Digital in the Television and Video Industries

A digital signal can mean different things to Video and CATV engineers, causing much confusion. The most common types of digital video and digital audio are as follows:

Uncompressed digital video and audio

Lossless compression of digital video and audio

Lossy compression of digital video and audio

Complex digital modulations schemes such as 64 QAM, 256 QAM, 16 VSB, 64 QPSK, etc.

SONET, ATM or other telecom base standards.

Serial digital interface or SDI

High definition or HD-SDI

Digital Audio or AES/EBU

The process of digitizing a standard NTSC video signal is straight forward.  The typical bandwidth of a video signal is 4.5 MHz.  Typically a sample rate of four times the video bandwidth is used or about 18 Mega samples per second.  The analog to digital converter or A/D typically have a sampling resolution of 8, 10 or 12 bits. This process generates a serial digital data stream of about 144 Mb/sec to 270 Mbps.  The video signal is typically encoded in a digital format at the video source or in the video camera.  Depending of the digital video format, the analog video will be samples at about 13.5 Mega samples per second and then encoded in one of several standards such as 4:2:2, 4:1:1 or 4:2:0.  While these encoding schemes are not referred to as compression, they omit or remove certain information to reduce the systems bandwidth requirement.  In the encoding schemes above, the three digits refer to the three common components of video.  The first component is luminance (Y) or the light intensity of the video signal.   The second is the color signal of red minus luminance or R-Y.  The third component is the color signal of blue minus luminance or B-Y.  These three components are referred to as YUV.  The numbers 4:2:2 have to do with the fact that twice the bandwidth is used for the Luminance (Y) channel than the two color channels. This technique is a form of compression that will be addressed later in the chapter.  HDTV or high definition video requires a data rate of 1.485 Gbits/second for one uncompressed signal.

The most efficient means of analog video transport utilizes analog to digital conversion.  Once video and audio signals are converted to digital information, many channels can be combined into one high speed data stream using Time-Division Multiplexing or TDM.   The high speed serial digital data stream is then converted to light via a laser or LED.  The block diagram below in Figure 19, shows the building blocks of an 8 channel video fiber optic transmitter.
Figure 19

The receiver unit performs the reverse function as shown in figure 20 The light or optical signal is received by a PIN photo detector. The optical signal is converted back into a serial data stream.  The data stream is de-multiplexed using TDM.  The digital data is then converted back to video and audio via digital to analog D/A converters.

Figure 20

Digital video transmission has many advantages over analog transmission.  Analog fiber optic system requires high linearity optical components that are expensive and require fine tuning and complex calibration procedures.  Once a video or audio signal has been digitized, it can be transported via fiber using readily available digital telecom optical components for both Multimode and Singlemode applications.  A digital system has a higher immunity to noise and superior performance characteristics compared to an analog system.  A digital signal can be regenerated and repeated virtually indefinitely without signal or performance degradation.

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