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This document provides information about creating high-definition Microsoft Windows Media Video files. Topics covered include an overview of the content creation process, a list of acceptable video sources, information about the capturing and encoding processes, and system requirements for encoding and playing a high-definition Windows Media Video file.
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Jennifer Winters Microsoft Corporation November 2004
Applies to: Microsoft® Windows Media® Encoder 9 Series
Contents
IntroductionBy using Microsoft Windows Media Encoder 9 Series, you can create high-definition Windows Media Video files in the high-definition television (HDTV) standard formats, such as 1280 x 720 progressive (720p), 1920 x 1080 interlaced (1080i), or 1920 x 1080 progressive at 23.976 fps (1080Psf). The process of creating a high-definition Windows Media Video file consists of the following steps:Capture the content from a high-definition source. Edit the content (optional). Convert the content to an .avi file. Encode the content.
Note that you can use software, such as Sonic Foundry Vegas or Discreet CleanerXL, to combine the four steps into a single step. For more information about these programs, see the Sonic Foundry Web site or the Discreet Web site.
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Capturing Audio and VideoThe first step in creating a high-definition Windows Media Video file is to capture the content from the audio and video source. This section lists acceptable sources, provides guidelines to follow during the capturing process, and lists hardware you can use for capturing.
Video Capture
Your video source can be:Film (35mm or 16mm) that has been transferred to a high-definition format tape by using a high-definition telecine process, resulting in a D-5, HDCAM, or DVCPRO HD tape. Video that you are capturing in real time by using a high-definition video tape recorder (VTR). Existing high-definition files, such as Cineon format files, Targa (TGA) sequence files, or tagged image file format (TIFF) sequence files. Before these files can be encoded, you must convert them to the Audio Video Interleaved (AVI) format. To do so, you can import the file into a video editing program, edit the video if desired, and then export the content to an .avi file. An existing high-resolution .avi file. A Motion Pictures Experts Group-2 (MPEG-2) transport stream. (Note that this process is not covered in detail in this article.) To source from an MPEG-2 transport stream, you must install an MPEG-2 decoder that is capable of decoding high-definition MPEG-2 streams on the encoding computer. For information about obtaining an MPEG-2 decoder, see the Plug-ins for Windows Media Player page.
You can capture the tape or live video directly from the VTR through a serial digital (uncompressed) connection to a digital disk recorder (DDR) device or a high-definition non-linear editing (NLE) system. The following NLEs and DDRs can be used to translate the data into a format that makes it appear to be an .avi file, therefore making it acceptable as a source file for the encoder:
In addition, note that AJA Video Systems is developing a capture card that can capture audio and high-definition video in real time.
Use the following guidelines during the video capture process:Capture content at the original frame rate to ensure that the audio and video remain synchronized during playback. Typically, movie content has a frame rate of 23.976 frames per second (fps). Capture to a YUY2 (4:2:2) pixel format to avoid color conversion problems during encoding. The Windows Media Video 9 codec uses primarily a 4:2:0 pixel format; however, if you choose to maintain the interlacing in your content (a new feature available with Windows Media Encoder 9 Series), then the codec uses a 4:1:1 pixel format internally. By capturing to a YUY2 pixel format, you can encode either progressive or interlaced content, because 4:2:2 is a superset of both the 4:2:0 and 4:1:1 data structures. It is important to note that if you capture to an .avi file with a 4:2:0 pixel format (for example I420, YV12, or IYUV), you will not be able to maintain the interlacing in the video because the 4:2:0 pixel formats do not carry chroma information on the alternate lines that contain the data for the second field. If you are working with progressive-scan source video or if you are working with interlaced source video that you plan to deinterlace, then a 4:2:0 pixel format is appropriate because the output file is smaller than an .avi file with a 4:2:2 pixel format. If your VTR has a hardware-based resizing option, you can use this to do real-time resizing and capture the video at the size you want. If a hardware-based resizing option is not available, capture at the source's original resolution and resize the video as described in the Editing Before Encoding section of this article.
Audio Capture
The audio can either be on the same tape as the video or on a separate medium, such as a digital audio tape (DAT) in either a DA88 or DA98 format. You can capture the audio in real time if your computer has a multichannel audio capture card, such as M-Audio Delta 1010 or Echo Layla24, and a multichannel audio editing program, such as Digidesign ProTools, Syntrillium Cool Edit Pro, or Steinberg Nuendo. If the audio track is on a DAT, it is very important that you synchronize the audio and video carefully. You can use the audio editing program for this purpose.
The encoder supports multichannel audio that contains 2, 5.1, or 7.1 channels. For detailed information about creating multichannel audio, see the articles Producing Multichannel Audio with Windows Media 9 Series and Creating 7.1 Audio.
The encoder can accept separate audio and video files. (To source from separate audio and video files, on the Sources tab of the encoder, click Both device and file.) If your audio consists of multiple (2, 6, or 8) mono channel waveform files, you can use the Mono to Multichannel Wave Combiner utility to merge the waveform files and the .avi file containing the video track into a single .avi file. You can then use that file as the source for the encoder.
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Editing Before EncodingEditing your captured audio and video is an optional step in the process of creating a high-definition Windows Media Video file. For example, as part of the editing process, you can correct the color of the video file, add effects, synchronize the audio and video tracks, or trim excess video from the beginning or end of the file.
Examples of video editing programs include Adobe Premiere, Adobe After Effects, and Avid|DS HD. Note that After Effects uses the RGB color format, while the Windows Media Video 9 codec uses the YUV format. Therefore, using After Effects results in extra color-space conversions, which requires extra time to process and may result in some loss in quality.
Depending on the resolution of the target display and the capability of the playback device, you may want to resize the video. It is important to use a program that features a high-quality resizer. You can also use the high-quality resizer option in the encoder. To do so, on the Tools menu, click Options, and then click the Performance tab. Next, set the When encoding to a file option to Better quality. Keep in mind that using the encoder to resize the video content greatly increases the encoding time.
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Encoding TechniquesThe final step in the process of creating a high-definition Windows Media Video file is encoding the content. The encoder includes profiles for creating high-definition 720p content. You can use one of the default profiles, or you can customize the encoding parameters and create your own high-definition profile.
Ensure that you are satisfied with your encoding settings by encoding a short section of the most complex portion of the content, viewing the encoded content, and then adjusting the settings as necessary before encoding the entire file. While you are encoding the test content, monitor statistics such as the encode time, CPU load, frame rate, and number of dropped frames, if any. (You should be able to encode an entire movie with no dropped frames.) After you have encoded enough of the test content, play the content to see whether the quality meets your needs. If necessary, adjust the encoding settings and encode the test content again until you achieve the balance of quality and file size (or bandwidth requirement) that you need. Use the following guidelines to help you adjust encoding settings:Match the format of the source video. High-definition video sources may vary by aspect ratio, pixel resolution, frame rate, and scanning method (progressive or interlaced). You should typically preserve the aspect ratio of the source video unless the content is anamorphic or you want to crop existing letterbox bars. If you do need to change the aspect ratio, be sure to maintain the pixel aspect ratio. In addition, the frame rate of the encoded content should match the frame rate of the source video, unless you need to apply the inverse telecine filter to return a previously telecined source to a frame rate of 24 fps. Match the format of the source audio. Audio sources may vary by sampling rate, bit depth, and number of channels. Typically, the sampling rate of the encoded content should match the sampling rate of the source audio. If you do need to use a different sampling rate, try to use an evenly divisible sampling rate. For example, downsampling from 96 kilohertz (KHz) to 48 KHz results in better quality than downsampling from 96 KHz to 44.1 KHz. Changing bit depth usually does not negatively impact the perceived audio quality of the encoded content. If your source audio has a bit depth of 20 bits, you will notice better results if you use 24-bit encoding rather than 16-bit encoding. However, if the playback computer only has a 16-bit sound card, the audio is requantized to 16 bits during playback. The source audio may have 2, 6, or 8 channels (which equate to stereo, 5.1, or 7.1 audio). It is recommended that you match the number of channels in your encoding session. If you encode 5.1 audio, the audio is automatically folded down to two channels when played on stereo speakers. Use a bit rate that is large enough to ensure smooth playback. For a 720p file, use bit rates of 3 to 7 megabits per second (Mbps). For a 1080i file, use bit rates of 6 to 9 Mbps. In order to determine the appropriate bit rate, you must balance quality and file size (or bandwidth requirements) to best suit your needs. Preserve nonsquare pixels. If your video source has nonsquare pixels, you should preserve the pixel aspect ratio. Typically, high-definition source video has square pixels, but this is not always the case. If the ratio of the desired playback window (for example, 4:3, or a 1.333 ratio) does not match the ratio of the x:y resolution of the video source (for example, 1280:720, or a 1.777 ratio), then the source has nonsquare pixels. (To preserve the proper ratio, do the following in the encoder: On the Compression tab of the Properties panel, click Edit, select the Allow nonsquare pixel output check box, and then click OK. Next, on the Video Size tab, click the desired pixel aspect ratio.) Windows Media Player 9 Series or later automatically scales the video appropriately during playback. Optimize video for the playback device. If your source video is interlaced, you can use the encoder to either maintain the interlacing or deinterlace it during encoding. (You can also use a video editing program to deinterlace video before you create the source .avi file for the encoder.) You should deinterlace the video if the target playback device is a progressive-scan device, such as a computer. You should maintain interlacing if the target playback device is an interlaced device, such as a television. If you deinterlace the video, Windows Media Player 9 Series or later can detect whether the computer's video adapter supports hardware deinterlacing during playback. If it does, the codec passes all of the information to the adapter for deinterlacing. If the adapter does not support hardware deinterlacing, the codec handles the deinterlacing process. Select an encoding mode that is appropriate for your distribution method. You can encode content at either a constant bit rate (CBR) or a variable bit rate (VBR). Use one-pass CBR when you plan to broadcast the Windows Media Video file live. Use two-pass CBR when you are encoding to a file or when you want to set up an on-demand streaming scenario. Use quality-based VBR when your highest priority is consistent content quality. Use bit rate-based VBR when you want to create files that can be downloaded before they are played, or when you want to control the size of the output file. Use peak bit rate-based VBR when you want to create content that will be played back on a device that has a constrained reading speed, such as a CD or DVD player. Adjust the key frame setting to balance video quality and file size. Increasing the distance between key frames increases the video quality for a given bit rate. When the quality is fixed (as it is in quality-based VBR encoding), increasing the key frame distance decreases the overall file size. If you use a longer key frame distance, remember that the distance affects the user's ability to seek in the file and increases the amount of time a user may need to wait for video in a multicast scenario. Use a large buffer. Typically, increasing the buffer improves overall quality. However, increasing the buffer also increases the delay between the time when the user requests the content and the time when the content starts playing. For download-and-play scenarios, increasing the buffer size does not noticeably increase the delay.
Encoding high-definition video takes a long time. For example, when you are encoding content by using one-pass CBR encoding, the encoding process may take 15 to 50 times the actual duration of the content; a two-pass encoding method takes twice that amount of time. If it is important to reduce the encoding time, you can adjust the encoder performance setting to achieve better encoding performance, as opposed to better file quality. To adjust the performance setting, on the Tools menu, click Options, and then click the Performance tab. Next, set the When encoding to a file option to Better performance. Note that changing this setting prevents the encoder from using the high-quality resizer. Therefore, you should not change the performance setting if you are resizing the content while encoding.
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System RequirementsThis section describes system requirements for both the encoding computer and the computer on which the content is played back.
Encoding ComputerTo achieve optimal performance for your encoding computer, use the following guidelines:Use a computer with fast or multiple processors. Because high-quality encoding requires more computing power, use a computer with the fastest dual processor configuration available (currently, a 3 gigahertz Intel Pentium 4 processor or an AMD Athlon XP 3000 processor). The encoder can use up to two processors to encode audio content, and up to four processors to distribute the video encoding load efficiently. Have sufficient storage on the hard disk. For example, a full-length movie requires at least 300 gigabytes (GB) of disk space. Use Microsoft Windows XP Professional. Microsoft Windows® XP Professional takes full advantage of a multiprocessor computer by automatically generating separate threads that enable processes to run different parts of programs on different processors simultaneously. This parsing of the work enables you to perform high-end encoding tasks without maximizing your CPU usage. You should install the latest upgrades and service pack for Windows XP and the most recent version of the Microsoft DirectX® End-User Runtime from the DirectX page. Use high-performance hard disks. For example, use multiple SCSI Ultra-160 hard disks with 15,000 rpm, stripped in a RAID 0 configuration. You need to use a high-performance hard disk subsystem to enable real-time high-definition video capture without dropping frames. Defragment the hard disks regularly. Install the encoder on a dedicated computer. After you install Windows Media Encoder on a computer, reserve that computer for encoding only. Quit all other programs before encoding, especially programs that access the hard disk. Turn off network and file sharing. Turn off video preview and postview. The Input and Output areas in the main encoder window show you what content looks like before and after it is encoded. Turn off these displays to improve encoding performance. Watch for direct memory access (DMA) buffer conflicts between the capture card and the SCSI card. These buffer conflicts may result in dropped frames. This type of conflict is less likely to occur in current computer configurations than in previous configurations. If conflicts do occur, one solution is to use a dual PCI bus motherboard configuration, in which the capture card and the SCSI card are on different buses.
Playback ComputerPlaying high-definition Windows Media Video files is a CPU-intensive process, due to the data rates involved and the large number of pixels that are rendered during playback. For this reason, it is important that you use a high-end computer for playback, such as a computer with a dual Athlon MP processor or a hyper-threaded Pentium 4 processor and a fast video adapter with current drivers.
Note that most of the next-generation video adapters will be compliant with DirectX Video Acceleration (VA). DirectX VA is an application programming interface (API) set that enables hardware-accelerated routines in decoding. Video adapters that are DirectX VA-compliant will be able to handle much of the high-intensity decoding, which will reduce the processor requirements for a computer to play high-definition video.
For optimal audio playback, the computer should have a multichannel sound card installed. (However, if the computer does not have a multichannel sound card, the audio plays in stereo.) For more information about setting up your computer to play multichannel audio, see the 5.1 Audio page.
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For More InformationThis section lists other resources.
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