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Kaylee
22nd January 2009, 10:51 PM
Is there a fool proof way to select 5 FM channels in this type of listening system (http://www.geminidj.com/uz1128.html)without using a trial and error approach? Basically I want to select 5 channels whose base carrying signal harmonics won’t overlap and cause annoying interference sounds. (I hope I expressed that correctly. I'm not an engineer nor am I a musician. )

One of my associates called the company that manufactured the sound FM system and they were of no help. I’m sure that this is possible because over 10 years ago I had purchased an 8 channel FM system where the channels were static (that is, could not be changed) and I was able to leave all 8 mic/transmitters on at the same time with no interference. A professional engineer was able to give me 8 channels that he said would work well together, and he was correct.

I no longer have the info, and the engineer has passed away. The company’s help department is of no help. I would think this type of information might be available on a chart somewhere on the internet.

I belong to a committee of a non-profit organization run by volunteers that has donated a listening system to a library branch of the New York Public Library, (http://forums.randi.org/showthread.php?postid=3109852#post3109852) that’s why I want the information. Any help would be greatly appreciated. Thanks!

69dodge
23rd January 2009, 09:12 AM
Do you know what the exact frequencies are that you can choose from?

The instruction manual I found at your link says that the range is 740-770 MHz, and that there are 700 channels, but it doesn't say what the frequency of each is.

Anyway, here are a bunch of groups-of-5 that I computed, assuming that you can choose the frequencies with unlimited precision. Of course you can't, so you'd need to round each to the nearest frequency that the system supports. I don't know how big a difference that might make.

These groups try to maximize the difference between harmonic frequencies:(741.6491368107963 747.0500933929579 755.7890247361502 764.527956079226 769.9289126613876)
(741.6102228519158 747.0108960591242 755.7493688689719 764.4878416777356 769.8885148854315)
(741.486244387459 746.8860147430096 755.6230267073261 764.3600386467297 769.7598090126412)
(741.3233519929927 746.7219360929448 755.4570286783855 764.1921212638263 769.5907053637784)
(741.1837649893423 746.5813325765994 755.3147803876782 764.0482281976438 769.4457957853883)
(741.1604595694225 746.55785744288 755.291030649445 764.0242038310971 769.4216017149156)
(740.9975671749562 746.3937787928153 755.1250326205045 763.8562864481937 769.2524980660528)
(740.8346747516189 746.2297001427505 754.959034591564 763.6883690156974 769.08339441719)
(740.7573071287188 746.1517690940891 754.8801919067337 763.6086147204915 769.0030766853743)
(740.6717823569197 746.0656214926858 754.7930365626235 763.5204516325612 768.9142907683272)

These groups try to maximize the ratio between harmonic frequencies:(741.4642275005754 746.0553814169252 753.4840244889492 760.9126675644075 765.5038214791566)
(741.1542508167913 745.7434853543527 753.1690228064544 760.5945602618885 765.1837947978638)
(740.8442741329782 745.4315892916638 752.8540211238433 760.2764529593696 764.8637681164546)
(740.5342974492232 745.1196932290331 752.5390194412903 759.9583456568798 764.5437414351036)
(740.2243207654101 744.8077971663442 752.2240177586791 759.6402383543464 764.2237147536944)
(745.7956047518892 750.4135785946564 757.8856172311716 765.3576558676759 769.9756297104977)
(744.9840752581513 749.5970241021132 757.0609321199154 764.5248401376903 769.1377889817231)
(744.1725457643879 748.7804696095554 756.23624700863 763.6920244076882 768.2999482529194)
(743.3610162706864 747.9639151170995 755.4115618974538 762.859208677688 767.4621075242248)
(742.5494867769376 747.1473606245563 754.5868767861903 762.0263929476951 766.624266795443)

I don't know which is better.

(They're easier to read if you make your browser window wide enough for each group to fit on a single line.)

Kaylee
23rd January 2009, 04:13 PM
Thank you! Thank you! Thank you!

I owe you a beer at the very least. If I remember correctly you’re in the NYC area, just send me a PM when you want to collect. :)

The first combination we tried from the second group (that maximizes the ratio between frequencies) worked. We had also tried the first combination from the first group, but that didn’t work.

Since you asked – the system supports a precision of up to the 3rd decimal place and usually lets you select a frequency at every .025 increment, but sometimes it doesn’t and it skips to the next .050 increment.

We ended up using these frequencies:
741.475, 746.075, 753.475, 760.925, 765.525

Now I have another question for you – how were you able to calculate this? And so quickly?

69dodge
24th January 2009, 07:21 AM
You're welcome.

I had help. Computers are fast. :D (And I know how to program them.)

Basically, I generated lots of groups of five frequencies each, more or less at random; for each group, computed a bunch of harmonics by multiplying each frequency of the group by 1, 2, 3, ..., 100, and checked to see what was the closest any of them got to any other (closer is worse); then picked the best ten groups.

It wasn't exactly at random. I used a method that I figured was likely to give good results. But afterwards I thought about it some more, and now I can't decide whether it was better than random, or worse! I may play around with it a bit more. Anyway, even if they tended not be so good, I tried lots of them and picked the best, and that seems to have worked ok.

I have no practical experience with these kinds of microphone systems. Did you try different frequencies before asking here, but always encountered interference? Maybe almost any numbers would have worked. Then it's not so impressive that mine did.

Kaylee
24th January 2009, 03:58 PM
That’s great!

You might want to consider releasing your program for the shareware market, and possibly also for the palmtop and palmtop competitors.

I don’t know much about those markets, but I’m sure I’m not the only one that got very frustrated by this question: what is the best way to select multiple FM frequencies that can transmit at the same time without interference? I would have thought that Gemini would have had charts available on their web site or at least answers available from their help department – but my associate claimed no luck. And as far as I can tell, Gemini’s competitors’ don’t make this information easily available either. I would think that anyone who uses a multi-channel wireless system would need this information.

Several people advised us just to try the hit or miss method. This included Gemini (the wireless system manufacturer), the wholesaler for the induction loop system and his local rep (a company that does electrical work) who installed it for us and helped us with the PA and FM system that it interfaces with. I think the statistical equation for the number of possibilities for a working 5 channel system out of a pool of 700 frequencies is 700 * 699 * 698 * 697 * 696? Ha! No thanks. Plus since we were working out of a branch of the New York Public Library this meant we had to reserve the time we worked on the listening system in advance and juggle several people’s schedules, and because of this we usually only had use of the room for relatively short periods of time. Since we were trying to resolve the problem quickly and satisfactorily before we had to decide whether to keep or return the system – we were really motivated to try to find out how to calculate the answer mathematically and decisively since I knew that a method existed. If we had to have returned the system, that would have lead to other difficult problems. I admit it was tempting because as far as I’m concerned the wholesaler and the local rep we were dealing with were not as knowledgeable as they should have been, and the installation process was pretty rocky. The local rep was being paid to handhold a few people during the process who were admittedly not technical and they really dropped the ball. We had a lot of other problems (since solved) that I haven’t described, and we simply shouldn’t have had them. The installation job was, I understand, actually a simple one for anyone who tends to be handy. It basically was not much more complicated then installing a home entertainment system, and putting up some bookshelves. The main reason we needed to have the local reps was because the library required the work to be done by a licensed contractor with insurance. /end rant

So to answer your question, we (the committee members) didn’t spend a lot of time trying different frequencies. I think the local contractor said they tried a few, but they don’t have a lot of credibility with me.

If you are curious to see how your computer generated frequency combinations work in real life – I’d be more than happy to reserve time in the library’s community room if you want to try them out. Just let me know when. It wouldn’t make sense for me to try them by myself because while I can hear the louder interference sounds, I can’t hear the quieter ones. (I just learned that there were additional quieter interference sounds I couldn't hear yesterday... Apparently after we tried the winning combination :), the contractor let me know that his combination had quiet interefernce sounds that I had never heard. We have had people without hearing loss also use the system, but they never mentioned it. I guess they wanted to be polite. Sometimes being polite is NOT helpful. /end second rant. ) The other people on my committee either couldn’t hear all of the interference sounds either, or they simply wouldn’t be interested. Now that the system works, they are ready to move on to other projects.

FWIW, the last combination that we tried was

740.975
741.975
762.975
743.975
744.975

I could hear some interference occasionally and apparently, as I just learned, the people that could hear well could hear some slight constant interference.

Also FWIW, I couldn’t hear any interference between 740.975, 741.975 and 743.975. I thought 762.975 frequently caused interference with the other channels and that 743.975 and 744.975 sometimes interfered with each other.

Thanks again for coming up with a winning combination! I think the system sounds good right now. One hearing person, (one of the contractors regrettably) agrees also. I’ll have more confirmation after our next committee meeting on the 3rd. A couple of people without hearing loss who are not contractors will be stopping by, so I’ll be able to get their opinions also.

69dodge
26th January 2009, 06:40 AM
So. I've done a bit more reading, and apparently I was completely confused before about the mechanism that causes some sets of frequencies to interfere with each other. I now have no idea why my numbers that you tried worked; they really shouldn't. (But I do understand why the numbers you originally tried didn't work; they shouldn't work either.)

It sounds like your project isn't quite finalized at the moment, so in case you aren't sick of the whole thing yet, here are some new and improved numbers that should work much better (unless I'm still confused, which is a possibility of course):(741.35 745.9 763.9 765.75 769.65)
(740.15 740.8 758.0 768.4 769.5)
(740.25 745.8 748.15 767.25 769.35)
(742.65 750.45 751.0 764.5 765.35)
(740.45 747.05 765.25 765.35 769.55)The theory is that interference among a set of frequencies is caused by intermodulation products. This means that you add together some of the frequencies, then subtract some others, and if the result is too close to any frequency of the set, that frequency will experience interference. (You're allowed to add or subtract frequencies in any possible way. You're even allowed to add the same frequency more than once, or subtract it more than once.)

For each set above, no way of adding or subtracting up to 13 frequencies yields a result that coincides with any frequency of the set. (Obviously, some repetitions are needed to get to 13, as there are only 5 per set.) The minimum distance for the fourth set is 0.05 MHz; for the rest, it's 0.1 MHz.

Kaylee
27th January 2009, 03:15 PM
I admit I didn’t understand your post. :o

Here’s my current understanding of this topic based on what I’ve been told by various people over the years. (Granted this is on a really simple level.):

* It’s the nature of sound to have harmonics. A sound whose base frequency starts at 2000 Hz will have harmonics at 4000 Hz, 6000 Hz and 8000 Hz, etc. Each successive harmonic will have less volume.

* Radio frequencies have harmonics also and it's calculated the same way.

* In a multi-channel system, to avoid interference you need to prevent the base signals and any of their harmonics from being very close to each other. If they are too close – one will hear interference sounds.

I’ve been given the analogy of tossed pebbles in a pond. The pond is the pool of radio frequencies available. Each pebble is a base carrying signal. Each ripple of the tossed pebble in the pond is a harmonic. The idea is to select the base carrying signals so that their harmonics won’t overlap, or to borrow from the analogy so that the ripples in the pond from the tossed pebbles won’t overlap.

So based on all that, when you told me in your 1st post on Jan 23 that the 2nd group of frequency combinations maximized the ratio of harmonic frequencies, that made sense to me.

But I admit that I don’t understand why it matters if the harmonics overlap if there are no receivers set up for the frequencies at which they overlap. So your first approach, maximizing the distance between the original or base frequencies made sense to me also.

I’m going to try to find more information on the web that can explain this in “laymen’s” terms.

Probably the sources you are using right now would be over my head, but if you can post or PM them, I’d be glad to give it a shot.

As per my PM, I’m finding out what times the room is free over the next few weeks and I’ll let you know.

This stuff fascinates me as much as it frustrates me, and I’m really curious to try the various combinations. I’m glad that you’re curious about this also!

JoeTheJuggler
28th January 2009, 10:08 AM
I don't know what your device is, but I've got a Shure wireless mic that has a bajillion possible channels. They've got a website where you put in the zip code where you're using the device, and it gives you the optimum channels. (Plus it gives them to you in the numbers used on the device rather than the actual frequencies. My best is channel 2 on group 2--next choice is anything else on group 2.) This takes into account the UHF TV broadcasts in my area. The receiver itself, though, also does a scan when you power it up and tells you whether the selected channel is clear.

A long way of saying, maybe the manufacturer can also give you some advice.

Kaylee
28th January 2009, 03:16 PM
My HLAA chapter has the Gemini UZ-1128 wireless system. We ordered one with 4 handheld mics and one lavalier mic. (FWIW, we’re going to switch to a headset-mic – the lavalier mic is useless.)

We can also choose many channels (700 choices between the 740-770 MHz band as 69dodge noted above).

But the Gemini Tech department is passing on erroneous information by saying that the only way to select channels is to just randomly pick a few and try them. They also offer no technical advice on how to make the selection and their tech people even go as far as to say that it’s not possible to do that – which I’m sure is wrong information.

So based on my chapter’s experience with this company, I wouldn’t recommend that anyone purchase a Gemini system. The Shure web site looks very helpful – I’m quite envious.

In addition to the Wireless Frequency Finder that you spoke about they also have charts that show which frequencies can be used together.

[ ETA: http://www.shure.com/ProAudio/TechLibrary/index.htm

See the Wireless Tools section. ]

Unfortunately none of their products deal with my system’s UHF band 740 – 770 Hz, so it doesn’t help me.

I do think that I got good results with the first group of channels in the second category that 69dodge posted in post #2 of this thread. Thanks again 69dodge!

69dodge
28th January 2009, 05:36 PM
But I admit that I donít understand why it matters if the harmonics overlap if there are no receivers set up for the frequencies at which they overlap.

Right. Exactly. That doesn't matter, which is why what I initially tried was wrong.

So your first approach, maximizing the distance between the original or base frequencies made sense to me also.

No, I think you misunderstood. Both approaches of my initial post dealt with all the harmonics, trying to keep them as far from each other as possible. The only difference was regarding the precise meaning of "far". Is 10 the same "distance" from 9 as 100 is from 90, or as 100 is from 99?

But, anyway, both are wrong, so it's not very important.

Probably the sources you are using right now would be over my head, but if you can post or PM them, Iíd be glad to give it a shot.

I looked at these web pages:

http://users.tpg.com.au/ldbutler/Intermodulation.htm (mainly the beginning)
http://en.wikipedia.org/wiki/Intermodulation
http://www.mwrf.com/Article/ArticleID/16649/16649.html

BTW, I'm pretty sure the last of those pages has a typo on the last line of equation (5). One of the two occurrences of cos (ω1 + ω2)t should be cos (ω1 - ω2)t.

In addition to the Wireless Frequency Finder that you spoke about they also have charts that show which frequencies can be used together.

[ ETA: http://www.shure.com/ProAudio/TechLibrary/index.htm

See the Wireless Tools section. ]

I just tested some of those frequencies with my latest program, and it seems to like them. So that gives me more confidence that I'm on the right track now.

Speedskater
28th January 2009, 07:09 PM
FM radios often use an intermediate frequency in receivers. A typical broadcast receiver uses 10.7MHz.

Kaylee
29th January 2009, 08:35 PM
69dodge and I met earlier today and we tried several sets of channels from his new and improved program. They all worked and I’m a happy camper! Thanks again 69dodge!

We discussed intermodulation a little bit also, and I was going to write a post about some of its mysteries – but I’ll save that for another day.

Today I think I found out why Gemini breaks every sales and marketing rule in the book by telling people that they have to manually pick their way through over 700 channels to come up with a combination of frequencies that will transmit without any intereference, and also why Shure mysteriously skipped over my system’s frequency range: 740 MHz – 770 MHz.

It may soon no longer be legal to use wireless microphones on the “ 700 MHz Band” or, more specifically, between the 698 – 806 MHz frequencies.

See: http://tinyurl.com/cr99bx, a Sennheiser web page:

Cut and paste:

“On February 17, 2009 all full power analog TV transmission is scheduled to cease. Digital TV broadcasts will be consolidated below channels 52. Channels 52-69 (698 – 806 MHz), generically called the 700 MHz band, will be reallocated to public emergency broadcast and telecom companies. The FCC will prohibit the use of wireless microphones in this range in the near future, perhaps as early as February.

Foreseeing these changes, we have been guiding customers to equipment operating on alternate frequencies for the past two years. Furthermore, at the beginning of 2008 we re-classified equipment operating on frequencies between 698 MHz and 806 MHz, such as Evolution Wireless C range, as special order items.”

I suspect that my chapter got royally f*ked.

Well, like they say, 'buyer beware.' When it was time for us to select brands, I had to take a few weeks leave of absence from the committee – but still I wish I had somehow found the time to do more research then.

Kaylee
29th January 2009, 10:54 PM
FM radios often use an intermediate frequency in receivers. A typical broadcast receiver uses 10.7MHz.

That's really interesting. I'm guessing they must transmit additional information in a packet so they can eventually reassign the broadcast to the correct station (which has its own unique frequency).

Kaylee
31st January 2009, 07:52 PM
I have a truly big headache. Not everyone on my planning committee understands the technical issues behind why having a wireless microphone system that operates in the 700 MHz Band is a problem. I’m tired of writing e-mails and I’m tired of explaining and just getting nowhere. So, I decided to write them a story and e-mail that to them.

If you are feeling patient today, dear JREF reader, please read it and let me know if you think it makes it clear.

The story ---

Everyone is probably wondering why we have a new problem with the wireless part of the library’s listening system.

That’s a good question!

I’m going to use an analogy to explain it.

Let’s pretend that we all live in a city called Smallville. Smallville has only one zoning board. The zoning board decides where it’s OK to have houses, stores, restaurants, factories, warehouses, etc. They also decide where it’s OK to drive a car, or anything larger than a car.

Back in 1998, the Smallville Zoning Board (SZB) decided that they had too much land zoned for factories and not enough land zoned for stores.

The SZB said we’re going to rezone (or reallocate) how 1,000 acres in the northern part of the city can be used. Starting on February 17, 2009, those acres can no longer be used to manufacture stuff, they can only be used to sell stuff.

The land is owned by the city, and leased on a long-term basis. The city announced that they would auction off long-term leases to sections of the 1,000 acres between the years 2000 and 2008. The winners would be free to sell retail goods (stuff) on those 1000 acres starting on Feb 17, 2009. Also on Feb 17, 2009 manufacturing within those 1,000 acres must stop immediately.

This got a lot of media coverage over the years. It got so much coverage that there was almost no one who didn’t know that after Feb 17, 2009 people would no longer be able to manufacture goods (stuff) in those 1000 acres, but only be able to sell goods (stuff) in those 1000 acres. This got so much coverage, that the 1000 acres got a nickname. It became known as Thousand Oaks.

What got almost no coverage in the media was that the SZB was going to make additional zoning changes. Extremely large semi-trucks could no longer be driven in Thousand Oaks, only cars and a few small delivery trucks. But the delivery trucks could only travel in that area if they had deliveries to make in Thousand Oaks. The SZB said that this law would not take place on February 17, 2009 but sometime afterwards.

The SZB announced this, but this was not widely reported in the media. Only the businesses that manufactured cars and trucks and sold them paid any attention.

Now, do me a favor and continue to pretend with me. Pretend that Smallville is located in an alternative universe and that in this alternative universe, cars and trucks are designed so that they are physically limited as to where they can be driven. That means that when you buy a truck, you not only have to know what kind of truck you want, but where you will want to drive your truck. You tell the dealer that information , and it’s his or her job to sell you the truck that will work with your requirements (or needs).

It is now October 2008. Bobby Bucker decides he needs a new truck. He goes to a dealer and tells him he needs a truck that can be driven between the states of Eaststate and Midstate. The dealer says, “No problem Bobby, I’ll set you up.” He calls him back later that day and tells Bobby, “I have the perfect truck for you!”

Bobby says “Great, I’ll take it.” He goes to the dealer to pick it up. And remember this is in the alternative universe, the dealer gives him a map and shows him which streets and highways he’s allowed to drive in that particular truck.

Bobby uses the truck. There are some problems along the way. He tries to drive down some highways and he experiences mechanical trouble because it turns out that his truck is not designed to be driven on those highways. He goes back to the dealer a few times between October and January and the dealer corrects his mistakes and gives him new maps.

Finally, things seem OK. Bobby is able to drive his truck. He always drives through Thousand Oaks when he goes between Eaststate and Midstate – not because he needs to be able to actually do anything in Thousand Oaks, but because that’s where his truck is designed to be allowed to be driven.

Then, by accident, Bobby finds out that after Feb. 17, 2009 he will no longer be able to drive his truck through Thousand Oaks. He is very upset! It’s the only way he can get from Eaststate and Midstate in his truck.

He finds out more information. He discovers for example that some dealers have stopped designing trucks that need to be driven through Thousand Oaks since 1998. Many other dealers have stopped selling trucks that need to be driven through Thousand Oaks since 2007.

Unfortunately for Bobby Brucker, he had not happened to have gone to any of those dealers.

Bobby also finds out that some dealers are saying the following:

• That while it will be illegal to drive through Thousand Oaks we don’t know exactly when it will become illegal. It will be after Feb 17th, 2009 -- but we don’t know for sure when.

• Even after it becomes illegal to drive though Thousand Oaks, there will be no effective regulation because it will be too difficult to regulate.

• However, everyone acknowledges that it will become congested with new small cars. On this alternative universe, when small cars and large trucks are on the same road – the small cars always gets the right of way with no problems. But, the trucks always gets into a noisy high pitched squealing accident that stops the truck and gives the truck drivers a huge headache. Eventually the truck drivers just won’t be able to drive their trucks through Thousand Oaks, at all.

Everyone knows that this will eventually happen, however no one knows for sure exactly when that will happen.

Bobby hasn’t called his dealer yet – but he obviously wants to exchange his truck for one that can be driven from Eaststate to Midstate without any problems. He wants to trade his truck for one that is designed to be driven on a different route and not through Thousand Oaks. A truck that is designed to drive through Thousand Oaks is no longer a good idea.

If Bobby doesn’t get to exchange his truck, he will soon have to buy a new one.

Bobby is very upset. Everyone knows that a truck is a major purchase and one that often lasts between 4 – 10 years.

His dealer sold him a truck that, because of regulation changes, will only have the same amount of usefulness for 4 and a half months.

==

In our story the cast was:

Smallville Zoning Board (SZB)- Federal Communications Commission (FCC)
Smallville - The entire radio frequency spectrum

1000 acres in the northern part of Smallville - 698 MHz – 806 MHz
1000 Oaks - “700 MHz Band”

factories - TV stations
stores - telecomm companies (Verizon, AT&T, Qualcomm)
trucks - wireless communication systems
truck manufacturer - Our wholesaler
truck dealer - The wholesaler’s local rep
Bobby Bucker - Us, the Planning Committee.




The drama in real life:

The FCC decided back in the 1990s that they were going to reallocate part of the frequency spectrum reserved for TV stations to other uses.

They decided that the bandwidth to be reallocated would be between 696 MHz – 806 MHz. It was referred to so often that the industry came up with a nickname for it, the “700 MHz Band.”

Auctions for pieces of the bandwidth were held periodically between the years 2000 and 2008. It got a lot of media coverage over the years. That will happen when the total yield from the auctions add up to 20 billion dollars. Eventually, almost everyone (even grade school children) knew that the 700 MHz Band would be reallocated from TV station usage to telecomm usage on Feb 17th, 2009.

However, many people didn’t know that this would also affect the wireless microphone systems. Basically only the wireless microphone systems’ manufacturers and dealers knew that. It didn’t get a lot of media coverage.

Some wireless microphone systems’ manufacturers stopped designing systems that would use the 700 MHz band back in 1998. Other wireless microphone systems’ manufacturers stopped selling them in 2007. Some are still selling them just weeks before the transition date -- Gemini is a good example.

Wireless microphone systems that operate in the “700 MHz Band” are affected because while they can coexist with TV stations for technological reasons, they aren’t expected to be able to coexist with wireless telephones, laptops, PDAs, etc. Even if they could, it will be illegal to operate wireless microphones in the “700 MHz Band” soon.

Many organizations use small wireless microphone systems as part of a listening systems. This includes churches, synagogues, schools, libraries, businesses for their meetings, etc. Wireless microphone systems that operate within the 700 MHz band will have interference problems in the future after the DTV transition date. Also, the telecomm companies that spent over USA $20,000,000,000 (twenty billion USA dollars) expect to have exclusive rights to the bandwith for which they have purchased their license rights. The FCC has "tentatively concluded" in one of their reports dated August 21, 2008 (Notice of Proposed Rulemaking and Order) that they will receive that. The major players in the industry has expected this for years and have planned for it.

==

OK dear JREF readers – you read it. Do you think the story is accessible enough so that the average technophobe will understand the problem?

Kaylee
2nd February 2009, 10:06 PM
I don't know what your device is, but I've got a Shure wireless mic that has a bajillion possible channels.

Good news. The company that sold us the listening system agreed to exchange the wireless microphone system part of it for another one that will operate in a bandwidth under 698 MHz.

Details to be discussed later -- that is they didn't give us brands/models yet -- they just agreed to do the exchange.

Joe, I really like the Shure web site. I like the way they supply tech articles and web tools, like the frequency finder. Do you mind telling me what model mic you have and over how large an area you use it for?

Thanks!

Kaylee
2nd February 2009, 10:55 PM
I'll just add a few more tech facts to the thread to help keep it appropriate to the Science Forum. :)

It turns out that the DTV transition date isn't just about television, its also about wireless microphone systems. The reason is because after the DTV transition date, the “700 MHz Band” will be reallocated from television broadcasts to telecom applications (such as cell phone calls and cell phone carriers’ data plans) and to public safety broadcasts (such as first responders’ broadcasts).

Wireless microphones systems can operate in the “white space” or empty space around television broadcasts, but they won’t be able to operate in the same bandwidth as telecom applications and pubic safety broadcasts. One reason is legal, the other reason is technical.

The legal reason, as mentioned earlier in the thread, is because the FCC is likely to disallow wireless microphone systems to operate in the spectrum sometime after the DTV transition date.

The technical reason is because the FCC has decided to preserve the "white space" in the spectrum below 698 MHz and to allow wireless microphones to operate in that spectrum on a secondary basis, that is as long as it doesn't disrupt televison broadcasts which have first priority. However, "white space" won't be intentinally preserved in the spectrum above 698 MHz, and improved technology means that the spectrum, above 698 MHz will fill up with more traffic and as the new owners build up their customer base there will be less "white space."

So even if the FCC decides not to enforce the predicted ban against wireless mic systems in the "700 MHz band", increased traffic will soon cause the wireless mic systems to experience interference and not be able to function anyway.

Even though the DTV transition date has been well publicized, since 2001 at least if not further back -- many wireless microphones consumers have not connected the technological dots and realized that this will render their systems useless they had purchased ones that operated below the 700 MHz band.

While some companies have been "good corporate citizens" and stopped selling wireless microphone systems that operated in the 700 MHz band for some time now (Shure and Sennheiser for the past 2 years, and Audio Technica has stopped developing new systems that used the 700 MHz band since 1998) other companies, like Gemini, are still selling them TODAY. They are doing that even though they will be in obsolescence after the DTV transition date and probably completely unusable shortly after that.

JoeTheJuggler
2nd February 2009, 10:59 PM
Good news. The company that sold us the listening system agreed to exchange the wireless microphone system part of it for another one that will operate in a bandwidth under 698 MHz.

Details to be discussed later -- that is they didn't give us brands/models yet -- they just agreed to do the exchange.

Joe, I really like the Shure web site. I like the way they supply tech articles and web tools, like the frequency finder. Do you mind telling me what model mic you have and over how large an area you use it for?

Thanks!

Mine's a Shure ULX system. The receiver is the ULXS4; the transmitter is ULX1-J1; and the mic is their headset mic. I need my hands free do juggle while I jabber away in my show.

Its frequency range is 470.000 to 952.000 MHz.

I'm not sure what you mean by how large an area. I don't travel much, so my shows are general just around the Greater St. Louis area. (The Shure website could give me recommended channels for other locations though.)

The distance from the mic to the receiver varies depending on the setup. Sometimes the receiver is right on stage with me (plugged into my Fender Passport). Other times it's in another room or a control booth as much as 100 yards away. (I've never had trouble with that distance.) The specs say its good up to 100 m.

This is the 3rd Shure wireless mic I've owned, and I've owned several other brands. The first Shure I had was a VHF, while the last two have been UHF. I've never had much trouble with interference on any of my wireless systems, but I find the Shures to be MUCH more reliable and able to stand up to a lot of use and abuse. Also, the sound quality on the 2 UHF systems is noticeably better than the UHF or the other brands I have (one of which is also a UHF).

I rely on my voice, and a little over a year ago, I had some lesion removed from my vocal cords, so I absolutely rely on my wireless mic and PA.

Having said that, when I bought this most recent Shure, it worked fine out of the box, but then went wonky after about a month. (I'd only get a very attenuated signal to the amp or whatever.) I sent it back under warranty, they did something and returned it. It worked fine again for about a weak, and then conked again. This time I sent it back, and they very promptly sent me a whole new system. I've used it for about a year now with no problems at all.

I bought this one from an internet vendor for the cheapest price I could find (less than half what it generally retails for in a music store). A music shop in town told me that they could match any bona fide internet price I found, but they'd still have to charge me sales tax (and the price I found included free shipping). Buying it from the store wouldn't have helped with the service. The nearest Shure service center is somewhere in Illinois, and they got it back to me very promptly (and apologized for not getting it right the first time--though it tested fine, and worked fine for a week, so I can't blame them for thinking it was OK).

JoeTheJuggler
2nd February 2009, 11:09 PM
I've never had much trouble with interference on any of my wireless systems,

In fact, I know of only two occasions. One was with a cheapie FM Radio Shack wireless mic. It once picked up a baby monitor somewhere in the apartment complex where I lived. (You could hear a music box or mobile or something making music and the baby's cooing and laughing--it was the cutest thing.)

Another time, I think it was a VHF mic (though I don't remember which) that I lent to someone else using it for a speaker meeting picked up the RF signal from a kid playing with a radio controlled car (in the same building in a nearby room). It sounded like someone playing an electric violin. . . but not well.

Kaylee
2nd February 2009, 11:14 PM
Mine's a Shure ULX system. The receiver is the ULXS4; the transmitter is ULX1-J1; and the mic is their headset mic. I need my hands free do juggle while I jabber away in my show.

Its frequency range is 470.000 to 952.000 MHz.

I'm sure what you mean by how large an area. I don't travel much, so my shows are general just around the Greater St. Louis area. (The Shure website could give me recommended channels for other locations though.)

The distance from the mic to the receiver varies depending on the setup. Sometimes the receiver is right on stage with me (plugged into my Fender Passport). Other times it's in another room or a control booth as much as 100 yards away. (I've never had trouble with that distance.) The specs say its good up to 100 m.

Thanks Joe, this is very helpful info. And 100 m will be just great for the library's meeting room (it's large.)


This is the 3rd Shure wireless mic I've owned, and I've owned several other brands. The first Shure I had was a VHF, while the last two have been UHF. I've never had much trouble with interference on any of my wireless systems, but I find the Shures to be MUCH more reliable and able to stand up to a lot of use and abuse. Also, the sound quality on the 2 UHF systems is noticeably better than the UHF or the other brands I have (one of which is also a UHF).

And more great info! Do you mind if I get noisy and ask what those other brands are? I cheerfully admit to being very lazy, and I'd much rather benefit from your past experience than from my future experience. ;) :)

I rely on my voice, and a little over a year ago, I had some lesion removed from my vocal cords, so I absolutely rely on my wireless mic and PA.

Having said that, when I bought this most recent Shure, it worked fine out of the box, but then went wonky after about a month. (I'd only get a very attenuated signal to the amp or whatever.) I sent it back under warranty, they did something and returned it. It worked fine again for about a weak, and then conked again. This time I sent it back, and they very promptly sent me a whole new system. I've used it for about a year now with no problems at all.

I bought this one from an internet vendor for the cheapest price I could find (less than half what it generally retails for in a music store). A music shop in town told me that they could match any bona fide internet price I found, but they'd still have to charge me sales tax (and the price I found included free shipping). Buying it from the store wouldn't have helped with the service. The nearest Shure service center is somewhere in Illinois, and they got it back to me very promptly (and apologized for not getting it right the first time--though it tested fine, and worked fine for a week, so I can't blame them for thinking it was OK).

And more and more great info! Do you like your UHF mic better than your VHF? I know that VHF mics generally cost less, but I've heard that the VHF bandwidth is much more crowded.


Thanks again!

K

JoeTheJuggler
2nd February 2009, 11:26 PM
And more great info! Do you mind if I get noisy and ask what those other brands are? I cheerfully admit to being very lazy, and I'd much rather benefit from your past experience than from my future experience. ;) :)

Currently, I've got a dead Sennheiser and Nady. (The Nady sounded like crap even when it was brand new--and it was in the same price range as my previous Shure, but not my current one.) I've had several other brands over the years, but don't remember which (besides the Shures that I liked and that really cheap Radio Shack. Oh yeah--I actually had a cheaper-than-cheap Radio Shack model that came with no receiver; you just tuned your FM radio to a certain spot on the dial--ugh!)

ETA: I've also used mics not owned by me in a number of situations, and I do like my own the best.


And more and more great info! Do you like your UHF mic better than your VHF? I know that VHF mics generally cost less, but I've heard that the VHF bandwidth is much more crowded.
The UHF models I've had sounded noticeably better, but I don't think that was a matter of the signal itself. I think the higher end models came with better quality mics. Also, the VHFs that I've had only came with a high impedence (unbalanced line) jack. The quality is much better with the low impedence jack. I appreciate having the choice, though, because I sometimes use an older keyboard amp that only allows the high-z.

For me, it's most important that I can plunk my setup down just about anywhere and not spend a lot of time fiddling and adjusting even though the acoustics of the spaces where I perform vary greatly.

(Once upon a time, I used a condenser mic on the lapel. When there was a sound guy with a professional mixer to do all the EQ for me--and we were given the opportunity to do a sound check-- it was fine. When I had to hit the ground running, as is usual for me, and use my own 30 band EQ unit, the results were pretty bad.)

I understand that a set-up that's fixed in a room is much easier to get a very good sound quality out of than the sort of thing I do.

JoeTheJuggler
2nd February 2009, 11:29 PM
OH yeah, on my ULX--when I power on my receiver (which I must do before powering on the transmitter) it scans the airwaves to let me know that the channel I'm on is available. If it's not, when I tune, it goes to the next available channel. I've NEVER had a channel reported as unavailable.

Also, I like the way Shure's headsets go on my head better than any of the other brands I've used.

Kaylee
3rd February 2009, 11:36 PM
Thanks Joe! I'm sharing the information with my chapter's planning committee. We are all hoping that we will have better luck with our 2nd wireless mic system.

The Shure ULX may be a little to pricey for most of the planning committee members, but I'm glad that we're in a situation where we can choose and where we aren't stuck with the first 700 MHz system.

The saga continues ... :)

DavidS
4th February 2009, 10:09 AM
That's really interesting. I'm guessing they must transmit additional information in a packet so they can eventually reassign the broadcast to the correct station (which has its own unique frequency).
Nope. Packets? We don't need no steenkeeng packets!

In this context, the intermediate frequency (IF) is entirely the receiver's business -- it can use any IF frequency it wants, the broadcaster has nothing to do with it. Or more than one -- "double conversion" using ~10.7 MHz first IF and 455kHz second IF is common enough.

The effect is that a signal at one frequency (or band of frequencies around that frequency) -- say, the broadcast frequency -- is "translated" to another frequency (or equivalent band of frequencies around that frequency) -- the IF.

That is, your favorite FM radio signal at (actually, around) 93.7 MHz gets "mixed" (multiplied) with the receiver's own "local oscillator" signal at 83.0 MHz. This produces a signal that's a mixture of (93.7-82.0)=10.7 MHz and (93.7+82.0)=175.7 MHz. It also might include other mixing products of the form [n*93.7MHz+m*82.0MHz], but that's not important because a filter discards all but the 10.7MHz part for further amplification and signal processing (which might include a similar conversion to, say, 455 kHz, or re-mixing with a different LO to shift the signal to another frequency for retransmission).

Doing it that way means only the initial receipt of the signal and the local oscillator need to be tuned together to work nicely across all the broadcast frequencies. The rest of the receiver can be tuned and optimized to work well at the single, fixed IF frequency.

Kaylee
4th February 2009, 11:53 PM
Nope. Packets? We don't need no steenkeeng packets!

:D

You know DavidS, I'm pretty sure that I understood your entire post. And I think I can see how it would be less expensive to design receivers that way. I find it a little scary how much I've learned about wireless microphone systems in less than two weeks.

But for right now, I'm going to stay focused on the consumer purchasing part of it. The wholesaler recommended a replacement that is lower in quality than the original. Just to give one feature as an example, the original system was a diversity system and the system offered as a replacement is a non-diversity system.

The saga continues. Eventually it will have to end. Even if its only because I pass away from old age first 30 or so years from now.