ATSC Ad Hoc Group on Broadcaster Requirements
Draft Report
8 May 2000
The ATSC RF Ad Hoc Group on Broadcaster Requirements has identified the following requirements for the DTV RF system. The requirements are intended to be in approximate order of priority.
Whatever the outcome of this inquiry into the RF delivery system for DTV, broadcasters require resolution of this issue quickly. Many stations are on the air, consumers have expectations and broadcasters will be left with no consumer demand for broadcast television, if the system does not work well, or if the public loses confidence because of ongoing debate about the DTV Standard. Therefore, this process should be concluded as quickly as possible.
Digital television service should be at least as good as the present analog system. It must allow broadcasters to reach their intended audiences on a robust, wireless basis without the viewer taking any extraordinary steps beyond that necessary to receive analog TV today. Included in this reception requirement is the ability to have DTV received both indoors and out-of-doors, using simple, small, low-gain antennas, without the need for "critical receiving antenna pointing accuracy" by the viewer when the signal strength is ìP1î [to be defined] dB above required receiver threshold levels in a multi-channel environment. The need for directional, high-gain antennas, mounted out-of-doors, should only occur in the far field where the characteristics of the antenna are being used to provide for gain based on coverage contours. The system must provide reliable operation in strong static and dynamic multipath environments (including Rayleigh fading) given a received signal power at the input to the receiver that is "P1" [to be defined] dB above the systems white noise threshold level.
System throughput should not be significantly compromised. The system should be capable of delivering services including high definition, multiple channel standard definition video, 6-channel audio and ancillary data, including closed captions, V-Chip capability, multiple language, program guides and other data services. The system should have a sufficiently high payload to support these services, and it should fit in a 6 MHz channel.
Related to the above requirement, there should be no changes to the DTV Channel Allotment Table. Any changes to the table would require regulatory action, which would be a lengthy process, extending the period of uncertainty well beyond what is acceptable.
The system must support portable applications.
Support of mobile applications is desirable. Operation in a mobile environment up to ìVVî [to be defined] miles/hour (or units of km/hour). The receivable data rate at this velocity should be ìMMî [to be defined] Megabits/second when the receiver input power level is ìP2î [to be defined] dB above the system's unimpaired white noise threshold.
The system must provide for the use of translators and/or on-channel repeaters to extend coverage and fill in areas of poor coverage.
The system must allow for a range of cost of receivers, including low cost receivers.
Hierarchical modulation is highly desirable. At least two data rates should be supported: a lower data rate ìrobustî mode that provides reliable reception of at least ìSDî [to be defined] Megabits/second when the receiver input power is at least ìP3î [to be defined] dB above the system's white noise threshold; simultaneous with the ìrobustî mode, a ìhigherî data rate mode that provides reliable reception of at least ìHDî [to be defined] Megabits/second when the receiver input power level is at least ìP4î [to be defined] dB above the system's white noise threshold. This implementation should be transparent to the user to assure ease of reception.
To assure the above requirements of the system are met, voluntary receiver performance thresholds should be defined by the ATSC. These thresholds should require successful demodulation, decoding and display in certain minimum reception environments described in terms of sensitivity, selectivity, noise immunity and operation in high level static and dynamic multipath. Broadcasters feel these voluntary thresholds would help the consumer manufacturers design and build DTV receivers to complete the RF delivery system from transmitter to display.
The ATSC RF Task Force process should be concluded ASAP, but some of the suggested 'requirements' require regulatory action. Under existing law, DTV broadcasters must transmit 8-VSB signals with the defined ATSC framing structure. There is no legal provision for a lower data rate mode, hierarchy, etc.
This is extremely difficult to define, as the RF Task Force has discovered in the ongoing debate about replicating NTSC 'coverage' or 'service'. There seems to be strong agreement that ITU Grade 3 video quality is NOT an acceptable benchmark for comparing DTV to NTSC, since many viewers watch NTSC pictures that are less than Grade 3 quality."critical receiving antenna pointing accuracy" must be defined, probably in terms of angular range of receivability. This angular range is also a strong function of antenna design. It will be much different for monopole antennas than for log-periodic antennas.
These environments must be defined. How strong is "strong"? How dynamic is "dynamic." And with what sort of Rayleigh fading parameters will receivers be expected to achieve reception?
Agreed, no changes to the Allotment Table. But some of the other proposed requirements will also require regulatory action.
"Portable" must be defined. To some, it means "a receiver which can be moved from one place to another, but is stationary while being watched." To others, it includes "pedestrian portability", like walking around with a laptop PC and receiving the signal while in low-speed motion.
If the TBD miles/hour is intended to provide "vehicular reception", this will likely require regulatory action. At least this requirement wisely acknowledges that the data rate will need to be reduced to support higher velocities.
The requirement for hierarchy wisely recognizes that an SD-only transmission is a lower data rate service and could be more robustly supported with transmission parameters that are matched to this lower data rate. The law only requires one SD service equivalent to the existing NTSC service, although there is an "expectation of some HD." As a precautionary note, if I were a broadcaster who planned to transmit ONLY one SD service and use my remaining bandwidth for other data services, I would be concerned about future regulatory action. You were given 6 MHz for the primary purpose of providing TV service. If you only need 2-3 MHz for TV service, how long do you think the Congress will allow you to keep 6 MHz? We are all well aware of the value of that spectrum on the auction block.
As was stated at the recent annual meeting of the ATSC, receiver manufacturers already effectively have a fundamental standard in the form or "no consumer returns." Consumer electronics is a relatively low margin business, and any significant number of returns quickly changes the P&L equation. These manufacturers also operate in a highly competitive environment and seem to be universally committed to making improvements -- if for no other reason than competitive market forces.
It has been stated in the RF Task Force meetings that what is really needed is more data on the nature of the RF propagation channel, so designers understand the consequences of their design choices. This does not imply that every receiver should be designed to the highest standard of features, ease of use, etc. There is ample evidence that existing receivers work well for many consumers. If minimum performance targets exceed those of existing and soon-to-be-introduced receivers, two disastrous consequences occur: (1) CE manufacturers are at risk of losing most of their investment in existing or soon-to-be-introduced receivers, which will discourage some of them from future investment in improved versions. (2) Consumers may be forced to buy higher priced receivers, with improvements and features that many of them would decide they really don't need and for which they do not wish to pay a premium.
To the extent that receiver standards might be adopted, I would argue that such standards should at least be tied to the intended application. The technical requirements for "stationary reception with some antenna pointing required" are not the same as "mobile reception with arbitrary antenna positioning". The implementation costs, of course, are also not the same either.
If the broadcasters achieve their desire for hierarchy, I could also envision a market for low-cost receivers which support only the low data rate (SD-only) mode -- a subset of the hypothesized new flexible standard. A certain chip manufacturer with whom I am extremely familiar has made a successful business in the U.K. by providing low-cost DVB-T receiver chips that support only the mode used in the U.K., rather than the entire DVB-T standard. This benefit to U.K. DTV consumers and broadcasters would not have been possible if there had been minimum receiver standards that required complete support of the DVB-T standard.
