AfterDawn: Glossary

Capture

Capture refers to the process of either sampling an analog signal to a digiatal format or saving a digital stream being broadcast. Capture generally refers to muxed video and audio sources, but video-only, or more commonly audio-only, sources can be captured as well. In fact, the process of capturing both the video and audio for a particular source is actually two individual operations behind the scenes.

Analog Video Capture
Capturing analog video such as NTSC or PAL television broacasts or videotape requires a special capture device capable of sampling the video to digitize it. The video is then encoded to a digital format by either hardware or software. The digitized video is then saved to a file. The hardware may be in the form of a capture card that plugs into a computer's expansion bus or an external box that connects to a computer via FireWire or USB. In some cases it may even be a standalone DVR or DVD recorder that doesn't require a computer at all. In addition to video, there is generally audio that must be captured and muxed with the video to create the final file.

ITU-R BT.601
Analog Video Capture is primarily based on the ITU-R BT.601 (Rec.601) standard for digitizing Standard Definition (SD) video, athough few consumer electronics devices, including capture cards, respect all the specifications in that standard. In order to understand the specifics of analog video capture it's best to include it in your reading.

Analog Audio Capture
In addition to the audio portion of video capturing, analog audio from sources such as records or analog tape. When performed as part of a Video Capture operation, audio capture may be handled by the video capture device or you may need to use your computer's sound card. For audio-only captures the sound card is also used. If you're computer has onboard sound (sound built in to the motherboard) you may need to add a separate card to get high quality audio captures.

Analog Tuners
If the analog video originates as a television broadcast you also need a tuner of some kind to convert it into a video signal that your capture hardware will understand. This may be a tuner built into the capture device itself, or an external tuner in a devices such as a VCR, with Composite Video or S-Video used to provide analog video to the capture device and RCA cables used for audio. SCART connections may be used for both video and audio.


Stream Capture
Rather than decoding to an analog signal just to capture back to digital, you can save the original stream, resulting in none of the errors inherent in both decoding and capture. There are two basic types of Stream Capture. The most common is Digital TV (DTV) capture, in which the streams for a single Program are Demuxed from the MPEG-2 Transport Stream, used to provide multiple channels in a single signal, and Muxed back into a file on a computer or Set-Top Box. In some cases, particularly for encrypted satellite TV signals, it may be necessary to use equipment provided by the television service provider (such as a Set-Top DVR) to capture streams this way. Find more information on Capturing Digital TV under Video Capture.

Stream capture may also refer to saving Streaming video to your hard drive. The specifics for this type of Stream Capture vary depending on the source of the stream and format of video and audio. Many streams found on the internet use some form of DRM, which tends to make playback of a captured stream complicated.


Related Guides
Digital Video Fundamentals - Resolution and Aspect Ratio
Digital Video Fundamentals - Frames & Framerates
Digital Video Fundamentals - Color Formats






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