As consumer electronics manufacturers begin to embark on the path to build cable-compatible digital receivers, they are faced with some confusion over the choice of standards that should be followed. Choosing the wrong standard as a basis for a cable-compatible digital receiver will lead to disappointment. Recently, the Consumer Electronics Association (CEA) approved a standard dubbed the "Cable Compatibility Standard" or EIA/CEA-818-C. The examination below shows how this compatibility standard is really incompatible with modern digital cable systems.
The CEA cable compatibility standard is divided into two parts. Part I describes the requirements for a compatible cable plant and, to a large degree, tracks with SCTE 40 2001 (formerly DVS/313r5, see On The Line, May 2001). However, it has several inconsistencies. Part II professes to describe the minimum requirements for a compatible digital receiver, but misses several key requirements and mis-states others.
With respect to plant requirements, EIA/ CEA-818-C permits an additional type of modulation. While quadrature amplitude modulation (QAM) was selected exclusively as the digital signal modulation for use on cable systems, the members of CEA's R-8 subcommittee chose to also include vestigial side band modulation (VSB), which is used exclusively by the ATSC broadcast system. It would seem that the allowance of this additional modulation scheme would be helpful, since it is permissive and not required. However, making it an allowed scheme on the plant adds unnecessary cost to a cable receiver by requiring a compatible receiver to include the added complexity of the VSB demodulator. Just such a superfluous requirement can be found in Part II under section 4.4.2(b).
Another serious flaw in the EIA/CEA-818-C standard is the treatment of digital closed captions. The CEA standard requires that all digital programs include captions transported according to ATSC A/53 while saying that captions transported according to SCTE-DVS/157 ãmay also be presentä (Part I, section 5.3). This is contrary to the requirements stated in SCTE 40 2001, where it clearly says that digital captions may be transported in "one or both" of the two transport schemes. What is the difference? The subtle wording difference of the CEA document seems to deny the most common type of digital content in the plant, which is now, and for the foreseeable future, captioned exclusively using the DVS/157 transport scheme. The implication is reflected in the receiver requirements of Part II, where the requirement to support the DVS/157 transport scheme is missing (Part II, section 5.4). Building a receiver without such a caption receiver will deny closed caption services on thousands of current digital programs delivered every week.
Part II of the EIA/CEA-818-C standard falls short in several areas that are necessary to protect the cable plant from harm. One such requirement is the need to limit spurious emissions from the receiver back into the cable plant. These emissions from a poorly designed receiver add noise to the plant and can prevent other receivers from operating. The CableLabs requirements for a compatible receiver state the limits as -37 dBmV. But Part II, section 4.3 (b) of the CEA standard permits spurious emissions up to 11 dB higher, from 70 to 130 MHz, and EIA-23, as referenced by the CEA compatibility requirement, permits levels as much as 22 dB higher at other frequencies. If a manufacturer builds a receiver that merely complies with the CEA standard in this area, that receiver will cause harm to the plant.
Copy protection is another area where the CEA compatibility standard falls seriously short in protecting the plant from harm. In order to provide interesting and compelling content for subscribers, cable operators must sign distribution agreements with studios that create the content. Often these distribution agreements include requirements that the content be protected against further copying by the subscriber. The CEA cable compatibility standard includes no copy protection requirements. Without an ability to protect against unlimited copying, a product built to comply only with the CEA standard would place the operator at risk of being in violation of his contracts.
The EIA/CEA-818-C cable compatibility standard is incompatible with modern digital cable systems in the areas of modulation, closed captioning, leakage, and copy protection. The modulation difference adds unnecessary cost to the receivers, the caption disparity denies services to the deaf community, the leakage shortcomings cause harm to the cable plant, and the lack of copy protection places the cable operator at risk of violating content agreements. These areas will need a major overhaul before this standard can truly be represented as a cable compatibility standard.
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