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Old 3rd July 2017, 08:25 PM   #21
Tubelab_com is offline Tubelab_com  United States
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you can never be sure that an exact copy will work first time.
That's true with just about everything from audio amps to automobile racing engines......The probability of success does improve by sticking close to a proven design, especially if it has been built by others successfully.

I do agree that RF has its own degrees of difficulty and verifying completely legal performance of a transmitter requires at least a spectrum analyzer and a competent operator of said analyzer. I also built my own 40 meter ham transmitter with an 807 and a surplus crystal when I was about 14 years old. It worked, and put out enough power to light up a 40 watt bulb being used as a dummy load. Back then you had to visit the FCC office to take the Novice ham test. My father promised to take me, but that never happened, so the TX got robbed for parts and tossed.

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With FM broadcast band you are restricted to around 10mw of power......rule is around 5-10mw power and you're safe from the FCC.
Not always the case. Back in the late 60's some enterprising students at the University of Miami (Florida) connected an FM wireless transmitter up to a small wire antenna in the top floor of a dorm room on campus and went on the air as "Radio Sunshine." The legal low powered transmitter covered most of the campus, but spilled over to the retirement community surrounding the school. It seems that their "hippie views" were not well received by the elderly listeners and complaints poured into the FCC. A visit ensued, in fact several, but Sunshine's operators were not breaking any laws. The school tried to shut down "Sunshine" so it started moving around, yet kept up random operations.

Things were about to get ugly since one of the Miami TV stations did a news segment on the "radio pirates" on campus. That news broadcast fixed the problem. Someone donated a 10 watt Gates FM broadcast exciter which was FCC type accepted for use as a stand alone low power mono FM station. The FCC fast tracked a broadcast license for the school, and WVUM 90.5 FM (Voice of the University of Miami) was born. With 10 watts, I could receive them from my house about 3 miles away. The school began offering radio broadcasting and other related courses. WVUM is still on the air, and a few years ago finally got a new TX that had far more power and stereo.

wvum | UM School of Communication
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Old 3rd July 2017, 09:47 PM   #22
Gnobuddy is offline Gnobuddy  Canada
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There is one area of RF that is (still) directly of interest to audio - wireless microphones. Commercial ones fall mainly into two categories - somewhat affordable and very unreliable, or prohibitively expensive and acceptably reliable. I've tried the former, and been burned (even my $150 AKG WMS 40 Mini failed within 2 years). I can't afford the latter (typically upwards of $500 for a single microphone).

I've also tried things like inexpensive Bluetooth transmitters and receivers of the sort you find on Amazon or Ebay; they turn out to be useless for live music, because latencies are in the tens to hundreds of milliseconds range. You sing or play now, and hear the sound emerge from the speakers a tenth of a second later. That makes it impossible to sing or play well.

I've looked for off-the-shelf RF transmitter/receiver modules to do what I want, but the ones I turn up all seem to be designed to transmit digital data, so they can't be simply interfaced with an analog microphone signal.

If anyone has any suggestions on a reasonably easy way to put together a legal, reasonably reliable, short-range wireless microphone RF link, of good enough quality for live vocals, I'd love to hear them. Plain old FM may be entirely adequate quality; if digital latency is involved, it needs to be under 10 mS, preferably under 5 mS.

When I was about twelve years old, I used to confuse my dog by talking to him via my dad's AM radio, and a one-transistor AM transmitter I threw together with a handful of junk-box parts. What I'm hoping for now is a better quality version of that.

-Gnobuddy
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Old 4th July 2017, 12:59 AM   #23
Tubelab_com is offline Tubelab_com  United States
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I turn up all seem to be designed to transmit digital data.........if digital latency is involved, it needs to be under 10 mS, preferably under 5 mS.
I have tried a couple of the cheap modules found on Amazon and also found the latency to be unacceptable. Not only is it audible, it is not always a constant time delay either. I am experimenting with wireless MIDI.

It should be relatively easy to do a decent FM transmitter with latency in the microsecond range. The main issues are interference and legality. The 915 MHz ISM frequency band (902 to 928 MHz) is allocated for almost anything in North America including Canada. There are some restrictions, but zero enforcement. There are tons of cheap Chinese transmitters in everything from cordless phones and wireless headphones to wireless video cameras (baby monitors). It is also legal for a licensed amateur radio operator to fire off 1.5 Killowatts in that band, and I have bounced a signal off the moon and off the ISS using about 600 watts. There are strong cellular and paging transmitters directly adjacent to that band.

For short ranges it may be possible to design a transmitter with 100 milliwatts or so, and a very good receiver to be capable of ignoring nearby cell towers. There would have to be several frequency choices in case there is a cordless phone on your first choice.

The uber expensive wireless microphones transmit on unused TV channels. The FCC allows this under certain restrictions, and only big companies with the required RF testing resources can play. Since unused TV channels theoretically have zero RF in them, range and interference issues should not exist. The frequency re-farming that occurred with the digital TV switchover in the US made TV frequencies more crowded, and lots of expensive microphones obsolete since they only had 4 frequency choices.

2.4 GHz is another possibility, but things get a bit more complicated as the frequency goes up.
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Old 4th July 2017, 03:42 AM   #24
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by Tubelab_com View Post
The 915 MHz ISM frequency band (902 to 928 MHz) is allocated for almost anything in North America including Canada.
Thanks for the hints!

I found some Linx modules in that band, and they're designed to accept analog inputs. But they're "Not recommended for new designs", and Digikey doesn't stock them any more:
https://linxtechnologies.com/wp/prod...eiver-modules/

So no joy there, probably.
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Originally Posted by Tubelab_com View Post
...frequency re-farming... made... lots of expensive microphones obsolete
Judging from stuff I read all over the Internet at the time, that made a lot of musicians and sound guys very upset. Quite understandably so.

-Gnobuddy
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Old 4th July 2017, 09:59 AM   #25
DF96 is offline DF96  England
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Originally Posted by Gnobuddy
If anyone has any suggestions on a reasonably easy way to put together a legal, reasonably reliable, short-range wireless microphone RF link, of good enough quality for live vocals, I'd love to hear them.
If it was possible, the internet would be full of designs. Making RF circuits which are small, low power and remain reliable when in the hands of a musician is a non-trivial problem. This is why the commercial offerings are expensive. In the UK you need type-approved equipment everywhere apart from amateur bands, and there you can't play music or do broadcasting, so a legal DIY route simply does not exist here.

I have used Sennheiser UHF wireless mikes in a church setting, but even in that fairly safe environment they still got broken within a few years. Design flaws were part of the problem, such as relying on solder joints to provide mechanical strength. However, when intact they worked very well - diversity reception was a big help.
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Old 4th July 2017, 06:08 PM   #26
AC439 is offline AC439  United States
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My journey into electronics started with RF projects first when I was very young. In the beginning, I got into building transistor radio (receive only) while my father was building audio tube amps. Back then, there was no cell phones but only a rotary pulse dialing landline phone. Not too long after that, we (me and some other electronics friends) wonder if we can keep in touch by transmitting. We got into 27 Mc briefly. Eventually got my HAM license. I like homebrew RF stuffs putting most my time on antennas....

Fast forward to now residing in FL, found out this is not a very HAM friendly region. With communication more readily available, I faded out of RF into AF. That's when I jointed diyaudio.com.

Building RF stuffs is different than AF. Depending on frequency you working on, a short piece of wire can be an antenna, transmission line or something else that can throw you off.

I never like wireless mic. They are not very HiFi. You have to modulate the RF signal with the AF signal so it can be transmitted wirelessly. Then on the receiving end, you demodulate it back to AF. Delay and distortion is inevitable. I do involve in live sound and I always prefer wired connection although many dislike the clumsiness. I prefer sound quality over convenient.

AC
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Old 5th July 2017, 05:53 PM   #27
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by DF96 View Post
Making RF circuits which are small, low power and remain reliable when in the hands of a musician is a non-trivial problem.
Agreed. At the frequencies currently legally usable, I wouldn't dream of trying to build entirely from scratch. I was hoping to find RF modules that would accept an audio signal.

I did find a few, but all the current offerings are designed to accept a digital audio signal, and many of these digital systems will have unacceptable latency for my intended purposes.

Given the vast number of commercial products that cost much less than $500, and still manage to transmit and receive audio, it doesn't seem like an impossible problem to me. Radio has been around for well over a century now, surely we can put together an affordable short-range wireless audio link with 60 dB SNR and 100Hz - 15kHz bandwidth?

I suspect the $500 per microphone price has a lot to do with the fact that we're talking about a specialty item, with somewhat limited competition in the marketplace.

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Originally Posted by DF96 View Post
However, when intact they worked very well - diversity reception was a big help.
My AGK Mini worked quite well, just long enough to get past the warranty period, then died, with no visible indication as to why it died. I have no RF test gear, and didn't even attempt a post-mortem.

At the other end of the spectrum, I recently attended the Vancouver Mini Maker Faire. The large building contained a number of exhibits featuring Tesla coils, Jacob's Ladders, and random high-voltage fluorescent or neon lighting. In that hostile RF environment, not a single wireless microphone worked, as far as I could tell. (Several exhibitors had brought them.)

-Gnobuddy
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Old 5th July 2017, 06:07 PM   #28
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by AC439 View Post
I never like wireless mic.
You have plenty of company. The majority of musicians dislike, or distrust, them.

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Originally Posted by AC439 View Post
They are not very HiFi.
Not compared to CD quality digital audio, certainly. But the weakest link in the live audio chain - by far - is usually the utterly horrible-sounding Shure SM58 microphones that the majority of musicians insist on using!

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Originally Posted by AC439 View Post
Delay and distortion is inevitable.
Digital systems often have very noticeable latency, even tens or hundreds of milliseconds. This is not inherent to an RF link, however, but rather to the nature of the digital communication protocol being used. The speed of light (RF) is incredibly high, and at 2.4 GHz, there is no inherent latency longer than nanoseconds.

But when you start to send multiple audio channels over one audio link, and add digital data buffering, and then add two-way communication, some sort of data protocol that negotiates handshaking, data transmission and re-transmission, and so on, and so forth, then you can end up with these unacceptably long latencies.

Keep in mind that sound takes about a millisecond to travel a foot through air. Musicians routinely perform with guitar amps or vocal monitors that are five to ten feet from their ears. This means a delay - latency - of five to ten milliseconds is entirely acceptable to (and not even noticed by) most musicians.

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Originally Posted by AC439 View Post
I do involve in live sound and I always prefer wired connection although many dislike the clumsiness. I prefer sound quality over convenient.

AC
If I had a dollar for every microphone cable I've uncoiled, plugged in, then later unplugged and re-coiled, only to have to do it again, and again, and again, I'd be moderately wealthy.

-Gnobuddy
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Old 6th July 2017, 11:19 AM   #29
DF96 is offline DF96  England
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Wideband FM provides acceptably low noise and distortion. If it is good enough for broadcasting the whole show then it ought to be good enough for one mike channel. Some devices include companding, to make the best use of the dynamic range.

I was happy with the Sennheiser offerings, apart from the need for repairs to solder joints from time to time. This was speech PA, so intelligibility was the main aim. We used two tie mikes (with belt clip transmitters) and one handheld. The main problem was trying (but mainly failing) to teach people that a handheld mike is not supposed to be used in the 'ice cream cone' position - I told them that if they could lick it then it is too close to their mouth.
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Old 6th July 2017, 01:53 PM   #30
Shhh its me is offline Shhh its me
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Originally Posted by Gnobuddy View Post
I suspect the $500 per microphone price has a lot to do with the fact that we're talking about a specialty item, with somewhat limited competition in the marketplace.
In terms of consumer electronics, you should see the cost of a mid range hearing aid. Makes a $500 price tag look very cheap.

So what do the TV broadcasters use for live panel discussions at venues that don't support booms? I see tie clip types, rarely a desk/stand mounted mic.
Perhaps wireless digital transmission, yet seemingly no latency issues with video sync or house PA.
Perhaps a heap of signal processing done to deal with this.... and top shelf equipment no band or pleb could afford.
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