DIGITAL RADIO RESEARCH (DRRI) INC.
EIA/NRSC DAB SYSTEM LAB TEST RESULTS: AN ASSESSMENT
Eureka 147 outperforms all in-band systems!
KEY FINDINGS
The Eureka 147 System produced results that were far superior to any of the IBOC systems with respect to audio quality, signal reliability and non-interference to existing analog services.
FM IBOC systems would produce unacceptable interference to their "host" FM station, as well as to nearby stations that operate on adjacent frequencies.
AM and FM IBOC systems would produce substantially-reduced service coverage, compared to that of their analog "host" stations.
The performance of FM IBOC systems degrades considerably, even to the point of failure, in the presence of multipath. The AM IBOC system cannot provide CD-Quality audio and produces impairments that expert listeners judge as "annoying".
BACKGROUND
On August 22, 1995, the Digital Audio Radio Subcommittee of the Electronic Industries Association (EIA) released the results of independent laboratory tests conducted on seven proponent Digital Audio Broadcasting (DAB) systems. (Two systems operated in t wo modes each, making for nine tests in total.)
Measurements and related audio recordings for each system were made at NASA's Lewis Research Center (LeRC) in Cleveland OH. Subjective assessments of the audio recordings were carried out at the Communicat ions Re-search Center (CRC) in Ottawa ON, under contract to the EIA. These tests are the first time all proposed DAB systems were assessed by an independent body using the same evaluation criteria.
This report outlines conclusions drawn by Canadian DAB experts who have reviewed the results and were present at a technical tutorial session in Monterey, California, from 24-25 August 1995.
TESTS PERFORMED
The main purpose of the laboratory tests was to determine the basic digital audio quality produced by each system, its reception reliability, and its ability to co-exist with other stations, including the "host" analog station. In co-operation with the N ational Radio Standards Committee (NRSC), the EIA developed a complex series of tests to determine these factors. Each proponent had the opportunity to propose system-specific tests that would best illustrate its operating features. All system proponent s took an active part in the subcommittee that developed the testing procedures. Each system was operated in accordance with the developer's specifications and tests were conducted using DAB encoders and receivers that were supplied by the proponents them selves. SYSTEMS TESTED
The DAB systems (and modes) listed in the Appendix were evaluated in the EIA tests. All comments and observations in this report relate only to the first seven system proponents listed. i.e. Eureka 147 and the six In-Band On-Channel (IBOC) proponents. T he AT&T In-Band Adjacent-Channel (IBAC) system is not a serious contender for a North American standard, as it utilizes adjacent FM channels and evidently would require significant frequency re-shuffling in most markets to make it practical. The VOA/JPL system is not discussed, since it is designed for satellite-delivered DAB in the 2.3 GHz band, allocated only in the USA and India.
TEST RESULTS
When the basic digital audio quality of each proponent is assessed in a lab setting, using strong signals and no induced impairments, the ratings for all system proponents, with the exception of the USA Digital AM IBOC system, are quite similar.
The Eureka 147 system (224 kbits/sec) rated the highest of all, even though the two USA Digital FM systems employ a higher data rate (256 kbits/sec) and use the same MUSICAM audio coding system.
Even with strong signals and no interference, the USA Digital AM IBOC system suffers audio quality impairments that experts judge to be "annoying"; consequently, this system is not capable of providing "CD-Quality" DAB service.
Although all DAB receivers require time to recover when signals fail or listeners change frequencies, the recovery time of IBOC receivers is far too long to be practical in a real-world environment. The Eureka 147 system generally recovers from signal loss in 1 second or less.
The IBOC systems can take from 5-9 seconds to recover.
When tested with five common household, portable, and auto receivers with known operating characteristics, IBOC FM DAB produces significant impairments to existing analog services on first and second-adjacent channels.
In a majority of the tests, expert listeners judged the stereo FM analog service to be "worse" or "much worse" when an adjacent-channel station, carrying an IBOC DAB service, is present. This interference tends to worsen when multipath occurs. FM stations operating one channel apart on the dial are said to be "first-adjacent", while those that are separated by two channels are "second-adjacent".
Multipath interference occurs when FM signals reflect from large objects, such as buildings and mountains, causing several time-delayed versions of the same signal to arrive at the receiver. When tested with five common household, portable, and auto receivers with known operating characteristics, IBOC FM DAB produces a significant impairment to the quality of the FM stereo audio on its "host" analog station.
IBOC signals produce objectionable background noise in FM analog re-ceivers. Many of the test reports from expert listeners said that the quality of the FM stereo analog service was "worse" or "much worse" when the station was carrying an IBOC DAB sign al. IBOC impairments to the FM stereo service are more substantial on home tuners than on auto receivers, probably due to the reduced bandwidth of the latter.
If two FM stations having a first or second-adjacent channel relationship (and standard geographical spacing) were both to imple-ment IBOC, their useful DAB service areas would be significantly less than their analog coverages (up to 32% for first-ad jacent Class C1 stations), in the zone between the two stations.
FM IBOC system performance and interference impairment worsens significantly in the presence of multipath.
Of the IBOC systems, the AT&T/Amati system performed best in a multipath environment, although failures still occurred under certain conditions. The USA Digital FM-1 and FM-2 systems generally produced degraded performance (or failed completely) whenever multipath was added to the signal.
If two neighbouring first-adjacent-channel AM stations were both to implement IBOC DAB, the digital signals would fail wherever the desired station's signal is not at least 34 times stronger than that of the undesired station.
Many AM stations in urban markets would experience DAB coverage that is substantially smaller than their AM service areas. Nighttime AM DAB service would likely be impractical for most stations, due to the presence of strong adjacent-channel skywave signals.
CONCLUSIONS
The independent test results provided by the EIA confirm that the digital radio concept that Canada has developed (Eureka 147 in a new band at 1452-1492 MHz) will indeed provide the highest quality DAB service. The tests showed the Eureka system to be fa r superior technically to any other proponent system and confirm the extensive evaluations conducted in Canada and Europe since 1990. Moreover, as Eureka 147 will operate in a new band, it automatically avoids any impairments caused to, or suffered from, existing analog services. The In-Band systems showed particularly badly with respect to the key attribute their proponents have always touted - their ability to co-exist in the AM/FM bands without causing interference to analog services. Demonstrations in carefully controlled env ironments may have produced promising results previously. But the independent lab tests show that IBOC fails when it is operated using simulations of real-world impairments, such as multipath and adjacent-channel interference.
The next step in the evaluation process is to examine system performance in the field. Current plans of the joint EIA/ NRSC testing committee call for this to be done in the San Francisco area later this Fall.
APPENDIXSystem NameSource CodingData RateSystem TypeProposed Band Used Tested(kbits/sec)
Eureka 147 MUSICAM 224 New-Band 1452-1492 MHz
Eureka 147 MUSICAM 192 New-Band 1452-1492 MHz
USA Digital FM-1 MUSICAM 256 In-Band, On-Channel (IBOC) 88-108 MHz
USA Digital FM-2 MUSICAM 256 In-Band, On- Channel(IBOC) 88-108 MHz
USA Digital AM MUSICAM92 In-Band, On-Channel (IBOC) 525-1705 kHz
AT&T/Amati LSB PAC 128 In-Band, On-Channel (IBOC) 88-108 MHz
AT&T/Amati DSB PAC 160 In-Band, On-Channel (IBOC) 88-108 MHz
AT&T PAC 160 In-Band, Adjacent Channel (IBAC) 88-108 MHz
VOA/JPL PAC 160 Direct Broadcast Satellite 2310-2360 MHz
For more information contact Wayne Stacey at (613) 830-6985.