Digital Video explained

Digital Video (DV) covers a large domain including everything related to video and computer. The most common use is to make an MPEG video clip, from a camcorder or TV set, and send it over the Internet or burn it onto a CD-ROM.

General scheme
The video source
Capturing a video clip
Editing a video
Processing a video
Encoding a movie
Storing a movie
Watching a movie
Recommendations

1. General scheme

Figure 1 shows the Digital Video processing chain. The capture process digitizes the video source to get a video file into the PC. The capture process can be an analog to digital conversion through a frame grabber or a digital file transfer.

This video file (AVI or MPG) will be edited to cut unwanted scenes, add titles and transitions and mix sound. The timeline (final movie clip) will then be processed to smooth the picture or clean up spatial or temporal noise.

Then the movie will be encoded to produce a file compliant with Video CD standard, the DVD standard or with Internet available bandwidth (Web streaming) so it can be stored in a usable format. The Video CD or DVD will be played on a TV set using a DVD player and the Internet video stream will be displayed with the help of a browser plug-in.

2. The video source

The video source will be captured from a camcorder (generally Hi8, DV8 or miniDV), a VCR, a TV set, a Satellite Decoder or a DVD-Video.

The common scenario is an analog video source which is digitized and converted into a file, but in some cases (DVD-Video, DV camcorder and DVB satellite decoder) we just have to transfer a video file to the PC hard drive.

The video source features will be defined by the TV standard it has been created for, PAL in Europe and NTSC in the USA, Japan and most other countries. The table below shows some specifications for Digital Video:

The frame rate is the number of pictures displayed per second. Actually, video is interlaced, meaning that odd lines are displayed then even lines. These 1/2 images are called fields, field rate is twice the frame rate.

Digital resolution is the number of lines which carry picture information (Luminance and Chrominance) and total lines include some lines used for synchronization and other hardware controls.

3. Capturing a video clip

Figure 2 shows the capture process which produces a video file from the video source.

The format of this video file will depend on the video source and further usage:

The most common format is AVI but the DV format will become more and more popular. MPG could become one of the major formats in a couple of years. Web streaming formats are usually based on MPEG-4 but there're still proprietary.

The video source is connected with Composite or S-Video (Y/C) plugs to a frame grabber which converts the analog signal into a video file, usually an AVI file but sometimes directly into an MPG file.

Capture cards are usually bundled with appropriate drivers, capture utilities and video editors which are recommended to for them, but there are two popular alternatives for capture: VirtualDUB and AVI_IO. VirtualDUB is particularly good at editing AVI files and many video filters are available off the Web. AVI_IO is generally used for long capture up to 400GB using multiple AVI files and can also capture a whole DV video tape (AVI_IO supports OHCI compliant boards or any other DV board as long as there is a Video for Windows capture driver installed for it, or a working WDM DirectShow wrapper).

A codec must be chosen for the capture and the codec code (FOURCC) of the codec is tagged in the AVI file so other applications will read the FOURCC and load the codec to read the video stream. Common software codecs for video compression are:

There are also MJPEG hardware codecs (Pinnacle DC10, DC30, Matrox) which provide perfect synchronization of audio and video (through hardware) and usually have better image quality than cheap-TV cards.

In early 2001, MPEG-2 real-time capture cards (WinTV-PVR) and software (PowerVCR II) are shipping at consumer prices. Real-time software encoding is as good as hardware for MPEG-1 (VCD). For MPEG-2 (SVCD), it is difficult to achieve hardware quality in software but one can capture I-Frame-Only-MPEG which is better than MJPEG at same bit rate. Thus, in a near future on 1.5GHz-CPU-based PC and with a bit more maturity, MPEG-2 capture will become very popular.

The video is compressed in real time onto the DV camcorder while recording. The video compression ratio is 5:1, encoding is I-MPEG (or I-Frame-Only-MPEG) with an adaptive interfield compression and audio format is 12 bits 32kHz or 16 bits stereo at 44.1kHz or 48kHz.

Thus the capture process is to transfer the data stream from the camcorder to the PC hard drive. This is done through an interface called IEEE1394 (or Firewire in the Apple world and iLink by Sony), this interface speed is 100, 200 and 400Mbps.

The data stream must be converted to a Type-2 AVI file to be compliant with editing software.

(...)