THE BIRO TECHNICAL BULLETINS
ON THE VALIDITY
OF THE FCC DEFINED 41 dBu DIGITAL
“NOISE LIMITED SERVICE” SPECIFICATION
FOR DTV COVERAGE
What’s Missing from the 41dBu Standard for HDTV
The very words “Noise Limited Service” imply perfect pictures on the screen of the HDTV receiver, verified by the 41 dBu reading on the fieldstrength meter. During the period of March 1 through March 10, 2005, Biro Engineering of Princeton, N.J. conducted intensive “Noise Limited Service” fieldstrength verification testing in the New York Metropolitan Area for a major CATV MSO to verify the projected 41 dBu service area of a new DTV only station.
What we found made us wonder whether this standard is enough to ensure optimal picture quality for HDTV customers. An HDTV audience is not interested in dBu fieldstrength, dBuV signal level or integrated power level recordings. They want real high definition TV pictures, free from frozen images on their $2,000 HDTV monitor and “NO SIGNAL” messages appearing on the screen.
Four (4) locations were checked on Staten Island, six (6) spots in Brooklyn and eight (8) measurement locations in Queens, New York. While more than 90% of the tested locations met or exceeded the Commission’s 41 dBu fieldstrength criteria for “Noise Limited Contour” service, only at one test site (out of a total of 18 test locations) were the desired pictures received by the HDTV receiver. However, even at that particular location, the pictures occasionally froze, and contained artifacts.
There seems to be a missing link between the 41 dBu minimum fieldstrength criteria and the concept of “Noise Limited Service”: the viewable HDTV picture.
The Right Testing Methodology
The Metropolitan Area field tests and measurements used the following test equipment and layout.
The Model 507 digital Signal Level Meter produced true integrated power level readings in dBuV, displaying the 10 MHz wide RF spectrum on the screen of the laptop.
The TEKTRONIX Model 2710 spectrum analyzer was used to check adjacent channel interference conditions. The interference reached extremely high levels in most areas because the vicinity of the Empire State Building, the TV antenna farm of most New York analog and DTV stations.
Furthermore, HDTV reception was checked with the aid of the Zenith Model 420 HDTV receiver and the pictures taped by the TV/VCR combo.
A typical adjacent channel interference situation is illustrated on the Signal Direction Sheet below.
The relative angle of interfering Channel 47, New York is only 27° and it is almost 10 times closer then the desired Channel 48, Kingston.
Projected severe interference conditions were verified with the aid of the TEKTRONIX Model 2710 spectrum analyzer.
The desired 5 MHz (one division) wide DTV spectrum in the center of the screen is just above the noise level. The Channel 47 adjacent channel video carrier on the left is more than 50 dB higher than the desired DTV station.
The high level of adjacent channel interference cannot simply be ignored! UHF tuners exhibit very limited or no protection against strong adjacent channel carriers. Reaching the demodulator section of the TV set will create undesired intermodulation products, an increase of the noise floor and damage to the HDTV pictures.
A second phenomenon, the irregular frequency response of the 5.5 MHz wide DTV power spectrum, will also affect DTV picture quality and the reliability of DTV reception.
FLAT, REGULAR RF RESPONSE
IRREGULAR RF FREQUENCY RESPONSE
The RF spectrum on the left, an ideal case of over-the-air DTV reception, displays a flat, 5.5 MHz wide frequency spectrum. The RF spectrum on the right has a deep notch and a high peak, caused by reflections and other wave propagation anomalies. No DTV picture was received at that test location despite the fact that the measured fieldstrength level exceeded the FCC set minimum fieldstrength of 41 dBu. There are no specifications or guidelines for acceptable frequency response irregularities, such as peaks and notches, tilts and deviations from the average fieldstrength
Making Sense of the FCC Criteria
And last but not least, on the subject of the FCC set criteria of 41 dBu fieldstrength.
Signal Level Meters, and Spectrum Analyzers provide signal level readings in dBmV or in dBuV, but not fieldstrength values in dBmicrovolt/meter (dBu).
Question: What signal level will produce 41 dBu fieldstrength, as measured at 677 MHz on the half-wavelength dipole, installed at 30’ above ground?
Signal level to fieldstrength conversions are performed in two steps. First, convert the dBuV into dBmV.
dBuV - 60
Next comes the formula establishing the relationship between the dBu fieldstrength and the received dBmV signal level:
dBmV + (20 * log f + 26.32)
Where the f is the center frequency of the DTV power spectrum in MHz.
Since our test antenna was a half-wavelength dipole, tuned exactly to the DTV station’s frequency, there is no need for antenna gain reduction. On the other hand, a 2.4 dB adjustment for the insertion loss of the antenna downlead is necessary since the signal level was measured at the end of the low-loss coaxial cable, not at the dipole’s terminals.
Calculating the received signal level:
|dBmV||= dBu – (20 * log 677 + 26.32)|
|= 41 – 82.93|
|= -41.93 or -42 dBmV|
Converting -42 dBmV into microvolts, it is exactly 7.9 microvolts.
If you are under the impression that this is a very, very low signal level, consider the additional 3 to 6 dB insertion loss on the coaxial cable between the rooftop antenna and the living room HDTV receiver of the audience. This will reduce the input signal level for the HDTV receiver to the more realistic 5.6 or 3.5 microvolts. Those are extremely low signal levels to produce a reasonably high Signal/Noise ratio, considering the 7 to 10 dB noise Figure of the UHF tuner in the HDTV receiver.
It is not the objective of this Technical Bulletin to create problems where no problem exists. However, picture quality limiting factors, such as:
cannot be considered acceptable engineering practices.
The magic 41 dBu “Noise Limited Service” requires an honest reexamination. After all, 41 dBu without perfect picture quality is the same as an iPod without content --it is only scrap metal.
For emergency engineering services:
Call: (609) 883-9866
Web site: www.biroengineering.com
PRINCETON, N.J. 08543