Download
1 / 37
370 likes | 480 Vues
DIGITAL CELLPHONE PROGRESS REPORT HIA Annual meeting Palm Springs, CA Feb 24, 2000 (Revised Aug 29) Mead C. Killion, Ph.D. Etymotic Research I. Background
E N D
DIGITAL CELLPHONE PROGRESS REPORT HIA Annual meeting Palm Springs, CA Feb 24, 2000 (Revised Aug 29) Mead C. Killion, Ph.D. Etymotic Research I. Background A. In 1994 my new European GSM digital cellphone knocked out TV from across a hotel room in Europe B. European cellphone mfgs took the position that hearing aid wearers shouldn’t buy cellphones. U.S. hearing aid wearers had no problem with U.S. cellphones at the time, which were analog. C. 1996 Sprint introduced digital GSM phones in Washington D.C. SHHH members reported they couldn’t use their hearing aids with (or even near) the new digital phones.
D. SHHH’s Donna Sorkin and others mounted a highly successful campaign, arguing that the Americans with Disabilities Act mandated that hearing impaired persons should have access to the “better” digital cellphones. In other words, this was not Europe, where the digital cellphone manufacturers could (and still do) tell the hearing aid wearers and manufacturers to buzz off. E. Sorkin asked me to be a “consumer’s representative” at the joint cellphone/hearing-aid-industry meetings. E. U.S. Congressmen became involved, and FCC chairman Hundt called a summit meeting, saying “solve it” to the several hundred representatives of cellphone companies and hearing-aid companies in attendance. F. Several meetings ensued, with over 100 engineers and lawyers in attendance. We broke up into groups of 30- 50 each to discuss how to solve the problem.
G. Barry Kranz, one of the cellphone representatives, made up this slide, which expressed an ideal result: cellphones would get down to 10 V/m and hearing aid immunity would go up to 30 V/m, giving a good margin of safety. It now appears unlikely that cellphones will get down to 10 V/m. The cellphone must radiate a fair amount of RF. (If progress does come, it will probably be motivated by a desire to reduce RF radiation absorbed by the brain.) The fact that cellphones now work with hearing aids is largely a result of hearing-aid design improvements.
H. NEED FOR A STANDARD 1. Harry Teder and I proposed a $500 “suitcase laboratory” comprised of Radio Shack sound meters and a boom box playing a special “buzz” CD we made. 2. The majority of the committee members decided that a more sophisticated measurement was required, and that both sides needed to agree upon a Standard defining how we were going to measure a) cellphones and b) hearing aids. 3. After countless meetings and some four years, we have a draft of a standard that is reasonable but not perfect. No engineer I know is willing to spend another year trying to perfect it.
RECOMMENDATION: 1. Once the hearing aid engineers you have been sending to these standards meetings are convinced that the standard is workable, give it your support. Either the dipole method (original U.S. preference) or the GTEM method (European preference) will permit hearing aid manufacturers to legitimately claim that a hearing aid passing the limits will work fine with nearly any digital cellphone, and especially a cellphone that also passes the cellphone limits in the standard.
FDA Given the past behavior of the FDA, if we approve a standard there is a fear that the FDA -- in a fit of enthusiasm -- may force us to test all hearing aids according to that standard, adding perhaps $10 to the cost of each hearing aid. Since manufacturers here and abroad report only about 1 in 1000 hearing-aid purchasers request digital cellphone compatability, this would mean a cost of $10,000 for each person needing such an aid. RX: Obtain assurance from the FDA that they will limit their enforcement enthusiasm to truth in packaging: If a hearing aid company wants to claim “useable with red-dot digital cellphones,” then (and only then) they must measure samples per the standard. If a manufacturer does not want to make any such claims, then that manufacturer has no obligation to do RF testing.
FCC Other than sabre rattling at the beginning, the FCC has done little beyond sending observers to the meetings. Prominent FCC engineers are no longer being sent to our meetings. The FCC receives $4 billion from cellphone companies each time they sell another frequency band for cellphone use. The FCC has not provided any funding, to my knowledge, for any outside research. (Most has come from the cellphone companies. Some has come from HIA.) The only progress on the cellphone front has been voluntary work by cellphone manufacturers who want to be able to say their cellphones are useable with hearing aids. While the FCC would be happy if cellphones didn’t interfere with hearing aids, it is not a high priority. The only hope I see for help from the FCC is if SHHH or AG Bell apply additional pressure. I have come to view the FCC as neither friend nor foe.
ENGINEERS (our heroes) The number of players has dwindled over the years. A few on both sides have hung in there, including Tom Victorian of Starkey Laboratories, Ray Kirchhoefer of Knowles, Horst Arndt of Unitron, Tom Scheller, Harry Teder, and Steve Berger of Siemens cellphone division. Why hang in there? Like the garden rake waiting to whack us on the forehead, the basic RF problem holds unpleasant possibilities if we don’t solve it. “Bluetooth” chips are nearing production, and are predicted to replace every wire except the 110V line cord in your computer setup. Your monitor, printer, network connection, and computer will all be connected via this 2.4 GHz digital transmission. Our (Etymotic/Motorola) early experiments indicate Bluetooth transmissions may not be a problem for hearing aids, but if we don’t have a standard way to measure the interference from present and future buzz sources, we will be always coming from behind.
ROOM FOR IMPROVEMENT? On a personal basis, I am still wrestling with the desire for an even simpler standard. At present we have several levels of “goodness” for both hearing aids and cellphones, which I liked as an engineer, but as a consumer representative I suspect it is more complicated than necessary. On the other hand, we have put about 100 times more manhours into this question than can be justified by anything except sheer stubbornness. To add more effort to make further improvements is hard to justify. Only for the future can one justify all this work, but the others and I believe that we will ultimately be pleased that we have generated simple instructions for measurements. These should assure that a hearing aid labeled “useable with red-dot digital cellphones” will work with any digital cellphone that carries the “useable-with-hearing-aids” red dot.
APPENDIX: THE SEARCH FOR SOLUTIONS A. Efforts to solve the digital cellphone interference problem led to possible solutions: 1. Cellphones with remote antennae or earpieces. Those solutions were rejected by SHHH and others as less convenient and creating a stigma. 2. Hearing aids with better immunity. It quickly became clear that: a. Older hearing aid designs could probably not be made immune to digital cellphone interference. b. New designs could be made immune. c. The microphone preamplifier introduced the most buzz of any element. It had to be bypassed before any other circuit solutions could work.
3. In some cases, by pure luck, older circuit designs were immune. K-AMP 100 V/m
5. Most hearing aid manufacturers offered their best new models with cellphone-compatable immunity, although only a fraction of older designs could be used with digital cellphones. 16% of older designs
B. Experiments to determine how much buzz were underwritten by HIA and conducted by two groups: 1. Harry Levitt and Judy Harkins, and 2. Harry Teder, King Chung, and Mead Killion. 20 dB SNR, 90% of Ss 25 dB SNR, 90% of Ss Levitt et al data Basic conclusion: For 90% of the 43 subjects to rate the sound as “acceptable for regular use” with 50 Hz (TDMA) buzz, an SNR of 20 dB was needed. An SNR of 25 dB was required for a 217 Hz (GSM) buzz. Levitt and Harkins reported similar results for 217 Hz buzz. Note, however: Nearly half of the subjects accepted a 10 or 15 dB SNR.
C. Serious experiments heading towards a workable RF source were undertaken by Marco Candiago of Unitron, Mike Sasha at Starkey, a few of us at Etymotic Research, and Harry Teder. We all found that what our RF friends warned us was true: the common “farfield” measurement in a “GTEM cell” was often not adequate. The cellphone is only a fraction of an inch away and produces a high-gradient signal. The only way to reliably check the immunity of a hearing aid was with a high-gradient (nearfield) source. We settled on a dipole. 1. Marco found that some hearing aids that looked good in the GTEM cell did not function well with real cellphones.
2. Interestingly enough, I believe that the same conclusion can be drawn from data generously supplied this week by our European colleagues at DELTA labs in Denmark. GTEM--> One would expect SNR to go up dB for dB once it gets above 0 dB. We don’t see that and the scatter is very large. Perhaps adding another test angle to the GTEM test would have helped?
I emailed an advance copy of this report to Allan Kristensen of DELTA labs (who produced these data). He was kind enough to respond quickly suggesting -- reasonably enough -- that things might be better than I indicated. The GTEM limits DELTA proposed gave reasonable SNR correlation and useable SNRs, as shown in the marked-up graphs shown below (Kristenesn). Our data and Levitt’s earlier data suggest 25 dB SNR is required for 90% of subjects such a 217 Hz buzz; DELTA’s 4 subjects appeared to accept an 8 dB SNR. Perhaps English is harder to understand in a buzz noise?? 10 dB worse than expected from GTEM (mck) Phones positioned for best understanding, not maximum loudness.
3. Gert Ravn and his colleagues at DELTA labs had earlier presented data showing the great sensitivity of the dipole method to the postion of the hearing aid. We responded that the data looked exactly as one would expect given the behavior of RF fields. The hearing aids simply had to be positioned carefully.
4. Marco Candiago developed a Teflon fixture that would allow the hearing aids to be precisely positioned relative to the dipole antenna.
D. Finally, everything in place, we commissioned TUV to measure the hearing aid immunity (dipole method) and the University of Oklahoma to check, on real people, how our rating method would work. The results were disappointing. A group of us went to Oklahoma to sort out the problems. We finally located most, but not all, of the problems in the experiment.
1. We were making some progress, however. We finally had enough nearfield data on cellphones to see what we were up against.
2. DELTA labs showed that four hearing aids with GTEM IRIL values below 33 dB (equivalent to 55 dB in 75 V/m E field) were nearly always judged as “very clear” by actual hearing aid users. Bad with E55 = 75 V/m Good
DIPOLE VS. GTEM ARGUMENTS One of the pleasant results of recent interchanges between U.S. and European engineers is that both sides have moved from the position that their method was Divinely inspired. Both sides now acknowledge that both the GTEM and the dipole approach have merit. And both have weaknesses. Several of us have long believed that the far-field GTEM measure does not adequately test a hearing aid’s immunity to digital cellphone interference. We believed that only if a hearing aid is well designed does the GTEM measurement assure good performance in real life, in which case we were depending on the design and not the measurement. We believed the U.S. dipole measurement was a real contribution to our field.
DIPOLE VS. GTEM MEASUREMENTS At this point, it seemed clear that what was needed was one more (!) set of data: How did the dipole measurements differ from the European preferred GTEM method? Having a chance to visit DELTA labs in Denmark, I saw their facilities and met with Gert Ravin and Alan Kristensen of that lab. It became obvious that they were as interested as we were in obtaining the truth. The following plan ensued: We already had dipole data from TUV in Minneapolis on some 18 hearing aids, and it looked as if we could obtain Dipole data from DELTA, along with GTEM data, on those same aids for perhaps another $10,000. Argosy, Beltone, Knowles, Phonak U.S., and Etymotic Research agreed to split this bill. Fortunately, several European companies (GN ReSound, Oticon, Siemens, and Widex) agreed to split the bill with us. Fortunately, because the final bill was more than expected.
3. All rf levels are given for an IRIL (Input Referred Interference Level) of 55 dB SPL referred to the hearing aid input. The typical audio signal level produced by a cellphone at full volume runs 85-90 dB SPL as it reaches the hearing aid microphone input, so an IRIL of 55 dB should allow good use of the cellphone with that hearing aid. For example, cellphone #35 could be used with any cellphone producing less than 33-36 dBV/m rf signal (depending on measurement and frequency) at the hearing aid location. E fields are dBV/m (Volts/meter); H fields are dB
SURPRISE! It seemed clear from the data that the GTEM measurement was usually harder to pass than the dipole measurement. In other words, a hearing aid barely passing the GTEM test at a given limit would typically pass the dipole test with flying colors. This was a surprising result, especially to those of us who believed the dipole method should be a tougher test.
The disappointing thing was that even with all this work we only had six hearing aids compared, and no U.S. GTEM measurements had been made on these phones. We obtained quotes from TUV, DELTA, and Texas Siemens (who had the GTEM facilities) for the cost of testing three more high-immunity hearing aids from the group of 12 that had already been GTEM tested at DELTA. These 3 or 4 would be given dipole testing at TUV and DELTA, and GTEM testing at Siemens’s Texas facility. The cost estimate was about $15,000, small compared to the estimated $640,000 I once estimate hearing aid engineers had spent on this problem to date.
After some discussion, Harry Teder argued that it was simply time to stop testing. “The problem is solved. Most hearing aid companies have models that work fine with digital cellphones. Why keep on beating a dead horse? Regardless of how we measure these hearing aids, they will prove to be immune.” As if to add fuel to the argument, in the meantime the European standards community has given up on the supposedly simple task of correlating GTEM and free-field measurements (which should be nearly identical). Any method has problems, but both dipole and GTEM measurements can verify that a good hearing aid is good (immune). Adding the GTEM method to the U.S. standard would not weaken it by the data we had seen. Conclusions: Draw up the U.S. Standard to allow both dipole and GTEM measurements.
Revised ANSI C63.19 proposed U.S. standard being prepared for ballot: 1. The dipole version will have four categories: U0 30-35 dBV/m U1 35-40 dBV/m U2 40-45 dBV/m U3 >45 dBV/m 2. The GTEM version will have the same categories and limits. Any company can choose which measurement method to use. Note: Combined with cellphone ratings, a sum of ratings of 3 or better should allow normal hearing aid use with that cellphone. (In practice, most cellphones fall in the U1 category producing <41 dBV/m.) If a company chooses the GTEM method, then the test results should be applicable in Europe, where it appears the IEC will approve two limits: 37.5 dBV/m (75 V/m) for general use with digital cellphones 20 dBV/m (10 V/m ) for immunity to “standby” problems
Perfect, No. Reasonable, Yes. Everyone that I know of who has been working on this problem for the last few years thinks it is time to move on. This standard will permit hearing aid manufacturers to openly claim what is true: Some hearing aid models are immune to digital cellphone interference.
July 21 was the 10th anniversary of the Americans with Disabilities Act, so an effort was made to have the standard approved or disapproved by that date (two weeks before the normal C63 deadline). That effort succeeded, and the standard is now being published for final comments. Once those comments are satisfactorily answered, the standard should go into effect. ____________________ This summary was made with the help and counsel of Tom Victorian, Steven Berger, Harry Teder, Tom Scheller, and many others who are, nonetheless, not responsible for any of the author’s remaining errors.
