Digital Video

Digital broadcasting is the sending of a digital signal through a common channel to a group of users that may be capable of decoding some or all of the broadcast information.

Digital Television (DTV) is a method of transferring video images and their audio components through digital transmission. There are several formats used for DTV including high quality digital MPEG and 28.8 video.

Digital video is the sending of a sequence of picture signals (frames) that are represented by binary data (bits) that describe a finite set of color and luminance levels. Sending a digital video picture involves the conversion of a scanned image to digital information that is transferred to a digital video receiver. The digital information contains characteristics of the video signal and the position of the image (bit location) that will be displayed. Digital television continues to send information in the form of frames and pixels. The major difference is the frames and pixels are represented by digital information instead of a continuously varying analog signal.

The first digital television broadcast license for the United States was issued to a Hawaiian television station in September 1997. Digital television sends the video signal in digital modulated form. Ironically, many television signals have been captured and stored in digital form for over 10 years. To transmit these digital video signals, they must first be converted to standard analog television (NTSC or PAL) to be transmitted through analog transmission systems and to reach analog televisions.

When digital transmission is used, most digital video systems use some form of data compression. Data compression involves the characterization of a single picture into its components. For example, if the picture was a view of the blue sky, this could be characterized by a small number of data bits that indicate the color (blue) and the starting corner and ending corner. This may require under 10 bytes of information. When this digital information is received, it will create a blue box that may contain over 7,200 pixels. With a color picture, this would have required several thousand bytes of information for only 1 picture.

In addition to the data compression used on one picture (one frame), digital compression allows the comparison between frames. This allows the repeating of sections of a previous frame. For example, a single frame may be a picture of city with many buildings. This is a very complex picture and data compression will not be able to be as efficient as the blue sky example above. However, the next frame will be another picture of the city with only a few changes. The data compression can send only the data that has changed between frames.

Digital television broadcasting that uses video compression technology allows for “multicasting” (simultaneously sending) several “standard definition” television channels (normally up to five channels) in the same bandwidth as a standard analog television channel. Unfortunately, high definition digital television channels require a much higher data transmission rate and it is likely that only a single HDTV channel can be sent on a digital television channel.

Figure 1 demonstrates the operation of the basic digital video compression system. Each video frame is digitized and then sent for digital compression. The digital compression process creates a sequence frames (images) that start with a key frame. The key frame is digitized and used as reference points for the compression process. Between the key frames, only the differences in images are transmitted. This dramatically reduces the data transmission rate to represent a digital video signal as an uncompressed digital video signal requires over 50 Mbps compared to less than 4 Mbps for a typical digital video disk (DVD) digital video signal.

Figure 1: Digital Video

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