IBOC: A Look From Every Angle
Oct 1, 2001 12:00 PM, By Doug Irwin
It’s a cool Monday morning; the first in October, in fact. After opening your office, and after downing the requisite two cups of coffee, you turn on your computer and open them: The dreaded capital budget spreadsheets.
But there is more to the story this year than you realize. Your office phone rings. It’s the boss. �Chief�, he says, �I know the capital budget is due this Friday, so it’s important that I tell you now. I’ve decided that we’ll start broadcasting IBOC at the end of next year, on the FM and the AM. I’ve been following the trades, and it’s just something I want to do this time. So make sure you figure out just how to do it and just how much its going to cost us.� Click. You didn’t even have a chance to get a word in. In fact, you sit there, stunned and slack-jawed.
After a few minutes, and a quick mental review of what you’ve learned in the trades and at the NAB convention, a plan starts to settle in your mind. According to what you have read in the press, the gear will be available next year. For purposes of planning this far ahead, it’s possible to ignore regulatory difficulties; in fact, in order to carry out the boss’ plan, you’ve got to assume everything is a go.
With a little help from your friends at iBiquity, you obtain a block diagram of the IBOC system for both AM and FM. (See Figure 1 for the AM and FIgures 2 and 3 for the FM.)
The FM side
Delivery: If you deliver your program audio to your transmitter site as analog discrete or analog composite, you are going to have to convert that audio to the AES/EBU format prior to hitting the input of the IBOC exciter.
Figure 1. An IBOC transmission system will contain several new elements. Shown here is an AM transmission chain. (Click image for larger view.)
Processing: Ibiquity has always assumed that a station would want to process audio for the analog transmission differently than for the digital transmission. Since the digital system does not make use of pre-emphasis, there is no need for HF limiting, as there is on the analog path. However, since the point of this article is budgeting, it does seem reasonable to point out that the same processing device could be used to feed the IBOC exciter as well as the analog exciter. One caveat: it is likely that the engineer will be able to get away with far more processing on the analog path than on the digital. PAC is not friendly to high amounts of equalization to the program audio. There is also some question as to the character of clipped audio after it passes out of the PAC decoder. Although one could use the same processor for both the analog and digital, compromises will need to be made between adjusting a single processor for best analog sound or for best digital sound.
RF stages: All of the major transmitter manufacturers will have linear amplifiers designed to work with the IBOC signals by NAB next spring. In the not-too-distant future, the same manufacturers will market transmitters that combine the low level IBOC carriers with the analog FM carrier, followed by high-power, linear amps that will amplify the IBOC carriers and the FM carrier alike. This common-amplification approach is a bit too far ahead for year 2002 budgeting.
Figure 2. If an analog FM transmitter is incapable of passing the IBOC signal, a high-level combiner system must be used. (Click image for larger view.)
At this point you have two options. The first is the high-level combiner that has been shown at the last two springtime NAB conventions. This combiner has four ports: one input for the analog transmitter and one output that feeds the antenna; one that accepts the high-level IBOC carriers, and finally one for a dummy load. The obvious advantage of this approach is that you use the same antenna for radiation of the IBOC carriers as you do for your analog FM carrier. There are several things to consider, though. The high-level combiner has about a .46dB insertion loss for the analog carrier throughput. You’ll need about an additional 10% RF output from your analog transmitter. Don’t neglect to consider the additional power needed by your transmitter, along with a larger heat load. The second input of the combiner accepts the IBOC carrier RF signals, but only couples 10% of their energy to the output port connected to the antenna. The rest is dumped into the dummy load.
What does this mean in terms of the size of the IBOC amplifiers? Well, let’s take, for example, an FM with an ERP of 100kW, and an antenna input power of 33.3kW. Since the average power of the IBOC carriers will be 20dB below the analog carrier power, in this example the antenna input power for the IBOC carriers is about 333W. Neglecting all transmission lines losses for simplicity’s sake, we need the IBOC carrier level going in to the combiner to be 3,330W. It is important to note that the linear amplifiers for the IBOC carriers will need to be able to handle at least 5.5dB above the average power level (albeit on a short duty cycle). So, neglecting transmission line losses, you need to purchase and install an amplifier big enough to put out 11,815W, just so that you can get 333W at the antenna input. Keep in mind the need for extra power from the AC mains, along with the extra heat load. In this example, you would probably need at least one full rack to house the amplifier for the IBOC carriers, and you’ll need space for the high-level combiner itself.
The second method would be to use a separate antenna solely for the purpose of transmitting the IBOC signals. This antenna should be optimized to closely match the main antenna pattern. Using a similar antenna design and paying attention to maintaining the same relationships to tower structural members can produce patterns within a few decibels of each other. Taking the same 100kW example as above, and using another antenna with the same amount of gain, (neglecting transmission line losses again) 333W would be required at the input of the antenna, and an amplifier that could put out 5.5dB over 333W, or about 1,181W. An amp this size will fit in a rack comfortably. No need to buy or install the high level combiner.
Figure 3. Newer transmitters, possibly with minor modifications, may be capable of passing the IBOC signal. In this case, a low-level combiner can be used. (Click image for larger view.)
Of course, the right choice will depend upon the circumstances. You could use the same antenna, and sink money in to a larger amplifier and combiner, and face larger electricity bills (and maybe even site rental), or you could buy smaller gear and install another antenna and transmission line. Obviously you’ll need to consider the cost of that on a monthly basis as well.
The AM side
Delivery and Processing: The requirements for AM IBOC are similar to that of the FM system. The audio that arrives at the transmitter site will need to be digitized into the AES format. The audio bandwidth of the analog portion of the transmission will be limited to 5kHz. Since this is not a requirement for the digital portion, it is desirable to use separate audio processing for the separate AM and digital portions of the transmission.
Transmission: Several transmitter manufacturers have already shown products at the NAB convention that can pass the IBOC carriers along with the normal AM carrier. Unless your transmitter is less than five years old, it is highly unlikely that it will pass the IBOC carriers, so if the boss is really serious (indeed this is the way to find out for sure) than make sure to budget for a new transmitter for the AM side.
Don’t neglect the antenna itself; the characteristics of the system, as seen by the transmitter, will need to be symmetrical �5kHz from center frequency. The frequency response of the antenna system will need to be symmetrical over the same bandwidth. If you had previously optimized your antenna system for stereo, then it will be optimized for IBOC as well.
Doug Irwin is director of engineering for Clear Channel San Francisco.
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