The field of television hasn't undergone so many profound changes in the last 40 years as we have experienced in recent years. Since the introduction of color television there has been continuous improvements in picture quality but they were all steady, continuous improvements . . . no quantum leaps. What we are seeing in the last 5 years, say, is radically different, with new standards set for broadcasting. Of course the most visible is the introduction of over-the-air HDTV here in the U.S. on November 1, 1998.

Most attention goes to the digital aspect of broadcasting nowadays, with compression algorithms, digital modulation, encryption techniques for Pay TV and ancillary data services like electronic program guides ranking high on the list of hot broadcasting technologies. There is a whole slew of analog techniques, though, that need to follow suit. Since most digital-TV designers today, quite naturally, have a background in digital hardware design, the analog issues sometimes tend to be forgotten. And I am not talking about the RF front end, which even if you use digital 64-QAM, QPSK or 8-VSB modulation still boils down to an analog signal. No, here is a case where we look at the basic video signal itself. How are today's changes in broadcasting affecting the video signal that travels between your camcorder, set top box, VCR and TV?

Remember when life was simple? The world was split into NTSC (USA, Japan) and PAL (most of Europe) countries (let's forget some oddities like France and the former Soviet Union for now who chose SECAM.) Bandwidths are around 6 MHz for NTSC, 7-8 MHz for PAL. The RS-170 standard of the EIA (Electronics Industries Association) back in 1957 defines the basic signal characteristics: a signal of 1 Vpp between blanking and full white when terminated in a load impedance of 75 W . NTSC and PAL deviate by using only 0.714V between blank and white for NTSC (0.7 V for PAL) and reserving the rest of the 1Vpp signal for the sync signals, including line sync present at the start of each video line (see Fig. 1.) So the ratio of video to sync is 10:4 for NTSC and 7:3 for PAL.

Figure 1

Essentially not much has changed since then; the color signal today still adheres to this early specification for monochrome video. This was accomplished by essentially putting the color information in parts of the video spectrum not used by the monochrome signal when color TV was introduced. Compatibility is guaranteed: even today's video can be shown on your 30-year old B&W TV, supposing the receiver is still up and running.

And now there is HDTV. At least in the U.S. Europe has seen its share of early HDTV hopes fainting away in the beginning of this decade with the unsuccessful and R&D-expensive HD-MAC (High-Definition, Multiplexed Analog Components) system, a hybrid of digital and analog transmission techniques, to the extent that even today not much support from service providers exists for HDTV. Japan is slowly fading out its own MUSE system. The US, relatively late in adopting any standardization for digital TV, is now leapfrogging by directly jumping to HD resolution. And here is where the analog ‘picture' changes . . .

. . . In digital HDTV, for the first time, the bandwidth of the signal transmitted over the air is significantly less than the bandwidth of the analog signal between your HDTV set-top box (if you're among the few ten thousands in the U.S. who bought one so far . . .) and HDTV display. Think about it. The signal is transmitted compressed over the air in the new 8-VSB-modulation format, which squeezes it into the same 6-MHz bandwidth of regular NTSC. However, after MPEG-2 decoding in your set top, its video bandwidth can be as high as 30 MHz. Gone are the days of composite video, enter the days of component video . . .

In component video, the signal is transmitted over 3 separate wires, much like for PC monitors today. While in PC environments these components are red, green and blue (RGB), broadcasting (MPEG-2 in particular) makes use of luminance (Y) and chrominance parts (R-Y, B-Y.) In use for some time already in broadcast and post-production environments to improve production quality compared to encoded NTSC and PAL, the advent of HDTV brings real component video for the first time to the consumer arena. So today's true HDTV set-tops have 3 RCA phono jacks for only video output, not just one for video and 2 for audio. Here's the quantum leap after more than 40 years . . .

CEMA (Consumer Electronics Manufacturing Association), part of EIA (Electronics Industries Alliance), has effectively standardized in the 2^nd half of 1998 the ‘analog component video' interface for consumer use.

Figure 2

The waveform of the analog HDTV signal according to CEMA/EIA shows essential (see Fig. 2) differences. First, the video levels. We still have a 1-Vpp signal including sync, but the blank-to-white range is 700mV, not 0.714V, making it a 7:3 ratio, not 10:4 (as in PAL.) Secondly, and more importantly, the shape of the sync is different: it consists of both negative and positive "excursions" with respect to the blanking level. By the way, there is an actual minimum rise and fall time on the edges of this new (tri-level) sync signal, just like in composite video, to limit the signal's bandwidth. So simply switching the signal with an analog multiplexer to a fixed level will not do the job, as digital designers out there might suggest . . .

This new 1998 CEMA standard is largely based on the older SMPTE (Society of Motion Picture and Television Engineers) 274M and 296M standards that define the picture format for the two predominant ATSC HDTV standards, in use today for HDTV in the US: 1080 lines interlaced (1080I) and 720 lines progressive (720P). As mentioned, analog component video is widely used, even for standard-definition TV, in broadcast studio & video post-production environments to avoid the artifacts (cross-color, cross-luminance, etc.) typically associated with decoding composite NTSC/PAL.

In the consumer HDTV set top box the analog component video output is typically generated after three D-to-A conversion of the MPEG-2 decoder's Y, B-Y & R-Y outputs. MPEG-2 by itself does not carry any horizontal sync of this nature, so designers of DVD players and set top boxes need to generate such a standards-compliant video signal in some shape or form if they want their equipment to be compatible with HDTV sets of different brands.

Over time the analog video interface might be replaced by an all-digital interface like P-1394. However, today analog video interfacing is the common denominator across different end equipments. In the world of digital video where bit-error rates, MPEG-2 timestamps and nifty software APIs reign, the analog details can be forgotten. Texas Instruments will have a solution for the new HDTV interface soon.

About the Author

Bart De Canne is a Senior Systems Engineer for Texas Instruments' Advanced Analog Products in Dallas, Texas. He is specifically involved in TI's data converter strategy for imaging applications. Before joining TI, he was working at Barco NV in Belgium, a supplier of high-end projection, and broadcast and cable headend equipment.

Mr. De Canne earned a master of science degree in electrical engineering and a bachelor degree in business administration from the University of Ghent (Belgium.)