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Digital PC camera design with IEEE1394 connection

Posted: 20 Mar 2000 ?? ?Print Version ?Bookmark and Share

Keywords:texas instruments? ti? ieee1394a? digital still camera? dsc?


238 International IC ? China ? Conference Proceedings Abstract Withtheboomingdemandforhighperformance,goodqualityvideo imaging, the need for a high-speed, high-bandwidth interface has becomeafocalpointforthenextgenerationofdigitalPCcameras. The IEEE1394a serial bus is ideally suited for this task, not only because of its 400Mbps transmission capability, but also because of the emerging need for various applications from the PC host side, such as video mailing, video conferencing and other high- bandwidth applications. User experience over the past two years shows that image quality and frames per second are the two most important technical factors in digital PC camera designs. This ses- sion will introduce the advantages of an IEEE1394a connection within a digital PC camera and give a brief introduction of general system architecture. A sample design with a currently available 1394a PC camera will also be covered. Introduction Thereare,generally,twodigitalcameracategories:digitalstillcam- eras (DSC) and PC cameras. In the marketplace, digital still cam- eras are positioned more as consumer electronics equipment, whereas PC cameras are PC peripheral devices. Unlike digital still cameras, PC cameras capture and process continuous image data then transmit those data in real time to the PC host, either WinTel or Mac, for applications such as video conferencing, video mailing or security. A typical digital PC camera design can be segmented into three modules: the image sensor, the video pro- cessing unit and the PC communication interface. Image sensor The image sensor-the analog eye of camera-is the first stage of image processing within a digital camera. The sensor converts optical signals into analog image data. Sensor technology can fur- ther be segmented into two categories: Charge-coupled device (CCD) sensors and CMOS sensors. The differentiating quality between these two types of sensors in terms of picture quality and performance are, generally, pixels per field and integration time. ThepowersupplycircuitryofCCDsensorscanalsobemorecom- plex than CMOS sensors,but with better quality and performance. The pixel depth of CCD sensors is generally 350K for most PC cameras on the market today, but can go up to over one million pixels. High-end CMOS sensors can reach 350 pixels. With tech- nologyinnovationandyieldimprovement,CMOSsensorsarenow emerging as the cost-effective performance leader for PC camera designs. Video processing unit With CCD image sensors, the image data is in analog form and must go through an analog shift register. These analog CCD data are then fed into a CDS/ADC (correlated double sampling, ana- log-to-digital converter) analog front end for data conversion. Bit resolution and sampling speed are the two key parameters for the ADC to convert the CCD image data, but there is a tradeoff of cost versus performance. After theADC, the digital-pixel data are then processed by a processor unit with various video imaging control functions, such as auto brightness, auto exposure, sharpness and gamma correction. From there the data are then sent to a video data compression unit for conversion into JPEG or MPEG1 for- mat. Each of the above functions entails a complicated image- processing algorithm for the pixel data to be processed. At the same time, the processor unit must also control the gain and timing of the CDS/ADC and the CCD sensor through a timing generator. This is needed in order to provide feedback to, and control the result of, the pixel data input. For advanced PC camera designs with motor-driven auto-focus functions, a motor interface is required. Motor interface stepping is also tuned by this proces- sor unit. With all of these processes to cope with, the calculation speed and efficiency of the processor unit inherently affects over- all PC camera performance. PC communication interface At least three communications interfaces are available to link PCs and PC cameras: EPP, USB and IEEE1394a. EPP, or enhanced legacy printer or parallel interface, is well known. USB 1.1, the current market share leader for PC camera interface designs, is a serial bus interface with a maximum data transmission capability of 12Mbps. The proposed USB 2.0 standard is defined to have a maximum data transmission rate of up to 480Mbps, but currently has no application or equipment in sight and will be more expen- sivethana1394ainterfacewhenitfinallystartsshipping. Coupled with reliability issues and the increasing cost of making the USB Digital PC camera design with IEEE1394 connection Creg Chang 1394 Technical Marketing Manager Texas Instruments Asia International IC ? China ? Conference Proceedings 239 2.0 compatible with existing USB1.1 devices-plus the fact that the target data transmission rates are not yet verified by industry-the USB 2.0 remains an iffy goal in the short term. The third communication interface for PC cameras is the IEEE1394a serial bus. The 1394a is a high-performance interface that can handle data transfer at 100Mbps, 200Mbps or up to 400Mbps. (The IEEE1394b standard will have a data transfer ca- pability of 800Mbps/1.6Gbps.) This high-bandwidth, industry- standard interface has been proven by various applications in both theconsumerelectronicandpersonalcomputingmarkets,inequip- ment such as digital camcorders, digital TVs, printers, scanners and other equipment. Advantages of IEEE1394a for digital PC cameras ForYUV 4:1:1 color format with 320 (H) x 240 (V) resolution at 30 frames per second, the data transmission bandwidth required to play a natural live video is around 37Mbps. In USB cameras, various compression techniques are embedded with the processor unit in order to transmit high video streaming through the USB 12Mbps bus. The compression proportion can be up to 20:1 for 640 (H) x 480 (V) resolution. The compressed data are then de- compressed on the PC host side using CPU time. For video conferencing, the load on the host CPU is to decompress the com- pressed video data from the USB camera, then compress it again and transmit the video data through either a private or public tele- communication network. With its high-bandwidth, the 1394a eliminates the need for a high compression rate on the PC communication interface. For YUV 4:1:1 color format with 640 (H) x 480 (V) VGA resolution at 30 frames per second, the data bandwidth requirement is around 148Mbps. With its 400Mbps data rate, an IEEE1394a bus inter- facecanefficientlytransferanuncompressed,qualityvideostream from the PC camera to the PC host with little compression load imposed on the host CPU. In the future, direct peer-to-peer con- nectivity with other 1394a devices will bypass the need for PC processing altogether. An IEEE1394a PC camera design example A 1394a node, such as in the below example, generally requires 4- layer support: application, transaction, link and physical layers. The application layer is where the application software calls for 1394a transaction service. At the transaction layer, functions will vary with different higher-level protocols based on 1394a bus pro- tocol. This layer can either be implemented with software drivers or be partially embedded in hardware. The link layer design varies as well depending on the application, but is implemented in the hardware. The link layer uses two types of packet transactions: isochronous and asynchronous. In general, 1394a isochronous packets send video data in periodic 125?s bursts with guaranteed bandwidth; whereas 1394a asynchronous packets exchange con- trol and status information with guaranteed data integrity from the bus. The last layer is the physical layer, where bus arbitration, speed signaling and data serializing are implemented. The PC camera solution shown in this example serves all four layers using hardware:theTSB15LV01fortheapplication,transactionandlink layers; and the TSB41LV01 as the physical layer device. Image light is captured by an image sensor with a lens in front of it. An interface to a stepper motor is provided for auto-focus control if desired. The CCD image sensor and driver shown in this example can also be found in many of today's USB PC camera designs. The serial output from the CCD sensor is an analog sig- nal, so an analog front-end (AFE) circuit is required to convert the signal into a digital signal using a CCD-specific correlated double sampling technique. Unfortunately, there is no unified interface among AFE and camera ASIC designs. The digital signals, or pixel data, are then fed into the camera ASIC for advanced image data processing. The following figure shows a functional block diagram inside a camera ASIC, the TSB15LV01, an integrated video signal processor for IEEE1394a PC cameras: TheAFE data interface provides a data path for pixel data into the integrated pipelined video signal processor. After image pro- cessing, the video data is then transferred into an RGB/YUV formatter block. This is a required process for the 1394a isochro- nouspackettransactiontoensureafixedpayloadsizetocarryvideo data exactly as in the original image. The formatted video data are then moved into a FIFO buffer to be processed by a 1394a link core, which is controlled by another required processor called an 240 International IC ? China ? Conference Proceedings asynchronous command processor. The data mover will transfer isochronous data in DMA-style from the FIFO through the link core out to an external physical layer device. On the PC host side, another 1394a node, usually with bus manager capability, receives the video data from the PC camera. The physical layer design is actually the same as in the PC camera. An OHCI (Open Host Control Interface) link layer, a hardware device under theWinTel environment, then extracts the video data from the channels assigned in the isochronous packet delivered by the PC camera. Then the transaction layer, which is contained in a driver stack (1394a bus driver, OHCI mini-port driver, TWAIN driver, and camera proprietary driver) will process the video data per the user's settings and requirements. Digital PC camera future trend As mentioned earlier, it is possible that CMOS cameras can lead the market once quality issues are resolved and yields meet mass- production requirements. However, CCD cameras will move for- ward with higher XGA resolution and rich features as long as the broadband public networking infrastructure is established. The speed of the IEEE1394a bus at the PC communication interface can then adhere to 800Mhz or higher for adequate transmission bandwidth. Digital PC cameras with DSC storage functions, or vice versa, is another camera design trend. It is also possible that PC camera designs may innovate further to transfer video directly into an external, portable 1394a AV-capable storage drive. This will be possible with a transport protocol called SBP-2 (Serial Bus Protocol 2), developed byANSI to adapt SCSI command sets into a serial bus environment such as IEEE1394, or using the AV/C (Audio/Visual Command) command sets defined for 1394 con- sumer devices. However, the tradeoff on cost versus performance with these applications will continually play a critical role for en- gineers doing good camera design. References ? IEEE1394a Standard (IEEE1394a-1995 & IEEE1394a.a), Institute of Electrical and Electronics Engineers, Inc. ? Digital Camera Draft 1.20 Specification, 1394a Trade Association ? Fire Wire System Architecture, Second Edition, Don Anderson, Mindshare, Inc. ? Information Technology - Serial Bus Protocol 2, American National Standard Institute, Inc. ? TSB15LV01 Product Preview, Texas Instruments ? TSB41LV01 1394a Cable Transceiver/Arbiter Data Manual, Texas Instruments ? TLV986, Area CCD Sensor Processor Data Sheet, Texas Instruments Author's contact details Creg Chang 1394a PC Camera and Repeater Technical Marketing Texas Instruments Asia 25F, 216, Sec. 2, Tun-Hua S. Road, Taipei, Taiwan Phone: 886 2 2376 2723 Fax: 886 2 2377 1460 E-mail: Presentation Materials International IC ? China ? Conference Proceedings 241 242 International IC ? China ? Conference Proceedings International IC ? China ? Conference Proceedings 243 244 International IC ? China ? Conference Proceedings International IC ? China ? Conference Proceedings 245 246 International IC ? China ? Conference Proceedings International IC ? China ? Conference Proceedings 247 248 International IC ? China ? Conference Proceedings

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