程序的安裝。這些通常是InstallShield或其它商

12、業公司提供的安裝程序。安裝程序將安裝包括所有應用的軟件組件。安裝程序經常也會伴隨著安裝視頻會議系統所需的視頻捕獲卡和聲卡。不幸的是，安裝程序有時會安裝失敗。如果懷疑程序安裝的有問題，請先卸載視頻會議系統的程序，然后重新運行安裝程序進行安裝。在絕大多數情況下，這樣操作都會有所幫助。 總結 視頻會議系統通過TCP/ IP網絡在個人電腦上的實現可能會涉及大量的個人電腦和網絡的安裝和配置。不幸的是，在個人電腦上安裝和配置系統的硬件和軟件是很復雜一件事。但在個人電腦上安裝和配置系統的硬件和軟件也不是像研究火箭科學那樣艱難。它需要有很多的步驟來去實現，特別是對于像視頻會議系統這樣復雜的產品。單獨拿出每一步

13、來說都很簡單，但很多簡單的步驟組合在一起，導致了一個相當復雜的過程。技術支持人員或用戶必須避免被繁雜的步驟和頻頻出現的各種奇怪的問題所嚇倒，哪怕問題是出現在Windows系統上的安裝問題。 Videoconferencing on Personal Computers with TCP/IPby John F. McGowan, Ph.D.Desktop Video Expert CenterApril 24, 1997IntroductionVideoconferencing systems for PCs over TCP/IP Local Area Networks and Wide A

14、rea Networks present substantial installation and configuration challenges. In addition to the videoconferencing applications, a videoconferencing system uses and frequently modifies five subsystems within a personal computer:· Video Display· Video Capture· Audio Output· Audio In

15、put· TCP/IP Network (both Network Interface Card and TCP/IP software)Each subsystem is complex in its own right. Thus, problems will likely occur somewhere during the installation and configuration of a videoconferencing system. As yet, videoconferencing hardware and software is not standard on

16、 most PCs. Installation must occur at the user site. Generally, PCs vary significantly with different hardware and software installed on each PC. Even an experienced installer can encounter a new situation on a particular users PC.All versions of Windows - Windows 3.1, Windows for Workgroups 3.11, W

17、indows 95, theupcoming “Memphis”, and Windows NT are built on a complex system of operating system components and vendor-supplied device drivers. Some device drivers manage actual hardware devices such as video display adapters, video capture boards, sound cards, and network interface cards. Other d

18、evice drivers share many properties with the hardware device drivers but implement TCP/IP protocols and other software features of the system.Device drivers are extremely powerful as they access the hardware and software at a privilegedlevel in all versions of Windows. Even in Windows 3.1 and Window

19、s 95, which can allow applications to overwrite memory within the operating system or other applications, device drivers operate at a privileged level which allows the drivers to do even more damage than an application. Because of this device drivers can cause severe problems, inexplicable crashes,

20、and strange conflicts between devices. Most configuration and installation problems involve the device drivers.While it is impossible to anticipate the many situations that will be encountered in the field, this document provides an overview of the installation and configuration issues for videoconf

21、erencing systems on PCs over TCP/IP networks.Video DisplayVideoconferencing systems make heavy use of the PCs video display: video adapter card and video drivers.Video display drivers can contain subtle bugs that can conflict with applications, including the videoconferencing applications, causing m

22、any problems including general protection faults and updating problems on the screen. In general, make sure to have the latest drivers for a video card. Most major video card and video chip vendors provide drivers on their Web sites and ftp sites. Use the video card drivers rather than generic video

23、 chip drivers where possible. For example, for Diamond Multimedia video cards, both Diamond and S3, the manufacturer of the video chips used in the Diamond Multimedia video cards, provide video drivers. Diamond has substantially enhanced the drivers for Diamond cards over the drivers provided by S3.

24、It can be very unclear that a faulty display driver is the cause of a problem in a PC.The digital video standards used in videoconferencing such as H.261 and H.263 work in theYCrCb color space rather than the RGB color space used by video cards and monitors. The latestgenerations of video chips prov

25、ide hardware color space conversion. The chip converts the decoded video from the YCrCb (luminance-color difference) video into RGB in hardware, providing a substantial speedup. Many chips provide hardware accelerated scaling of images. The need or desire to install a video card with these advanced

26、features may arise.Under Windows 95, color space conversion and scaling are accessed through the DirectDrawAPI. The DirectDraw HAL (Hardware Abstraction Layer) provides the actual device driver. UnderWindows 3.1, the standard Graphic Device Interface (GDI) provides no support for hardware color spac

27、e conversion or scaling. The standard device drivers for video cards under Windows 3.1 are GDI device drivers. Intel defined a standard called DCI, Display Control Interface, that supports advanced features such as hardware color space conversion and scaling. Under Windows 95, the correct DirectDraw

28、 device driver for a video card must be installed to access the hardware color space conversion and scaling features. Under Windows 3.1 or Windows for Workgroups 3.11, a DCI driver must be installed to access these features. The videoconferencing application must support DirectDraw or DCI as appropr

29、iate. An application that uses the GDI calls cannot access the hardware acceleration features in general.Videoconferencing applications often include documentation, readme files, and so forth that list known conflicts or problems with video display hardware and drivers.Things To Do for Video Display

30、 Problems· Reboot.· Check for IRQ or other resource conflict.· Check the videoconferencing system documentation for notes on problems with the video card.· Make sure that you have the correct or latest version of the video display adapter device driver.· Reinstall the video

31、card using the Windows Control Panel.· Reinstall the video card using the video cards installation program.· On Windows 3.1, manually install the drivers by editing SYSTEM.INI (make backup of original SYSTEM.INI)· Install a new or different video card.Video CaptureVideoconferencing sy

32、stems require a video capture system. A video capture system includes a video camera, typically an NTSC or PAL composite video camera, and a video capture card, either an ISA bus or PCI bus capture card. Usually, these are not provided pre-installed on the PC. Thus, installation of a videoconferenci

33、ng system on a PC almost always involves installation of a video capture card and camera. Usually, the video capture hardware is provided with the video conferencing applications.Installing a video capture card requires opening the PC case and inserting the card in anavailable ISA or PCI slot as ava

34、ilable. Seating problems may occur, where the card appears to be seated in the slot but the electrical contact is imperfect or broken. In this case, the software will report a failure to find the video capture card, or other symptoms may occur.The video capture card usually has either a composite vi

35、deo input or a composite and an S-Video input. The camera must be attached to this input. In some cases, the electrical connection will be imperfect even though the cables appear plugged in. In these cases, the installer must manipulate the cables until a working connection is established.Some ISA v

36、ideo capture cards require selection of the IRQ interrupts through jumpers on the ISA board, although this is becoming less common. Other ISA cards may be configured through software. Plug and Play compliant ISA and PCI cards may sometimes experience resource conflicts due to bugs in the Plug and Pl

37、ay implementations.Video capture is handled through Video for Windows drivers. The most recent version of the 16 bit Video for Windows for Windows 3.1 and Windows for Workgroups is Video for Windows 1.1e. A version of Video for Windows with 32 bit video compressors and unknown other differences from

38、 Video for Windows 1.1e ships with Windows 95. This Video for Windows (95) provides the video capture functions in Windows 95. ActiveMovie 1.0, which ships with Windows 95 OEM Service Release 2 (OSR2) and can be installed on earlier releases of Windows 95, does not provide any support for video capt

39、ure. The video capture card installation software should install a Video for Windows video capture driver. This driver is identified by the line:in the drivers section of the Microsoft Windows SYSTEM.INI file.Things To Do for Video Capture Problems· Reboot.· Check for IRQ or other resource

40、 conflict.· Check the cables connecting the camera to the video capture card. Jiggle and adjust the cables in case there is a faulty electrical connection.· Check that the video capture card is properly seated in its slot on the motherboard.· Check the videoconferencing system documen

41、tation for notes on problems with the video capture card.· Make sure that you have the correct or latest version of the video display adapter device driver.· Reinstall the video card using the Windows Control Panel.· Reinstall the video card using the video cards installation program.· On Windows 3.1, manual