The CGC Communicator Newsletter Web Posting

With respect to this Letter to the Editor item that appeared in CGC #828...

******************************************************************

  ON THE PROPOSAL TO INCREASE HD RADIO TRANSMITTER POWER

  More information on the proposal to increase HD Radio
transmitter power levels by up to 10 dB may be found at the
URL below. However, one CGC Communicator reader suspects that
analog receivers have typically blended to mono when HD cuts out,
so the request for more HD power is a bit disingenuous. The
reader states that, "The quality of this monophonic reception
is not comparable to that of the stereo reception HD provides."

  http://tinyurl.com/39sjyv

******************************************************************

...the following comment was received on February 12, 2008:

Hi Bob,

I enjoy the coverage that the CGC Communicator provides. Thanks for providing this service to the industry.

In your last issue of the Communicator there was a question as to why the broadcaster wants to increase the HD radio sideband power if the radios blend back to analog at loss of digital coverage.

The real driving issue is to improve the reliability of the secondary, multi-cast channels carrier on the HD Radio signal. Many stations have implemented HD Radio multi-casting where there is no fall back to the analog of signal when the digital signal fails. Up to a 10dB increase in digital sideband power greatly improves the coverage of these multi-cast services in the suburbs and beyond.

Harris provided the FlexStar HDx exciters that were used to do the testing since the Split-Level combining feature in the FlexStar HDx allows variable sideband injection level with real time, digital adaptive correction.

Adding 10dB to the IBOC sidebands is quite a technical challenge since the ratio of sideband level to the existing maximum level of 3rd order RF IMD products at the output of the transmitter has to improve by 20dB, not just 10dB. We have found that with good digital pre-correction, that about 50% additional back-off of a common amplification transmitter from the -20dB sideband level is required to get to the -10dB level and still maintain the RF IMD's at the existing, -75 dBc below the unmodulated, analog, FM, carrier level.

It is interesting to note that the Public Radio stations and NPR are some of the main proponents of increasing the sideband power level, because they were some of the first broadcasters to implement multi-casting.

73,
Geoff - W8GNM

Geoffrey N. Mendenhall, P.E.
Vice President
Transmission Research and Technology
HARRIS Corporation
Broadcast Communications Division

Geoffrey.Mendenhall (at) harris.com

______________________________________________

In response to the Mendenhall letter, the following letter was received from NPR Labs:

We appreciate the Communicator's coverage of broadcast issues, but felt the need to clarify for your readers NPR Labs' position in improving HD Radio coverage as summarized in Geoff Mendenhall's recent letter. While we would consider ourselves a leader in technical implementation of HD Radio, we are not "a main proponent of increasing the sideband power level" and in fact are completing a thorough investigation of the potential interference impact as well as the coverage effects of elevated HD Radio transmission power. Although our investigations into this area coincided with the field testing carried out by Clear Channel, Greater Media and CBS, we were not involved in that study. Only when we're comfortable with our independent studies and feel confident about more fully understanding the science of the digital radio coverage improvement options and interference consequences will we adopt a position on the matter.

As part of our year-long research project, three basic avenues of potential HD improvement have been studied: (1) receiver and receiving antenna performance, which poses no risk of increased interference; (2) single frequency networks, i.e., boosters; and (3) increased digital sideband power. Given the potentially-closer spacing afforded by contour protection allocations in the educational reserved FM spectrum, the second strategy poses the least risk of objectionable interference to 1st adjacent analog stations, a fairly important consideration since 99% of our listeners are still tuned to the analog signal only. (This was discussed in our paper for the 2007 NAB Engineering Conference.) Interference can be especially important to public radio listeners, particularly in rural areas where a single public radio signal may serve as the "public radio lifeline." It would be inaccurate, based on our research activities, for NPR Labs to be considered an unqualified advocate of increasing sideband power level.


Mike Starling (CTO, NPR Labs) & John Kean (Senior Technologist, NPR Labs)
February 22, 2008

______________________________________________

In clarification of his original letter, Geoff Mendenhall wrote as follows on February 27, 2008:

In my previous note to you regarding the reasons for investigating elevated HD Radio sideband levels, I did not mean to imply that NPR was driving the investigation into the benefits of higher HD Radio sideband injection levels. In fact, the initiative for the testing of elevated sideband levels originally came from commercial broadcasters. I know that NPR has an interest in improving the HD Radio coverage of both the main and multicast channels, but I did not mean in any way to speak for them. Elevating the sideband levels is only one of several different technologies that NPR is evaluating so it would be too soon to say which would be the preferred method.

Regards, Geoff

Geoffrey N. Mendenhall, P.E.
Vice President
Transmission Research and Technology
HARRIS Corporation
Broadcast Communications Division

Geoffrey.Mendenhall (at) harris.com

Posted by Steve Blodgett
Earthsignals.com