Digital Data in Analog Signals
Total Page：16
File Type：pdf, Size：1020Kb
Digital Data In Analog Signals WalterS. Ciciora, Ph.D. Technology Consultant Southport Connecticut [email protected] Abstract designers of the television system, the National Television Systems Committee The NTSC television system was (NTSC) was to facilitate affordable consumer designed primarily to make television television receivers while maximizing the receivers affordable in the era of number of channels possible. This, of course, vacuum tube circuits. In the age of is a difficult trade off. A consequence of this digital and analog integrated circuits, difficult challenge is that there is spectrum much more aggressive signal capacity to harvest now that more complex processing is possible. This allows electronics can be brought to bear. some of the signal inefficiencies accepted when NTSC was created to The Data Application be overcome. It is now possible to add significant amounts of digital data into Dau( transmission was the initial an analog signal in a compatible application for ancillary signals because data manner. The FCC has authorized the rates which fit into the under utilized spectrum use of several of these systems and has spaces could produce useful results while not encouraged the development of others. placing unrealistic demands on circuit design. Some of these systems simply use One of the first ancillary signal the Vertical Blanking Interval, VB!. applications was Closed Captioning for the Others add additional carriers. At hearing impaired. The conservative design least one system takes over portions of that characterized consumer electronics was the video screen and adds digital data extended to the captioning system. A very in its place. low data rate was employed. Fortunately, that This paper will review the is all that is required for this application. methods proposed for compatible data Teletext is a more aggressive data insertion into NTSC signals and application which puts text, graphics, and explore possible applications of this even photographs on the television screen. approach. Additionally, Teletext can "download" software to computers either internal or INTRODUCTION external to the television receiver. While Teletext has failed in the U.S., it has enjoyed Ancillary signals are additional signals considerably greater reception in Europe. which may or may not have anything to do The successors to Teletext are the with the video program content. Ancillary systems currently becoming available for signals are a "bonus" which allows additional access to the Internet using television information and services to be provided. receivers or set top boxes. In other related These signals were initially impractical due to applications, the signal directly feeds a the expense of early receiving circuits. computer. Because of the massive computing Ancillary signals are possible because the power available, much more interesting spectrum utilization of the television signal is images can be delivered. Personal computers relatively inefficient. The goal of the 1997 NCTA Technical Papers -275- are now available with one or more tuners the HDTV transrmss10n rate is around 19 built in. Mbps (in 6 MHz), six 3.0 Mbps movies can be carried in the same spectrum. At 1.5 Mbps, The Video Application double that number is possible. Since cable has a more controlled spectrum, it can Recent developments in the television approximately further double these numbers industries have focused upon the transmission leading to perhaps twenty four movies in 6 of High Definition Television (HDTV) which MHz. This is even more practical in systems requires a substantial increase in transmitted that use statistical multiplexing. information and hence would greatly expand The development of HDTV and its the required analog video signal bandwidth. acceptance as a future broadcast standard has This is unacceptable. Digital approaches are led to the requirement for a transition period the solution. Great progress has been made in between broadcasting the present analog TV the area of digital TV bandwidth compression to that of compressed digital HDTV. and a national standard has been selected by It is quite likely that digital the Federal Communications Commission in transmission of the current NTSC (standard December 1996. These HDTV developments definition) television signals will also become have, by a combination of techniques, attractive for some applications, particularly in substantially reduced the bandwidth required the transition period. Digitally compressed for fully digital transmission of the video NTSC is now being delivered in several media information. For instance, a single HDTV with bandwidths much less than the traditional channel can be transmitted within the analog 6 MHz per channel. Since the transmission of Broadcast TV channel assignment of 6 MHz standard analog NTSC will remain for many rather than the tens of megahertz once thought years before complete transition to digital high necessary. In the case of cable's well-behaved definition, the availability of a technique spectrum, double the data transmission rate is allowing simultaneous, non-interfering possible. Two HDTV signals can be carried transmission of digitized NTSC-resolution in6MHz. signal(s) within the same spectrum as an This same technology which makes analog NTSC signal could result in a two (or HDTV in 6 MHz possible allows multiple more) times expansion of channel capacity in standard definition digital signals to be stuffed the existing broadcast frequency assignments. into 6 MHz. Movies have several advantages If more efficient means of bandwidth over video in this regard. Movies have twenty compression emerge, even the transmission of five frames per second versus video's thirty. HDTV simultaneously with analog NTSC is This alone is a twenty five percent reduction an attractive, though distant, possibility. in data requirements. Movies have the further significant advantage that they can be VBI TECHNIQUES processed iteratively. That is, the movie is run through the processor several times with The original display device used in adjustment of the processor made to minimize television receivers was the Cathode Ray the creation of artifacts on a scene by scene Tube, CRT, which uses an electron beam to basis. Very good results have been obtained stimulate a phosphor coating on the inside with movies at data rates of 3.0 Mbps. Quite face of a vacuum tube. This vacuum tube is acceptable results have been seen at 1.5 Mbps. appropriately called a picture tube. The When compared to the video obtained from a electron beam is scanned horizontally to form commercially recorded VHS cassette, the lines and vertically to paint a complete image. digital results have some advantages. Since The strength of the electron beam is in inverse 1997 NCTA Technical Papers -276- proportion to the strength of the television development funded by the department of transmitter power and regulates the amount of Health, Education and Welfare (HEW). As a brightness in the picture. The deflection of the result of this work, the FCC reserved line 21 electron beam can be accomplished by of field one of the television signal for the electrostatic forces or magnetic forces. Most transmission of closed captions in the United television display devices used magnetic States in 1976. In 1979, the National deflection. Magnetic deflection circuits Captioning Institute (NCI) was founded to require time to move the electron beam back caption programming and to further the cause to the left side of the screen. During this time, of captioning. In the early 1980s, Sears the electron beam must be turned off or Roebuck stores carried a captioning decoder blanked to prevent unintended stimulation of in set top box configuration selling for about the phosphor screen and the resulting $250. In 1989, NCI contracted for ITT interfering light. The period of time during Semiconductor Corporation to develop a cost which the electron beam is turned off is called effective caption decoder microchip for use in the "horizontal blanking interval." When the television receivers. In 1990, Congress passed electron beam reaches the bottom of the the Television Decoder Circuitry Act screen, it must be returned to the top of the mandating new television receivers of thirteen screen to continue the process of making inch diagonal display measure or greater to pictures. Just as in the horizontal case, the include caption decoding circuits after July 1, electron beam must be blanked to prevent 1993. Approximately twenty million disturbing light patterns on the screen. This television receivers per year are covered by period is called the Vertical Blanking Interval this r~quirement. 1992 saw NCI work with (VBI). The VBI is much longer than the the FCC and the Electronic Industries horizontal blanking interval. The combination Association (EIA) to develop captioning of the two blanking intervals constitutes technical standards. The 1996 around twenty five percent of the total time. Telecommunications Act requires the FCC to This time cannot be used to convey pictures, institute rules requiring closed captioning on but it can be applied for other useful purposes. video programming but allowing exemptions for programming that would suffer an "undue Closed Captioning burden". The Closed Captioning (CC) system is The first United States attempt to use called "closed" because it is turned "on" or the VBI for ancillary purposes was in 1970 "off' depending on who is using the television when the National Bureau of Standards (NBS) receiver. Those without hearing impairments proposed to use it for the distribution of and who understand the spoken words need precise time information nationwide. The not be disturbed by text on their screens. The ABC television network was a partner in that CC system uses very low speed data in order effort. While this initiative did not result in a to minimize the impact of transmission path service, ABC recommended a captioning problems such as reflections and interfering service for the hearing impaired. The First signals. The data rate for the CC systems is National Conference on Television for the 503.5 Kbps of binary (two level) data.