Wednesday, January 18, 2006
HD digital radio LLL-LIES, LIES, AND MORE LIES!
"P.E. Timothy Cutforth of Vir James Engineers spoke of severe 2nd adjacent interference posed by the 6 Denver AM’s that were operating with HD Radio. Some of this was within the 2 mV contour of the "desired" station. Tim also charged that the present STA operations do not properly comply with the NRSC-2 mask, which uses a 10 minute peak detection."
The comments and replies on HD Radio-IBOC in general, and the NRSC-5 standard in particular, continued to roll in right up to the August 17 deadline. Nearly 1000 comment postings are listed in Docket 99-325. A number of Consulting Engineers and smaller broadcasting companies expressed concerns about interference with the AM system. P.E. Timothy Cutforth of Vir James Engineers spoke of severe 2nd adjacent interference posed by the 6 Denver AM’s that were operating with HD Radio. Some of this was within the 2 mV contour of the "desired" station. Tim also charged that the present STA operations do not properly comply with the NRSC-2 mask, which uses a 10 minute peak detection. Tim stated: "The very manufacturers setting up IBOC equipment are specifying that the spectrum analyzer should be set for average not peak detection as specified in NRSC-2. The new proposed NRSC-5 standard calls for several allowable spikes 20 dB above the (averaging detector) mask level. The present FCC rule upon which the IBOC STA operation is based specify peak detection and NO SPIKES above the mask level over a ten minute observation period. Again the broadcaster and the FCC has been mislead into believing that the IBOC STA operation complies fully with the present NRSC-2 emission mask when in fact it may miss the mark by 20 dB or more as actually installed and measured."
Broadcast Company of the Americas (BCA), which programs 50 kW 1090 XEPRS, was particularly concerned. Thanks to favorable propagation over the ocean, this Mexican AM has a 5 mV/m signal over much of Los Angeles, but received severe interference over large areas within that contour when Infinity's 50 kW KNX 1070 Los Angeles and Disney's 50 kW KDIS 1110 Pasadena turned on their HD exciters. BCA mirrored Cutforth’s concerns: "Whereas NRSC-2 used peak weighting and 10-minute spectrum storage for spectrograms, the proposed NRSC-5 uses average weighting and 30-second storage. The relatively lax NRSC-5 standard thus tends to gloss over what the ear actually hears as "hiss/noise" and what the spectrum analyzer displays. The NRSC-2 spectrum was never "maxed out" within a few seconds of storage time. It took many minutes to build up the NRSC-2 spectrum mask, unlike NRSC-5’s instant build up with constant digital signals. Additionally, NRSC-5 allows for two discrete "spikes" within 75 kHz of the carrier frequency to be 10 dB above the emission mask, with the result that a hybrid transmission that is barely meeting the proposed mask can claim compliance with the NRSC-5 standard. The ear hears these vast differences in digital and analog sidebands." (XEPRS was formerly "The Mighty 1090" XERB, with the inimitable Wolfman Jack.)
Having done hundreds of AM NRSC measurements over the years, I can verify that a station that meets the mask in a 10 minute peak test will normally have average sidebands many dB lower. The mask was never intended to be a limit for "good" operation, but a maximum for a "worst case" station operation. Some really lousy sounding AM stations with very audible sideband splatter can still pass the NRSC mask. While I’m still optimistic about the FM system, after reading many of these comments I’m concerned for the future of AM IBOC with the Ibiquity system. If adopted as-is, multiple lawsuits from aggrieved and irate parties seems likely.
HERE IS THE LINK: