In this Friday’s Intel Brief consulting engineers Ed Bukont of Comm-Struction and Services and Marty Hadfield of The Hadfield Group discuss, respectively, the use of GPS-enabled IBOC exciter and audio network products for use in analog AM and FM broadcast, and FM HD Radio programming considerations for your HD3 channel.
FM HD Radio programming considerations for your HD3 channel
By Marty Hadfield, President, The Hadfield Group
You have probably heard it said that in the programming battles with XM & Sirius, one significantly competitive edge FM HD Radio has going for it is the comparatively better fidelity over satellite programming.
While that may be considered to be true for your HD1 & HD2 channels, it would be a stretch of the imagination to apply that thought to your HD3 channel. The reason? Every time another HD channel is added to your initial HD1 on-air bit stream, the available program fidelity is reduced. By the time you are adding HD3, while keeping the fidelity relatively high on HD1 & HD2 channels, the quality available for HD3 approaches that of a poorly equalized telco loop.
The typical programming response to this situation is to head toward a talk based format, thinking that voice sounds better than music under low bandwidth conditions. However, there are important differences between an old analog telco loop and your HD3 that makes a talk format perhaps one of the worst choices.
The telco loop example usually just limits the lower and upper frequency response, making music sound poor, but speech was carried through as reasonably acceptable. In contrast, with virtually all digital encoding, the digitizing process eliminates small slices of the fidelity of the original program content to produce a form of reduced bit rate encoding. With most music programming there is so much content in the original source material that the overall process is designed to “mask” a great deal of the sliced out pieces. On the other hand, with the spoken word, our hearing is easily annoyed by those missing slices of voice fidelity. This is particularly disturbing to the human ear when coupled with other audible side-effects of low bit rate audio, such as your announcers sounding like they are underwater with swirling/gurgling sounds. If their speech goes on & on, it can be really distracting. If you have an opportunity to listen to SAT Radio programming sometime, you will probably hear all of these effects. One other cautionary note about HD3 – while most music formats may sound pretty good, oddly enough, Rock formats often sound quite bad at HD3 transmission rates.
To date, I have only heard one spoken word format that I believe sounds reasonably acceptable on HD3. Some Non-Commercial/Educational stations are running World News and other BBC programming on their HD3 channel. The resulting programming sounds reminiscent of listening to distant broadcasts on the Shortwave Bands, which is where most of us in the Western World would have heard the BBC.
On a closing note, under most conditions, each of your HD channels should use audio processing appropriate for both the type of program content and the available bit rate. There are many audio pre-processors available specifically for use on FM HD Radio. These are available through your usual list of broadcast equipment vendors. If you have a really sharp engineer that is well versed in the art of preconditioning audio for reduced bit rate transmission, they can probably knock together a custom processing package.
The Hadfield Group, LCC is a provider of technical consulting and project management services. Consulting services include signal coverage analysis and recommendations for facility improvements.
The Hadfield Group was formed in 2007 by Marty Hadfield after 17 years as VP of Engineering for Entercom, where he led their technical growth initiatives from an 11-station group to the present count of 114 stations in 24 markets. Hadfield was instrumental in the design and construction of dozens of studio facilities, high-power AM & FM transmitter sites, new and rebuilt tower sites and has launched 75 FM HD Radio installations.
Marty Hadfield, a Seattle native, began his broadcast engineering career in 1976 at KIXI, Inc as Chief Engineer of AM/FM combos in Seattle and Tri-Cities, WA. Subsequently, Marty became Corp DOE for Bellevue based SRO Broadcasting, then a TV/Radio Special Projects Engineer for Fisher Broadcasting.
Contact: [email protected]
www.thehadfieldgroup.com
Voice: 253-351-8922
Do I really need to install this right now?
A justification to consider the use of GPS enabled IBOC exciter and audio network products for use in analog (terrestrial) AM and FM broadcast.
By Ed Bukont
Do I really need to install this right now? That is a question sometimes asked about the little GPS antenna that comes with most IBOC exciters. It may be a challenge to install the GPS antenna(s) where the studio or transmitter site are in a multi-story building. (And yes, you can extend that cable on some antennas. In other cases, you may be able to split and existing GPS antenna). This consideration is sometimes overlooked in new and existing installations when using configurations such as E2X (Exporter To Exgine) where the Exporter, which requires the GPS, is at the studio site and the exciter, which formerly required GPS, is at the transmitter site. We have heard comments from station personnel that they were told by someone that there is no real reason you need to install the GPS antenna right now. For some, there can be considerable expense to that installation or to acquiring the GPS signal by an alternate method. Yes, in the short term, the system may run without the GPS signal, but there are indeed distinct reasons why you would want to install that antenna and use the GPS signal now.
Especially when there is a significant expense involved, it behooves the manager to ask what other benefit is there to the GPS beside proper operation of the IBOC signal? For both AM and FM terrestrial broadcasting there can be a positive impact to the reception of the analog signal when GPS is used as the reference for the carrier frequency. Starting with AM, this article looks at how a station’s ANALOG signal may immediately benefit from use of the GPS signal incorporated with the IBOC exciter, even if the IBOC signal is not turned on.
The critiques of AM IBOC broadcasting often cite the potential for energy in a station’s IBOC sidebands to be a source of objectionable interference in the reception of an adjacent channel. This problem however may only be an increase, rather than wholly new interference. While the requirements of the AM hybrid IBOC (Mode MA1) emission mask as referenced in (NRSC-5) are tighter than the emission limitations of analog-only transmission (NRSC-2), noticeable interference can occur when there is energy in that portion of sidebands shared by adjacent stations, especially if there is asymmetry in the transmission or reception of the sidebands. Properly known as Hermitian symmetry, the concern is to have energy in the sidebands and around the analog carrier appear to the receiver in such as way as to exploit the math of RF reception that would cause either the IBOC carriers or analog carrier to be removed from the receiver section where necessary. The result provides only the desired analog or digital information to be further processed for a usable output. This concern however is NOT at all new, but rather the presence of IBOC carriers makes the condition more evident. The problem of receiving the desired channel when subject to adjacent interference has always been a problem for AM reception, especially in fringe areas and under conditions favorable to DX reception. Until the advent of IBOC, this problem tended to be more evident as co-channel interference.
Now you may ask, how can the problem be co-channel and adjacent channel at the same time!? In the crowded, overlapping and narrow bandwidth of AM, that is entirely possible. Forgetting for the moment about any IBOC considerations, let us review a concern for reception. Small differences ( under 10Hz) in the respective carriers of two independent stations on the same channel will cause both to appear as discrete signals within the passband of a receiver. The receiver cannot tell the difference between the two carriers as both occur within the range of frequencies for which the receiver is designed to recognize as one channel.
The receiver will try to demodulate both signals, resulting in a new signal (properly known as heterodyne) that is the difference between the two inputs to the receiver’s mixer stage. (It should be noted for clarity that heterodyne is also the method by which radio receivers are made to tune across the band, using a fixed intermediate frequency to beat against the desired channel and extract a resulting lower frequency product for demodulation.) At AM broadcast frequencies, a difference of only 2Hz between co-channel carriers is sufficient to cause considerable disruption to proper reception.(1). Unfortunately, FCC regulations only require the carrier of an individual station to be within 20Hz of its assigned frequency. Especially if a station is running older equipment or using out of calibration test gear to maintain compliance, the chance for and likely hood of co-channel or adjacent channel interference is very high, even though your station may meet FCC and NRSC limitations. This result is usually heard as a low frequency beating sound within the audio which interferes with the intelligibility of with the desired demodulated audio. Some radios are equipped with a tunable beat frequency oscillator (BFO, so named as it mimics the beating frequency described above) that is designed to generate a carrier which can be introduced to the mixer section specifically to cause a cancellation of the interfering carrier. The BFO however is not usually a viable function in HD reception. The receiver is furthermore not likely to cancel the sidebands of the analog carrier as the off-frequency content would prevent the sort of sideband symmetry necessary for that to occur. If we now add in the multiple carriers of IBOC, and their frequency division relationship around the analog carrier, it should be easy to see how a minor difference in frequency between two carriers could generate considerable noise within and adjacent to the channel.
To be fair however, the IBOC standard as proposed by iBiquity specifically spaces carriers in such a manner as to reduce the harmonic relationships that would give rise to far worse interference. Again, this is a problem for analog reception of co-channel stations even without the presence of any IBOC carriers. Because the IBOC carriers consist of many small partitions that appear farther out than the primary carrier, the presence of IBOC can appear as interference to both the overlapping (adjacent channel) and to the primary (co channel). When the co-channel stations are both running IBOC carriers, but those carriers (and for that matter the main channel) are not within better than .1Hz of each other, the interference to each other can be enough to make the channel unusable. This has been a problem for co-channel stations since nearly the inception of the industry.
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Broadcasters can do little about improvement to the reception system, but there is something that can be done in transmission that will improve useful reception. Smith and Moore propose in their paper that terrestrial broadcast could benefit from an improvement of 4 to 10 times in the interference limited listening area by the use of a national time-base standard to generate all AM carriers within 0.001 Hz of the assigned frequency. Differences then between co-channel stations would be almost invisible to the heterodyne constraints that plaque receivers. The recommended time base standard would be derived from a GPS signal. As it turns out, the IBOC standard for digital broadcasting, as approved (though not mandated) last year by the FCC (2) and described in NRSC-5 (3) already proscribes the use of GPS in several ways to generate the various timing signals and maintain their relationships, which are essential to IBOC and analog signal generation. We would digress momentarily to mention that the generation of a very stable time base is essential to the proper construction, modulation, reception and demodulation of AM and FM IBOC transmission.
For multicast (currently only available in FM hybrid), there is also another concern where GPS is used at the studio. With regard to the more immediate use of the GPS function offered in IBOC capable exciters, the author of this article made an inquiry to Smith and Moore regarding the suitability of the IBOC GPS reference for use in the generation of an analog AM carrier to their specifications. Mr. Smith replied, “After a conversation with representatives of iBiquity, it would appear that the licensed commercial AM HD Radio exciter units will generally be within 0.01 Hz of exact frequency, since they are nominally locked to GPS. We would actually prefer a lock to within 0.001 Hz (1 mHz), especially for the Class-C local channels, but the 0.01-Hz lock accuracy for HD stations should provide a significant benefit over the current non-locked regime.”
Therein lies a benefit for an existing analog AM broadcast station that can be realized now in the migration to IBOC technology products and the installation of the GPS antenna even if your business or technical model does not currently allow for the implementation of IBOC broadcasting. Discounted equipment is available from some vendors and discounted HD licenses are available from iBiquity that may be attractive to some who would entertain a modest expense to help improve the usable reception of AM broadcasts. You would have a benefit today to your analog signal and a discounted progression to the provision of HD services which are expected to provide additional benefits to stations in the future. This is but one of several ways in which analog can benefit from HD conversion.
The current state of the art for use of GPS in FM is a bit more complicated. If your station is making the conversion to HD, and intends to provide multi cast (HD 2, HD 3) channels (FM Mode MP3), it is important to understand that the GPS time base is critical to the generation of the AES timing parameters used in the choreographed assembly of the packets between networked devices that create an HDC frame containing the primary (HD 1) and secondary (HD 2/3 etc) audio and all data related functions. AES 44.1kHz is used not only for audio sampling purposes, but in order to ensure that the audio gets priority, the AES frame data is also used to generate the timing of all data assembly, including non-audio services. For various concerns related to the precision of the various internal clocks and how framing information is generated and disseminated, it is important for the clock to have the GPS reference, rather than the internal reference.
Analog FM can benefit as well from the use of GPS derived time base. Again, we are looking at how use of GPS in origination will affect reception of the analog signal. What was discussed about the possible improvement to AM reception holds true as well for FM, although the improvement is not nearly as dramatic. Where the difference can be noticed however, are two fold. First, as radio receivers evolve and receiver manufacturers, led by the auto industry, seek to improve performance, there is increasing use of frequency rather than audio relevant algorithms to assist in the search for channels. This is in part due to the incorporation of HD technology but also the realization that a good portion of a channel now contains data, such as SCA, that are some distance away from the center carrier. In a search, it is those frequencies around the carrier which may be apparent to the demodulator first, rather than the nominal carrier frequency. Because these ancillary services tend to have a fairly low modulation and limited bandwidth, they make good, predictable markers for use in search algorithms (versus the variations in audio level and stereo pilot that were once preferred).
An algorithm is likely to contain various lookup tables which tell it what the meaning is of a particular piece of frequency data (such as discriminating between noise, carrier, pilot etc). There in lies the first reason for use of GPS. A precision reference can assist your FM signal in standing out from the dial during a search. Those stations with a clean and predictable quality to the RF and data properties of their signal, which is to say the signature expected by the receiver algorithm, have a greater chance of having the receiver stop at their channel during a search or scan. Second, stations running HD stand a chance of being passed over in a search, either manual or automatic, if the receiver cannot achieve a lock to the HD carrier within a reasonable amount of time after acquisition. This is a function of the generation of both the OFDM signals that comprise the RF portion and the AES timed frames that comprise the data portion of the signal. Both the RF and data domains are derived from the GPS clock. Some users of HD receivers may already note marked differences in how a receiver scans in each direction and which nominally HD stations the receiver may, or may not, lock into during a scan. Chances are that the station that takes longer to lock is a station this is not receiving the benefit of GPS derived timing. That precision need not be reserved for HD only but is a benefit available to analog reception through the use of the higher precision offered in the use of a GPS derived time base. The sequence of plant improvements recommend by iBiquity for the conversion to IBOC, starting from the transmitter, backwards through the STL and finally to the studio is supported in the suggestion that a station begin with incorporation of those benefits which can be obtained now, such as GPS derived timing and begin the conversion to IBOC.
Ed Bukont CSRE, CBNT, is President of Comm-Struction and Services, LLC
P.O. Box 629 Bel Air, MD 21014
Voice 410.879.5567 Cell 240.417.2475
[email protected]
Footnotes:
1 (Stephen Smith and James Moore, ‘A Precision Low Cost GPS-Based Synchronization Scheme for Improved AM Reception, Oak Ridge National Laboratory, 2007)
2 Digital Audio Broadcasting Systems and Their Impact on the Terrestrial Radio Broadcasting Service, Second Report and Order, First Order on Reconsideration and Second Further Notice of Proposed Rulemaking, MM Docket 99-325, FCC 07-33, released May 31, 2007.
3 IN-BAND/ON-CHANNEL DIGITAL RADIO BROADCASTING STANDARD NRSC-5, National Radio Systems Committee (CEA/NAB), April, 2005
