AfterDawn: Glossary

Video Capture

Video Capture is either the process of digitizing an analog video signal or simply selecting and saving a digital video signal from a digital broadcast. In both cases the goal is to copy the video to a computer hard drive for storage, editing, encoding, burning, and sometimes playback. In addition to video, there's normally audio that needs to be captured as well. For digital (stream) captures this is essentially the same operation as capturing the video. For analog captures this may require additional hardware.

Analog Capture
The traditional video capture methods were developed for working with analog video streams, including OTA (Over-the-Air) and Cable Television, as well as analog Videotape. While the same procedures can typically be used to capture digital video from a converter box, it won't result in the same quality as capturing the actual digital stream. Whenever capturing an original digital stream or the same stream converted to analog video you should stick with the original.

Macrovision
When capturing broadcast sources there shouldn't be any concern about copy protection. However, if you're attempting to capture commercially produced videotapes you'll likely run into Macrovision copy protection. Some capture devices will be stopped by Macrovision, while others will ignore it.

Capture Devices
Analog video (and audio) capture requires a variety of components. To begin with the computer must be able to receive an incoming video signal. This may mean a tuner, or simply a device with S-Video/Composite video inputs, in addition to whatever audio inputs are necessary to receive the source. Capture devices may be built into a PCI add-in card. For internal cards it's become common to include hardware encoding, but this isn't universal. PCI capture cards may require software to do the actual encoding for them during capture. It's become increasingly common to see external capture devices that use either a FireWire or USB interface. External capture devices always use hardware encoding to convert the video and audio streams before transmitting to the computer.

Capturing Audio
For capture cards that rely on software for video encoding it may also be necessary to use a separate sound card to capture the audio. The capture card may have outputs that plug directly into sound card inputs. The capture software will need to control both the video capture card and the sound card to create a file with synchronized video and audio. External capture boxes always Encode the audio, as well as Muxing it with the encoded video.

Capture Software
Most capture hardware comes with at least a simple capture program. If the hardware also handles the encoding this software may be necessary to adjust internal quality settings. You can also use many capture cards with HTPC software Like SageTV, BeyondTV, MythTV, and Windows MCE. While some software only supports specific hardware, other programs support a large range of VfW (Video for Windows) or DirectShow (DirectX) devices. VfW and DirectShow are frameworks used to encode, Decode, and process video and audio. If you're looking for capture software and have a capture card with a DirectX (DirectShow) driver you should try VirtualVCR. If your card uses a VfW driver you're better off starting with VirtualDub. Both are free.

Capture Codecs
All analog video must be encoded to some digital format. Depending on your capture hardware, and possibly software, you'll be using a hardware or software encoder to create a muxed video and audio file. For hardware encoding MPEG-2 is the most common format. It's well suited to realtime encoding, albeit at higher bitrates than you might get in two or more passes, and encoding hardware is available with a relatively low cost for relatively high quality.

Software Encoding
Both VirtualDub and VirtualVCR support a number of high quality codecs. Generally either lossless codecs like Huffyuv are used or relatively lossless formats like MJPEG or DV. There are also software MPEG-2 encoders used with some capture software. It's generally best to avoid high Compression formats like MPEG-2 or MPEG-4 if capturing with software encoding.

Analog Capture of Digital Sources
In some cases there's no option for capturing a digital signal directly. In these cases you may have settle for using an analog output signal to record DTV. For example, encrypted satellite or cable transmissions can't currently be captured digitally.







Capturing Digital TV
Most DTV is broadcast as MPEG-2 Transport Streams. This allows multiple channels to be broadcast in a single signal, with each Channel as a single program in a Multiple Program Transport Stream. Like analog capture, you must have a card or other device capable of recieving the broadcast, as well as capture software that can either save the entire transport stream from a single station (Single Program Transport Stream) or isolate a single program (channel) from a Multiple Program Transport Stream and save it as a separate file. The resulting capture will be another TS (Transport Stream) file, usually with MPEG-2 video and AC-3 (Dolby Digital) audio. Since transport streams aren't compatible with all software, it's often necessary to either Demux to Elementary Streams or remux into a standard MPEG-2 Program Stream Container.

Encryption and DRM
Unlike analog television, simply having access to a digital broadcast doesn't automatically mean a computer with a DTV tuner (receiver) can decode it. Encryption can be used to protect subscription services from unauthorized access. Although there's been a number of efforts to force hardware manufacturers to implement various DRM schemes, which would allow broadcasters to disable transfer to other devices, or even all playback if the broadcaster decided to. The most famous example of this is the so-called "Broadcast Flag" mandated by the FCC in 2003, but later struck down for being outside the scope of FCC authority by U.S. Federal Courts. Various attempts have been made since then to legislate Broadcast Flag compliance by adding provisions to other legislation, but to date they've been unsuccessful. This is primarily due to the efforts of groups like the EFF, who have let the public know what legislation is being proposed.

Broadcast Flag Timeline

Capturing
Rather than converting an analog signal to digital, capturing DTV simply involves isolating the appropriate program in the broadcast TS and saving the associated streams to a TS file on the computer's hard drive. This can usually be accomplished using software included with your capture card or third party PVR or HTPC software. For capturing DVB (Terrestrial, Satellite, or Cable) DTV broadcasts you'll need a card designed for the appropriate standard, DVB-T, DVB-S, or DVB-C. DVB standards are used for DTV in most parts of the world, although only DVB-S is found in the U.S., and isn't the only satellite transmission standard in use.

DTV In The U.S.
Digital television in the U.S. is somewhat more complicated because no single unified set of transmission standards exists. Terrestrial broadcasts are in ATSC format, digital cable signals are in QAM format, with most also being encrypted, and satellite broadcasts may be in DVB-S format, or use a competing format such as DSS. With the introduction of HDTV DVB-S is finally being adopted across the industry. Unencrypted DVB-S can be captured with most standard DVB-S capture cards, although systems such as Dish Network add encryption that these cards can't handle. ATSC and QAM require different

CableCard
Although most digital cable television channels in the U.S. are encrypted and require a special receiver which can be programmed by the cable provider to be decrypted, relatively new technology called CableCARD partially solves this problem by allowing other devices to access the digital streams in the encrypted QAM signal. However, at this time only HDTVs and set-top boxes like TiVo's are capable of taking advantage of it. Withoiut CableCARD support most cable channels can't be captured digitally.

Capturing Via Set-Top Box
A number of set-top boxes are capable of capturing video. These boxes, including both retail DVRs and those distributed by cable or satellite television providers may allow you to transfer video across a network or firewire connection, or even burn captured video to DVD or other media. Some devices even include CableCARD support, enabling you to get lossless (stream) captures from them.






Watching Captured Video
Depending on how you captured your video you may need to do some additional processing to get it ready to be played on a DVD or High Definition DVD player, or even a computer.

Playback Guides
How to play TS files

How to play .TS content on Xbox 360

How to Play MPG and MPEG files

How to play MKV files

How to play AVI

Processing Analog Captures
Analog captures may be captured in a low compression format like HuffYUV or a relatively high compression format like MPEG-2. Extremely efficient formats like the various MPEG-4 profiles aren't generally used for realtime encoding, and therefore aren't well suited to capturing. With many modern capture solutions utilizing hardware MPEG-2 encoding you can often just author to a simple DVD with no menus and play it in any standalone DVD player. If you don't have DVD-ready MPEG-2 files (or you're not satisified with your capture card's compression) you can re-encode, either with MPEG-2 again or with a more effiicient Codec like XviD or x264.

Processing Digital Captures
Unless you're going to watch your capture on a desktop or laptop computer (or HTPC) you'll almost certainly want to move the streams into a different container. Transport streams are important for transmission, but not used in most consumer formats, and often not supported by tools you may want to use. Depending on what you'll be doing with your video you may want to demux to elementary streams or simply remux into a MPEG-2 PS (Program Stream), Matroska (MKV), or other container. Since HDTV captures can be very large it's common to further compress them into a format like AVC.

Editing Captures
Processing TS captures with free tools

Creating MKV files with mkvtoolnix

VideoReDo Plus Installation and Configuration

VideoReDo Plus - Loading, Analyzing, and Repairing

VideoReDoPlus - Removing Commercials

Digital Video Fundamentals - IVTC

Authoring
Depending on the format and properties of the video you may want to author it to DVD, HD DVD, or even Blu-ray's AVCHD format. While there are some tools that will author the standard BD-MV format used in commercial releases, due to the wording of the Blu-ray license for player manufacturers BD-MV discs without encryption aren't supported by all players, and aren't expected to be supported at all in the future. For this reason it's not recommended to author to BD-MV.

Authoring Captures
Convert MKV to DVD with freeware

Basic TMPGEnc DVD Author Guide

MKV
Perhaps the most common container for HD captures on a computer is MKV (Matroska). MKV files support a wide range of video formats, including MPEG-2, MPEG-4 Part 2 (DivX, XviD, etc,...), and MPEG-4 AVC/H.264. A variety of audio streams can also be used, including the standard AC-3 (Dolby Digital) used in most DTV broadcasts.

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