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This third article in the High Definition (HD) Video series describes the process for capturing HD video in real-time from a video source to a computer's hard disk drive, where you can use digital media tools to edit and encode the video. Read this article to become familiar with HD video capture protocols, some popular hardware and software solutions for HD video capture and editing, and common HD video capture issues.
The previous two articles in this series, Zero to HD in 60 Seconds and Understanding HD Formats, describe important concepts about HD formats and storage needs. If you are unfamiliar with HD formats and storage needs, it is recommended that you read these previous two articles before reading this article about capturing HD video.
A few common protocols are used for capturing HD video. A protocol is a combination of connectors and cabling, and the data that goes over them. These connectors and cabling can transmit one or more bit streams of video data, depending on the protocol.
The professional format for HD video is high definition serial digital interface (HD-SDI), also known as SMPTE292M. HD-SDI uses the same BNC coaxial cabling as standard definition SDI, and supports all of the 1080i/p and 720p HD formats discussed in the article Understanding HD Formats. HD-SDI is typically 10 bits per channel, although most capture cards support real-time conversion of 8 bits per channel for video capture.
HD-SDI can provide great quality with one major limitation capture solutions today produce uncompressed video, which requires large amounts of storage space. Many vendors are working to improve HD-SDI capture solutions for systems running Microsoft® Windows®. For example, Cineform, which is discussed later in this article, has developed a lossless compression scheme for HD-SDI video capture that is expected to double the amount of content that you can save to a storage array.
The other cable type used for HD video is the industry-standard 1394 connector, also known as FireWire or i.LINK. This 400 Mbps serial connector is used with all the compressed HD formats. While often associated with the digital video (DV) format, 1394 is a general data standard that can transmit video at many times the data rates of the DV format. Today Windows supports DV25. Windows XP Service Pack 2, which Microsoft is planning to release in the near future, is also expected to include support for DV50.
1394 supports the following video capture standards:
HDV. The most popular 1394 video capture standard is the HDV format, which is an MPEG-2 transport stream on conventional DV tape. Because MPEG-2 uses interframe compression, HDV can provide reasonable video quality at much lower bit rates than uncompressed professional formats. JVC manufactured the first HDV cameras, which provide HD video capture for approximately $4000, U.S. currency. Other major camera manufacturers are also expected to offer HDV cameras in 2004.
While today's HDV equipment, like the JVC HDV cameras, supports either 480 60p or 720 30p, HDV itself supports 1080 formats and other standard frame rates. The major drawback of HDV is the difficulty of frame-accurate editing for interframe compressed video. JVC provides a limited editing application with their cameras. However, using video editing tools such as Adobe Premiere Pro requires transcoding to an intraframe-only compression format like that provided by CineForm, which is described later in this article.
DV100 and HDCAM. Today, DV100 (DVCPRO-HD) and HDCAM formats are primarily used with HD-SDI. However, since both formats are compressed on tape, it would be much more efficient for editing tools to capture the compressed bit stream rather than converting it to uncompressed video by using HD-SDI.
Some DV100 and HDCAM editing solutions have been discussed, but no product is currently available that provides support for standard editing tools at a reasonable price point. This type of solution would be valuable to users who want the improved storage capacity and performance provided by compressed formats. Microsoft has announced that Windows XP Service Pack 2 is expected to include support for DV100.
Component Analog HD
Professional HD production is almost always done in digital form. Component analog HD does exist, but it's used almost exclusively to drive consumer displays, and even this use is being rapidly replaced by the consumer digital visual interface (DVI) standard. Commercial capture cards do not support component analog HD, but HD-SDI converters are available. The AJA HD10A converter is often recommended. It even supports the legacy 1035i analog format for input.
While some video editing systems are capable of capturing live video into the Microsoft Windows Media® Format, for the most part, these tools process captured video in an intermediate format and then encode it. This workflow enables you to use the superior 2-pass encoding method, which analyzes the captured video during the first pass through the encoding tool and compresses the video during the second pass.
Most video editing systems use either AVI or MOV file formats. The format you will use depends on the formats supported by your encoding tool. For example, Microsoft Windows Media Encoder 9 Series does not support MOV source files, although other encoding tools such as Canopus ProCoder and Discreet cleaner XL do support this format.
In 2004, a number of new products are available for authoring HD video on Windows. The following paragraphs describe two lower-cost, high-quality video editing solutions that use Adobe Premiere Pro, an HD nonlinear editing (NLE) tool that can export directly to Windows Media 9 Series.
BOXX Workstation with a Bluefish Capture Card
BOXX delivered the first uncompressed 1080i/p video editing workstation for Adobe Premiere Pro. The BOXX workstation combines a preconfigured computer, a Bluefish 4:4:4 capture card, custom drivers, and RAID storage. This workstation currently uses dual 3.06 GHz Intel Xeon processors. The storage array is a 10-drive RAID3 of 250 GB Parallel ATA drives, providing approximately 2 TB of online storage.
The bandwidth required for uncompressed HD video is too great to allow real-time effects, so some rendering will occur for anything beyond cuts-only editing.
Figure 1. Sample capture on a BOXX workstation
While HD video is often thought of a high-cost, professional solution, consumer HD video solutions based on the HDV format are now available. Like the DV format for standard definition video, the HDV format provides "good enough" quality using a FireWire-based workflow at a reasonable price point. As described previously in this article, the HDV format uses MPEG-2 transport streams and interframe compression. While Adobe Premiere Pro and Adobe After Effects can import MPEG-2 source files with the correct DirectShow filter installed, MPEG-2 does not provide the performance necessary for real-time effects and frame-accurate editing.
Cineform offers the following products for converting HDV content into AVI files with visually lossless compression:
Connect HD. Connect HD, also known as HDLink, is HDV capture and transcoding software that you can run as a stand-alone application or integrate with other HD video editing applications. Connect HD does not provide real-time effects; it provides transcoding and is ideal for use with non-real-time applications like Adobe After Effects.
The Connect HD workflow is straightforward. Connect HD captures the MPEG-2 stream from tape and transcodes it to the CineForm visually lossless 4:2:2 codec in an AVI file. The conversion starts while the application is capturing the video. On a computer with a fast processor, the AVI file is available soon after the application completes the capture process. The AVI file is available immediately on a professional workstation that performs real-time conversion. You can then use this AVI file in any Windows-based application that reads AVI files, such as Windows Media Player or Adobe After Effects.
Aspect HD and HD RT effects and transitions. For Adobe Premiere Pro users, CineForm also offers Aspect HD. This software enables real-time editing of multistream HD video content in both 720 and 1080 formats. It includes all of the Connect HD features, as well as providing a new rendering pipeline that supports a robust set of real-time effects and transitions from within Adobe Premiere Pro. While the real-time performance is only in the CineForm-provided effects and transitions (other codecs work, but require rendering), a very full set is provided.
Typically, HDV, like DV, is a video capture format, and the final delivery of the HD video will be in a different format. However, the CineForm tools also install presets for Adobe Media Encoder that can convert the captured video to a HDV/DVHS-compatible MPEG-2 bit stream from the Adobe Premiere Pro and Adobe After Effects timelines. While the video quality produced by this process is quite good, be aware that this process is not lossless.
Prospect HD. Prospect HD is a new flavor of Aspect HD that provides similar rendering and editing performance for HD video. The main difference between Prospect HD and Aspect HD is that Prospect HD provides capture support directly from professional-level capture cards through its HD-SDI interface. The codec used in Prospect HD is also a 10-bit version of the 4:2:2 codec used in previous products. This codec achieves approximately 10:1 compression into a visually lossless format that not only supports the HD RT rendering and editing but also significantly reduces storage costs.
Today, CineForm supports only the 1280x720 resolution for JVC HDV cameras; they also intend to support 1080 when it becomes available.
The following list identifies some manufacturers that produce capture cards that support video editing with Adobe Premiere Pro.
AJA. AJA is a long-time manufacturer of video capture and processing hardware. They've recently introduced the XENA, a Windows-based line of PCI capture cards for SD and HD. The XENA HD supports 8 bits and 10 bits per channel. The XENA has some very useful high-end features, such as advanced dithering mode for 10-bit to 8-bit conversions and on-the-fly conversion between 720 and 1080 formats.
Blackmagic Designs. The Blackmagic Design's DeckLink HD card revolutionized HD pricing with a $1995 (U.S. currency) uncompressed HD card for the Macintosh, and it's now available for Windows. The DeckLink HD is a PCI-X card, so it requires a newer, high-end motherboard. It supports the full range of 1080 and 720 formats.
Matrox. In the near future, Matrox is expected to announce a new capture card for Adobe Premiere Pro that will support 10-bit video capture with real-time 10-bit effects. This solution will be especially useful for video editing.
Digital Disc Recorders (DDRs) are a professional-level solution for video capture and editing. While HD models are based on computers running Windows, they provide a degree of integration and simplicity that goes beyond what an NLE tool can provide. DDRs emulate a tape deck, including deck control, and support embedded audio in HD-SDI, which many PCI-card based solutions do not. DDR products typically capture video to an internal format, and then export the video to other formats, including Windows Media Video.
Until recently, DDR solutions were the only option for reasonably priced lossless HD capture for Windows, although this is rapidly changing with the introduction of Adobe Premiere Probased solutions.
The following DDRs support Windows Media Video 9 Series integration:
Accom. Accom DDRs are based on a 2.4 GHz P4 processor running Windows 2000. The Accom DDRs provide a wide variety of features, including good Windows Media Video support. They support video exporting using any .prx file (the profile used by Windows Media Encoder) for configuration. They also provide the ability to start an encoding session during an active capture session, saving some rendering time.
Drastic Technologies. Drastic Technologies VVW Series DDRs are similar to the Accom DDRs. Drastic Technologies is expected to release a free feature update that will include simultaneous capturing and encoding. One advantage of the Drastic Technologies DDRs are their native AVI file format, which means that no transcoding is necessary to import the video source files into Windows Media Encoder or other AVI-based tool for encoding.
While today's solutions capture video to an intermediate file for later encoding, another iteration or two of Moore's law will put us within grasp of real-time compression from a video source to Windows Media Format without needing an intermediate file. This type of live-capture system has obvious advantages storage requirements largely vanish as a concern, and live HD broadcasting becomes possible. However, one drawback to live-capture solutions is that they only support 1-pass encoding.
Digital Rapids is the first company to offer a live-capture solution for Windows Media Video. Other live-capture solutions are expected in the near future.
The following paragraphs describe some common issues that you may experience when you capture HD video.
Converting from 10 Bits to 8 Bits per Channel
Windows Media is an 8-bit Y'CbCr format (where Y represents brightness or luminance, and Cb and Cr represent the color difference signals). Therefore, if you plan to compress your video content, capture that content at 8 bits per channel. If you intend to process the captured video and the processing software supports 10-bit rendering, capture your video at 10 bits per channel. However, 10-bit rendering is not a common feature of video processing tools for Windows.
Keep in mind that video processing tools vary in the quality of their 10-bit to 8-bit conversion. Hardware that supports 10-bit to 8-bit dithering can provide superior results when you capture video directly to 8-bit, rather than capturing to 10-bit and then converting to 8-bit by using an NLE or encoding tool.
4:2:2 vs. 4:2:0
HD-SDI uses a 4:2:2 sampling rate, whereas Windows Media Video is typically encoded at 4:2:0. Therefore, capture your video source at 8 bits per channel, unless you intend to do a lot of motion graphics work. When you capture video at 4:2:0 using hardware that supports this sampling rate, storage requirements are slightly reduced. The 4:2:2 to 4:2:0 conversion is very smooth, though, so there is no quality advantage to capturing video at 4:2:0.
Converting Between 1080 and 720 Formats
Some hardware supports automatically converting between 1080 and 720 formats. Down-sampling from 24p projects 1080 format to 720 format. Going from 1080i to 720p can make sense if the output is going to be 720, because the storage requirements are somewhat reduced. Up-sampling from 720 is not useful for encoding, because scaling to more pixels than the video source will result in a poor-quality playback experience.
Hopefully this article has provided you with useful information about video capture formats, tools, and systems. You can use this information to help you find the video-capture solution that best meets your HD video needs.