Wireless Microphone Receive Antennas

WE HAVE MOVED! Please navigate to the current article links below to see the latest from dBB Audio!

  1. Wireless Microphone Dead Zones?!
  2. DIY Wireless Microphone Antenna Distribution – Antennas
  3. DIY Wireless Microphone Antenna Distribution – Introduction
  4. DIY Wireless Microphone Antenna Distribution – Part 2

 

As we know wireless microphone systems use radio frequency, RF, waves to link a wireless mic and the receiver together. We can think of RF as the same thing as audio waves just in much higher of a frequency. To give you an example of how high of frequency, the human hearing spectrum is from 20Hz to 20,000Hz. The Shure wireless frequency band of “H5” starts at 518MHz which is 518,000,000Hz. Obviously this is way to high for us to hear, but we can treat it the same way we do as audio waves.

Antennas for RF can act much like microphones, we need to select the right microphone for the source. Well we have our omni-directional antennas such as a 1/4 or 1/2 wave dipole (which most wireless systems use), this acts almost the same way a omni-directional microphone does receiving signal from all angles.

Then we have our shotgun or hyper-cardioid antennas, which we call a directional antenna. These antennas include the two most popular Yagi-Uda (which we simply call a Yagi antenna) and a Log-Periodic. These have multiple elements (or wires) and are directional meaning they are more sensitive in one direction than the other.

One of the more popular directional antennas in the wireless microphone industry is the Log Periodic antenna. It is a multi element directional antenna with a wide bandwidth which means it has good sensitivity to a wide frequency range. Shure’s version is pictured below.

This is the Shure PA805SWB Directional Antenna.

Before get too involved in the technical lingo of antennas, lets bring this back to microphone talk.  What is the benefit of a directional microphone such as a cardioid or hyper-cardioid microphone? It is to record the source only and reject all of the background noise.  A directional antenna does the same thing, just with RF.

Time to get into the dirt of all of this stuff.  I am going to be talking about the frequency range of 518MHz-542MHz which happens to be Shure’s H5 band for their wireless microphones.  Anything I talk about here applies to any frequency for RF be it higher in frequency like 900MHz or lower in the 100MHz area.

The Federal Communications Commission, FCC, has quite a fun job of keeping all of the RF frequencies coordinated.  If they didn’t we wouldn’t have our Wi-Fi or Cellular devices working like they do.  Taking a look at the 518MHz-542MHz range we find ourselves in the middle of the TV spectrum.  The FCC decided to put our wireless microphones in the same spectrum as our TV stations.  The Shure H5 band fits into TV channels 21-27.

This is the FCC allotment for the frequencies for the television channels 21-27 which lay in the same frequency band as Shure’s “H5 Band.”

These are the different channels that Shure has in the H5 Band which ranges from 518MHz – 542MHz.

While this doesn’t seem like much of an issue at first, I will now explain why this can be such a large problem.

Television just made a large transition from analog to digital.  If you are like me, I enjoy the high definition DTV stations.  Each TV station gets 6MHz of space to themselves for use.  So channel 22 has 518MHz to 524MHz.  Before the switch to digital us audio engineers had an easier time finding spots for our wireless microphones because there were empty spaces in each TV channels spectrum.

A spectrum analysis of an analog and digital television transmission. Source: http://www.comsearch.com/newsletter/images/analog_tv_ch_display.gif.

In Figure 1 of the above illustration, we see 3 spikes on the display.  Going from left to right these are the Luminance Carrier, Chrominance Carrier and the FM Carrier.  Then we can see our digital TV spectrum in Figure 2.  We can see that the digital transmission takes up all 6MHz of the channel.

Before a sound engineer could place a wireless microphone in between the Luminance and the Chrominance carriers in frequency and have no issues with the TV station causing interference with the microphone.  As we can see with digital there is no spaces in the channel.

Lets take a look at channel 24, which in the Phoenix, Arizona area is KTVK-DT aka 3TV.  Channel 24 ranges from 530MHz to 536MHz.  Here is a photograph of the transmission spectrum from a different station, KTVP-LP (channel 22), from my RF Spectrum analyzer:

This is a photo of the screen of my RF spectrum analyzer showing channel 22, KTVP-LP.  KTVP-LP is a 42 kW station and even at this “lower power” can make a large impact on a wireless microphone. The digital transmission of television now takes up the entire bandwidth of this channel ranging from 518-524MHz.

As you can see the transmission takes up all of the frequency range there. Now you may be thinking, “Well that’s fine, I will just put my wireless microphone on top of that frequency, after all my wireless mic is closer to my receiver than that TV station.”  While this will sometimes work, the power output from the KTVK-DT station is 1000 kW, which is also known as 1,000,000 watts.  Now we can look at our Shure SLX wireless system.  It has a 30mW output, or 0.03 watts of power.  This is a VERY large difference in a audio reference, this is like listening to a flute playing a ballad next to a fighter jet taking off at an airport.

So how can we help our wireless transmitter win in the fight of David and Goliath?

1.) Use a higher output wireless transmitter (however the FCC limits this at 50mW and then you have to worry about battery life)

2.) Move the receiving antenna closer to the wireless microphone

3.) Use a directional antenna that points toward our wireless microphone and away from the TV station

4.) Move our wireless microphone frequency to a different “channel” where there is no interference with a TV station

As you can see, we have a few different options and I will be going more in depth about these in future posts.  Please if you have any questions feel free to post below.

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Dead Zones?!

WE HAVE MOVED! Please navigate to the current article links below to see the latest from dBB Audio!

  1. Wireless Microphone Dead Zones?!
  2. DIY Wireless Microphone Antenna Distribution – Antennas
  3. DIY Wireless Microphone Antenna Distribution – Introduction
  4. DIY Wireless Microphone Antenna Distribution – Part 2

 

All of us remember the advertising of Verizon Wireless where the technician is walking around to different areas saying “Can You Hear Me Now?”  Well, they never let us hear the other side of the conversation…  Over the next few weeks, I am going to be dedicating a few posts to wireless microphones and how they work.  Specifically, the Radio Frequency part of the wireless microphone system.

A few months ago, I was listening to the pastor speaking at my church and I kept hearing a small spot of sound where he wasn’t coming through the PA.  Our church uses a Shure wireless microphone system on the pastor.  Whenever he would walk around, it seemed that there was a dead spot in the antenna coverage in one small area.  During those brief moments, no sound from the pastor was coming through the speakers.

Being a ham radio operator, KD7QCU, I am intrigued in the idea of trying to devise a cost effective way of solving the reception issues.  In my next blog posts I will be talking about my ideas for solving these issues in a cost effective way.  So stay “tuned”, pun intended!

Polarity issues in CD recording from bad wiring

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Balanced audio signals are a great thing for the audio industry. If you don’t know much about balanced vs. unbalanced signals, I highly suggest doing some reading on Wikipedia.

Balanced signals have a differential amplifier on the output and the input side. They basically carry a mono signal down 3 conductors. Two wires carry the signal and then the last wire carries the ground. The beauty of the differential amplifier is that two wires that carry the signal. One wire carries the normal signal, then the other inverts the polarity. So, the two signals are out of polarity of each other. At the receiving end of the balanced signal, there is another differential amplifier which inverts one of the wires and sums them together to get the signal back to normal.

If there is any noise injected into the wire on its run, it is injected into both wires in polarity. When the signal reaches the receiving end of the wire, that differential amplifier inverts the polarity of one of the wires which cancels out the noise. This is because the noise is now out of polarity with each other and, in the summing process, cancels out itself.

Illustration from http://www.ians-net.co.uk/images/articles/balanced/balanced.gif showing how noise is rejected in a balanced signal.

At North Ridge Community Church, NRCC, we have a Tascam CD recorder which we use to record the spoken word of our services. I noticed one day that I had to turn up the CD really loud in my car to be able to hear it. This surprised me because earlier in the day I was almost clipping the meters with the CD recording levels. I decided to rip the CD into Cubase 5 to see what was going on. Here is what I found:

Out of Polarity signals which were recorded to the CD like this.

As you can see, the left and right channels are recorded out of polarity to each other. Basically, they are cancelling each other out from the inversion.

After seeing this on the screen, I went back to the church to find that the CD recording feed is coming from a balanced mono send off of a matrix on the Allen & Heath ML4000. The cable then goes into a 1/4 inch stereo to R and L RCA adapter. The way this adapter works is for a headphone cable which takes the left from the tip and sleeve and the right from the ring and sleeve. With a balanced signal, the tip and ring are inverted in polarity. So this is sending the + into the left while sending the – into the right.

I recorded a video of myself explaining what is going on when you have a signal that is out of polarity. In the video, I accidentally mention that this could be called 180 degs out of phase which is incorrect. The only term to call this is out of polarity.

After making the correct type of adapter with the balanced to RCA, the recordings will now have full volume on the CD recording with a full spectrum sound. This is another reason not to trust any installed wires without checking them.

Drew Brashler

“Vocals are the most important, so I’ll start with the Drums”

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I heard this saying a long time ago and I love it and still say it to this day.  “The vocals are the most important part of my mix, so I will start with the drums.”  The way you approach mixing may differ from mine, but I think of audio as a triangle.  Just like the food industry thought up the food group triangle, I am going to make one for you here for audio.

First and foremost, we have the Drums & Bass at the bottom of the triangle making a very good – sorry for the pun – base.  Next the keyboard and piano would go, some of the other backing instruments for fills could either go in this level or the next up.  Then acoustic & electric guitar on the next step up.  Then at the very top there is the vocals, which is what most listeners are primarily hearing.  When someone walks away from a show, not many people (excluding drummers and sound guys) will say “that is the best kick drum I have ever heard” most of the listeners will say “man xyz vocalist was great today.  The idea behind this is if you have a bad mix on any one of these levels the one above it will not be supported correctly.

Enough theory; lets get into some changes.  At North Ridge Community Church, NRCC, we have an acoustic Pearl Session Custom drum kit, in front of that we have a plexy drum shield which helps a little for the front row of our worship center.

Next come the drum microphones… this is where it is lacking big time.  We had some off brand microphones that were made by Audio-Technica.  There is no data on the mics and they honestly sounded horrible the way they were set up.  We had a mic on the Snare, Hi Tom, Low Tom and Kick.  I started with the kick drum and pleaded for two microphones to be purchased, working with a budget that was far too exhausted for the year is tough when trying to get funds for microphones.  I settled on the Shure Beta 91A and the Shure Beta 52A and they got approved.

From http://www.microhone-data.com. Here is the specs of the Shure Beta 91A.

From http://www.microhone-data.com. Here is the specs of the Shure Beta 52A.

I place the Beta 91A inside the drum on a small towel and the Beta 52A is just inside the sound hole of the front head pointed a bit off center from the beater.  The combination of these two microphones is amazing.  You can really hear the body and natural sustain of the kick with these mics.  Here is a recording of a resent Sunday at NRCC worship song is called Happy Day:

Next, I grabbed an SM57 out of our amp room and switched out the no-name mic on the snare drum, this made a great difference as the SM57 is an awesome mic for the toms or snare drum.  My placement is having the rear of the microphone pointed at the hi hat to reduce bleed from the hi hat (cardioid microphone pattern).  I put the mic about an inch above the top head and an inch or two in from the rim pointed down toward the center of the bottom head.

From http://www.microhone-data.com. Here is the specs of the Shure SM57.

Next was the hi tom, with the old no-name microphone on there. I compared an SM57 with it, but I liked the sound of the older microphone there.  I just changed the placement a bit.  Placement is an inch above the top head, inch in from the rim, angled down pointed closer to the mic than the center of the bottom head.

Floor tom, we replaced with a Sennheiser e906 with the switches set to normal.  This is about two inches away from the top head about two inches in from the rim, pointed down toward the center of the bottom head.

From http://www.microhone-data.com. Here is the specs of the Sennheiser e906.

Lastly, there was no microphone on the overhead.  While in some venues you can get away with this, I want my main source of audio being from one point source.  We have a mono system so there is no use for stereo panning, but when your mind hears the same audio coming from two different places it does tricks.  So I added a AKG SE300B with the AKG CK91 capsule on it.  It is a small diaphragm condenser mic that we had in the back room which sounds really nice as an overhead.  Because we have a mono system in our worship center we do not need a stereo mic setup for the overheads.  I have the single overhead about 1/2 ft above the cymbals and equidistant between the kick and snare.

From http://www.microhone-data.com. Here is the specs of the AKG CK91.

Up at the FOH we have two DBX 1064’s which are quad channel compressors.  I decided to put two of the channels on the snare and kick running a pretty high compression ratio getting about 8dB of gain reduction on peaks of the snare with a limiter just incase our drummer decides to do a bunch of rim shots.  The kick drum is running about 8-10dB of gain reduction with no limiter.  I wish I had controls over the attack and release however the DBX compressors are doing just fine.

Overall, the drums have improved tremendously in comparison to before.  The drum set could use a new set of heads, our church budget is renewed in a month or so.  In the room, the kick drum is filling the room quite a bit more and really sounds great.

Spending a few practices trying different microphones out, changing mic positions, adding compressors and tweaking EQ settings can really make a huge difference.  I will say this time and time again, if you are coming into a church as a new lead engineer do not take anything for granted, check every mic cable, every microphone, just make sure that everything is setup the way it should be.

Drew Brashler