Transmission: Implementing IBOC

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Transmission: Implementing IBOC

Oct 1, 2000 12:00 PM, Doug Irwin

Digital radio: coming to a station near you Over the last several years, U.S. broadcasters have been deluged with press about the coming of digital audio broadcasting and specifically in-band on-channel (IBOC) DAB. The number of proponents whittled down to one since Lucent Digital Radio and USA Digital Radio recently merged to form iBiquity Digital. The proponent's test methodologies and results have been reported, and many people have had a chance to hear demonstrations at various conventions. Also, the sense of urgency is waxing since satellite digital radio is at hand.

So, let's jump ahead just a bit. Let's assume that the iBiquity system works and is subsequently approved by the commission. There are several questions that every station will ask: How will IBOC work on my station? Will stations need to replace every single piece of RF hardware, or will IBOC work with what is currently used? Will the current audio processing chain work with IBOC? Will the newly purchased digital audio delivery system and its audio compression sound good on an IBOC radio?

From the studio end The iBiquity system most likely will use Lucent's Perceptual Audio Coder (PAC) at a data rate of either 96kb/s or 128kb/s for the FM band and 48kb/s for the AM band. PAC also will be used by the satellite radio providers. IBiquity wants to maximize the likelihood the receiver manufacturers will build receivers that will work not only with satellite-delivered digital radio, but with AM and FM IBOC as well.

I spoke with a well-known audio processor designer while researching this article. I was told that the audio being fed to the IBOC system should never be compressed to less than 384kb/s (referring to MPEG-1/Layer II). This designer went on to stress that audio fed in to the IBOC system should be uncompressed.

Now consider all the ways in which broadcasters use data compression. The most pervasive application of data compression is the storage of program material in the form of data files on a computer. How compressed are your audio files? Do you receive program material from a network or satellite feed that makes use of data compression? Do you receive remote broadcast feeds or talent feeds via an ISDN codec or perhaps via e-mailed MP3 files? And, perhaps most importantly, does your STL use a compressed data format? These are aspects of your operation that will need some attention. Right now, most broadcasters agree that transcoding loss, the audible effect from sequential use of different encoding and decoding algorithms, will render the final audio product unappealing at the consumer end, where it counts the most. This would obviously defeat the purpose of DAB, which is to provide a superior audio product.

Another operational consideration is that of the long delay inherent to the IBOC system. In the USADR system, when the listener turns on an IBOC receiver, the first audio heard will be an analog signal. The digital receiver takes some finite time to acquire the signal and begin decoding. Once the digital signal is acquired and reliably decoded, the digital signal will blend gently with and replace the analog signal in time. Another reason this is done is so that if the digital signal is lost, for example when a mobile receiver travels under an overpass or around a large building, the receiver will blend in the analog signal while it reacquires the digital signal. The audio delay allows the receiver to complete this blend before the listener notices a signal problem. Once the digital signal is reacquired, digital audio will once again replace the analog.

From a station-operation standpoint, the halcyon days of listening off-air are rapidly coming to an end. There already are some stations with an air monitor that switches to a separately processed feed when the jock turns the mic on. There are, of course, stations that use a profanity delay for on-air protection against errant lingual occurances. Now, however, there will be a long delay on the order of 8.5 seconds (4.5 for the DSP and 4 for time diversity) all the time. Stations with off-site traffic or news reporters will need to plan an alternative method of monitoring the station because off-air monitoring will be impossible. Remote broadcasts via ISDN codecs have faced this challenge for some time already. A pre-delay air monitor system will be required.

Processing and STLs The designers at iBiquity anticipate that broadcasters will want to process analog audio separately from digital audio. For this reason the IBOC system has two audio inputs. This means that two audio processors will be needed. For complete audio chain redundancy, four audio processors will be necessary. (Perhaps you should reconsider donating your old processor to the local college station.)

Stations that enjoy the convenience of having all of the audio processing located at the studio most likely will have to buy a new STL or two. There now will be two separate air chains to be delivered to the transmitter site. Undoubtedly, STL manufacturers will devise a clever and straightforward way to accomplish this. AM-band stations will need an STL to carry the processed analog program (bandwidth limited to 5kHz mono) in addition to the PAC-encoded digital audio stream, which will be around 100kb/s after forward error correction is added. FM-band stations' STLs will have to carry the analog signal (still 2 channels of 15kHz audio) along with the PAC-encoded digital data stream, which will require about 240kb/s after forward error correction is added. Are you thinking capital budget yet?

The most economical way to get around the STL issues mentioned above will be to place the processors at the transmitter and deliver a single audio feed to the transmitter.

At the transmitter There are three ways to accomplish the transmission of IBOC on the FM band. What an individual station does to facilitate the process will depend completely on the particular circumstances at the transmitter site.

The first approach uses a combiner fed by the station's analog transmitter and digital transmitter. Similarly, it has two outputs: one for the combined RF signal with the analog and digital signals, and one output for wasted power. The combiner system has substantial losses on both the analog and the digital path. For example, for a station with 10kW total power output (TPO), the transmitter power will need to be increased to about 11.1kW to make up for the combiner loss. In this example, the combined digital power component will be about 63W because the ratio of the digital input power to the digital output power on the combined port is 10dB. The digital TPO needs to be 630W. It is important to note here that this will be the average power. Unlike the FM carrier familiar to most broadcasters, the digital transmitter output will vary in amplitude. In fact the peak-to-average ratio can be up to 6 dB. Ibiquity currently is working on a method to greatly reduce this variation. What this means is that the digital transmitter must be designed to handle 10 times its average output (albeit on an extremely small duty cycle).

The implications here are fairly obvious. Many stations do not have an analog transmitter capable of an additional 10 percent of output power to make up for the combiner loss. Additionally, it is not hard to imagine that the digital transmitter I mentioned in the above example will take up a considerable amount of floor space or rack space, which is precious at many transmitter sites. An additional transmitter at any site has its own requirements for main power, backup power and HVAC.

The second way to implement the IBOC scheme is by using a transmitter that can pass both the analog and digital signals all the way through, so that they can be combined at a lower power level. This sounds very good, but, not surprisingly, there are implications. Since the digital input signal varies in amplitude, the final amplifier that comprises this theoretical transmitter will need to reliably pass this signal. Instead of having one relatively small transmitter capable of passing the digital component operating in conjunction with an analog transmitter as in the example above, the entire transmitter must be capable of transmitting both the digital and analog hybrid signals. In the 10kW TPO case, the new transmitter would need to have the capability of delivering about 13.4kW, with a peak-to-average ratio of 6dB.

While this methodology eliminates the combiner, it complicates the transmitter requirements dramatically. In most cases, stations that want to transmit an IBOC signal in this fashion are going to need to replace their transmitter. If system redundancy is required, the cost will increase very quickly.

The third methodology is to use a separate digital transmitter feeding a separate antenna. Assuming that this digital antenna is physically close to the analog antenna portion (to forego any complicated licensing issues) the situation is simplified and made much less expensive by eliminating the high-power combiner requiring a lower-power digital transmitter. Station groups in a market can take advantage of the efficiencies of scale. Up to five digital transmitters can feed a low-power combiner connected to a single, broadband antenna. This saves floor and rack space, and it reduces power consumption and air conditioning load.

Fortunately, the bandwidth requirements for filters, combiners and antennas will remain the same. The overall bandwidth of the FM IBOC signal will be 200kHz, and current filter/combiner and current VHF antenna technology will handle this increased bandwidth.

Saving AM Many broadcasters feel that AM IBOC will save the venerable band. Those that have heard AM IBOC demonstrations will agree. Will it be easier or less expensive to implement AM IBOC than its FM counterpart? Not surprisingly, the answer will depend upon the individual station's circumstances.

The transmission schemes for AM and FM IBOC are very similar, but one of the major differences is that some of the many carriers used to transmit the digital signal on the AM band will actually fall within the normal, analog passband (which, as was mentioned earlier, is 5kHz). (All of the digital carriers in the FM system fall outside of the normal passband.) Like its FM counterpart, these carriers are modulated in pairs with the same data. Half of the pair will be above the center frequency, and the other half below. For this reason, symmetry in the frequency and phase response of the AM phasor and ATU system will be crucial beyond 10kHz, just as it was for AM stereo. Poor system symmetry will lead to a higher bit error rate at the receiver. In fact, an AM transmission system optimized for AM stereo already is optimized for AM IBOC.

One lesser-known benefit of AM IBOC is the fact that the receiver does not use the main carrier in the detection process, instead relying completely on the low-level digital carriers that are transmitted along with the main carrier. The result should be good IBOC performance even in a directional station's nulls.

Like in the days of AM stereo, one of the most crucial performance aspects of a transmitter will be that of incidental phase modulation. This will result in an increase of the bit error rate at the receiver, thus limiting the system performance. Most older transmitters are not going to be able to meet this requirement.

Transition from hybrid stage So far, this article has discussed only the AM and FM hybrid systems. Eventually, stations will abandon the hybrid schemes and transmit a completely digital signal. Ibiquity has also developed completely digital transmission schemes for the AM and the FM bands.

With the current state of AM, I can see a day in the not-too-distant future in which a completely digital signal will be a reality. An AM owner may have little to lose to attempt a digital-only experiment. In a sense, the entire scheme is simplified because the need for backward compatibility is obviated. In practice, the digital signal in the digital-only IBOC carrier will be 13dB greater than the digital carrier in the hybrid system. The antenna requirements remain the same.

Final transition The coding and data rate will remain the same as in the hybrid AM system at 48kb/s using Lucent's PAC algorithm. There will be an additional data stream available in the all-digital IBOC system, but it will be limited to 16kb/s, with a slight time delay.

Like its digital AM-band counterpart, the all-digital FM IBOC signal will contain a secondary data stream, also slightly delayed in time and limited to 24kb/s. The main signal will still be transmitted using the PAC algorithm, with a data rate of 128kb/s or 96kb/s. The level of the digital carrier will be increased by about 10dB compared to the FM hybrid system. The use of only one type of transmitter obviously eliminates the need for high-power combiners or separate antennas.

Changing times It is important to note that while this article was being written, the Lucent Digital Radio and USA Digital Radio teams are in a post-merger technical review, and aspects of the operation of the iBiquity system as described are subject to change. The best elements of each system will be used in the final product.

It is obvious that there are many issues to face at the average station prior to the implementation of IBOC. The expense is great enough, for either AM or FM, to prevent IBOC implementation from being a slamdunk capital budget issue - even at the major market level.

The one crucial element being considered by every station is to what degree do consumers want digital radio. No company manager will approve a large capital expense without at least some reasonable assurance that the investment is going to pay off. Although satellite radio promises to be a competitive threat to some stations, at least it will afford stations a way to measure the level of interest of consumers in digital radio. I hope consumers snap it up.