Ed's view. Are We Being Ripped Off? I have heard and read several remarks over the past few years concerning the negative impact on HDTV quality due to the number of sub-channels being broadcast on the station's digital channel. So, is our HDTV quality being compromised by all of this "multiplexing" activity? In other words, are we being ripped off. The answer to this (in my view) is "it depends." If you accept this answer, it is not necessary to read further, and you may move on to other non-goal director behavior (pause)... So, the subject has piqued your curiosity, as it should. For, as HDTV grows, we must act as "policemen" to stem attempts to compromise quality. To be sure there are, and will be, many different quality levels due to production variances, HDTV production equipment limitations, and the effect of the production "learning curve." As production experience is gained and equipment improves, the quality will only get better. However, the question remains whether this positive production improvement curve will be, or indeed is being, compromised at the transmission end - regardless of the physical "transmission" mode - OTA broadcast, cable, DBS, etc. Probably not, but again it depends. Why? First of all, we must realize there is nothing in the FCC Digital TV rulings dictating that the digital OTA broadcast service must be HDTV. The rulings only prescribe the digital channel frequency assignments, transmission power levels, and the ATSC formats for OTA video services (e.g. MPEG 2 compression, 8VSB modulation etc.). The MPEG 2 encoding parameters allow for a 20Mps video channel that was determined, through many ATSC sponsored subjective visual evaluations, to be "HDTV." However, broadcasters are free to use their allocated bandwidth for services other than HDTV, as long as one service is ATSC video. As we know, many are doing just that, usually transmitting an "HDTV" program alone with three or four news, weather, public events, etc. programs. Although other services than video is possible, OTA broadcasters have yet to find a successful business model for them. That gets us back to the base question: is all this multiplexing, regardless of what it is, compromising HDTV quality. It certainly can, and in some instances it may be (again, it depends) but in actuality it will not, nor is it likely. Why? The first reason is political - that means you and I and our elected representatives. From its inception, what we now know as HDTV was "sold" to the American people, and thus politically, as a TV system delivering very high quality images. Therefore, the FCC and congress have looked with a very jaundiced eye on supporting any broadcast spectrum allocation that does not pay off on the HDTV promise. The second reason is economical. In the very competitive world of commercial television production, and with the traditional broadcast networks watching their viewing share decrease with respect to cable networks, these networks are forced to respond to any competitive element. As HDTV becomes more popular, its ability to influence even a fraction of the viewing share becomes very important to the network programmers. Indeed, there is no doubt, since ABC's recent announcement that Good Morning America will be produced in HDTV this fall, that NBC will be forced to follow with Today in HDTV. The third reason is technical. By approximation mathematics, still (non motion) 1080 ATSC images can be transmitted with maximum definition using only 5Mb/sec, and that assumes that every pixel has a different brightness level, which is virtually never the case. Anyway, the last I checked, television is a motion picture medium. If a broadcaster were to limit its "HDTV" sub-channel to 5Mb/s and use the remaining 15Mb/s to multiplex a variety of other services, the impact on the HDTV image would not be at all acceptable. As the image elements move, more bits are needed to transmit the compressed motion data. If the encoder is "starved" of bits to support this motion data, the results are not a pretty picture. The image elements in motion would tend to break up into non-coherent noise, call "mosquitoes." This noise, at a distance, resolves itself to the human eye as a loss of resolution of the image edges. Also, the motion elements would tend to "smear," the faster the motion, the more the smear. If the bit starvation were severe, the image would break up into blocks. To minimize the artifacts, the HDTV sub-channel video could be severely pre-filtered before being encoded, but this would limit the video frequency response, and it would no longer appear as "HDTV" to the viewer. (Note that some pre-filtering is always done prior to encoding to minimize artifacts). However, the saving grace in this technical dilemma is a neat piece of encoder technology called Statistical Multiplexing or "statmux." Statmux technology dynamically allocates bits to the multiplex sub-channel that demands them at any particular instance. This is the same technology that allows DBS and Cable to transmit thousands of programs with practically no bandwidth. The idea is that statistically, at any instance, all of the multiplex sub-channels will not require the maximum number of bits to support an artifact free image. Therefore, those sub-channels requiring lots of bits can be taken from those sub-channels that do not. As a result, with Satmux, the broadcaster can multiplex a high definition sub-channel, along with several SDTV sub-channels or services without any appreciable artifacts. But, there is an "it depends" caveat here. Not all encoders are created equal, nor is the expertise of the broadcast (or Cable) engineers managing the encoding process. Therefore, the encoding quality depends on those factors. However, as greater encoder efficiencies are developed and broadcast expertise increases, HDTV quality improves. There is probably some degree of "let's see what we can get by with" attitude existing with some broadcasters. So, again, it is up to us to be vigilant regarding HDTV quality. If we notice compromises, we should call and complain. Therefore, to some extent,whether of not we are "ripped off" defends on us. If we accept this task, HDTV will only get better. Ed