DATE: JANUARY 15, 2001
TO: NAB, MSTV, COFDM/8VSB STEERING COMMITTEE
FROM: NAT OSTROFF, VICE PRESIDENT, NEW TECHNOLOGY
SINCLAIR BROADCAST GROUP, INC.
SUBJECT: NEW AND DISTURBING EVIDENCE CONCERNING THE ACCURACY OF THE
COFDM DATA
Sinclair's technical teams, as well as many others on the Steering Committee, were surprised at the poor performance of the tested configuration of COFDM based on previous experience in Baltimore and elsewhere. The belated emergence of information concerning a defect in the COFDM receiver used in the test has caused us to pursue further investigations.
The defect in the COFDM receiver is associated with its front-end design and not the fact that it is a COFDM receiver. In short, the receiver was configured with an unprotected high gain preamplifier connected directly after its antenna input. We at Sinclair speculated that this preamplifier, which covers from low band VHF to the top of the UHF band (50 MHz to 800 MHz) and has a gain of 18db, is being overloaded by any strong signals in that spectrum, including VHF, UHF and FM broadcast stations as well as cell phones. We have reported these concerns to the Steering Committee earlier.
On Sunday, January 14, 2001, after obtaining two of the tested COFDM receivers in questions, we undertook some independent field-testing.
The results of the field-testing for both receivers proved, beyond a reasonable doubt, that the COFDM receivers and, consequently the COFDM data taken from the tests, are not representative of the correct results. In short, the COFDM results are simply wrong!
The field tests on Sunday were undertaken at seven (7) locations using the two COFDM receivers and a COFDM signal from WBAL-DT. The test was structured to measure the COFDM receivers using an antenna at 30 feet, with and without a filter inserted before the receivers' inputs. This was done to learn if the COFDM receiver outputs were being adversely affected by strong signals anywhere between 50 and 800 MHz that could overload the front-end preamplifier.
At six of the seven tested sites, located between 10 and 15 miles from WBAL, it was found that the COFDM signal could not be received reliably when the receivers were connected to the 30 ft. antenna without the filter. This is consistent with prior results. However, with the insertion of the filter, all seven sites were able to produce successful reception.
Further investigations showed that the receivers were being desensitized by 3db to 7db at all seven sites without the filter in place. This is strong evidence of a failure of the receiver's preamplifier to handle the complex broadband high power RF environment. It appears that just connecting the 30 ft. antenna to the receiver caused it to lose 3db to 7db of sensitivity.
In our opinion, this new data sheds a disturbing light on the Phase I COFDM results. In short, the Phase I COFDM results do not accurately represent what that technology is capable of achieving in terms of coverage and ease of reception. The desensitizing of the receiver by the overloading of its unprotected preamplifier created an artificial additional carrier-to-noise (C/N) penalty against COFDM of not 4db, but 9 to 11db.
Such a penalty explains the failures at 40 miles when signal levels were thought to be adequate and the failures indoors under similar conditions. It should be noted that the 8VSB receiver used in the tests does have the equivalent of a front end tunable filter.
We urge the Boards of NAB and MSTV to not publish this report in its present form. The errors it contains can now be shown to be massive and deceiving. If the report is published without a clear acknowledgement of the errors it contains, the engineering honesty and credibility of MSTV and our associated organizations will be put into serious doubt.
The use of the COFDM receiver that was chosen without a protective channel filter was, in our judgement, a gross engineering error that has destroyed the COFDM portions of the report as an objective, scientific document. Given that the eyes of the world are on this report, special efforts must be expended to ensure its accuracy. Anything less will be an embarrassment.
If the 8VSB/COFDM report is published without addressing this issue, Sinclair does not want to be associated with it and requests its name and the names of its employees be removed from the document.
If the Boards decide to not publish or to modify the report or comment on it to reflect this problem and launch a fair and objective study to determine the truth of the matter, Sinclair will provide additional funding over its original commitments.
It is regrettable that this information has been developed so late in the process. We believe, however, that in this case, it is better late than never. We need to realize that the world's engineering communities will scrutinize the COFDM/8VSB report and it needs to be as accurate and correct as possible. Anything less should not be acceptable to the sponsors of this project.
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