Wireless - Setting Wireless Frequencies

[DavidDaMonkey]

I dug through the archives, so I apologize if I missed this in there. When setting up frequencies for wireless gear I have never used the groups, channels, etc. I always just manually scroll through the frequencies, trying to stay at least 2 MHz apart when possible and trying to use ones that aren't showing any errant RF signal. Is this a bad way of doing it? For the show I'm working on right now we are using Sennheiser ew100's. I know there must be something I'm missing, but I'm wondering if someone could enlighten me as to what it is.

[epimetheus]

At my church, our main wireless distributor ran a frequency scan in our area and gave us a list of clear frequencies. We work off that as we add more wireless channels. Occassionally we have to discontinue using a particular frequency because some new source of interference has come up, but for the most part this has worked for us. We've got something like 30 channels of wireless throughout the building.

[DaveySimps]

What you are not taking into account in your current methodology is intermodulated frequencies, that is, those pesky secondary spikes in RF that appear when several different wireless units are used at one time. The more wireless signals (carrier frequencies) you use, the more intermods you get. They are the by product of the combined carrier frequencies being used. To avoid this you need to choose your specific frequencies carefully, depending on what exactly you have available. Also, do not forget to take into account other RF in your area such as TV signals, hearing impaired systems, etc.

The easiest way to go about this is to the prediction software that is offered free by the Manufacturer. Sennheiser's is located below. Remember that Ew100 can only have a maximum of 12 compatible frequencies per frequency bank - best case scenario. They also have the automated frequency scan feature that searches for the best available frequency, that will take the information I mentioned above into account. Just make sure you leave the transmitters for the previous units on while you are scanning each additional transmitter.

http://12.148.18.149/

~Dave

[mbenonis]

It's a way to do it but not necessarily the best way. As folks have alluded to above, you may end up with potential spurs on frequencies you are using. The best way to avoid this is to run the calculations and ensure that you're not getting any products on frequencies in your system. Read through my FAQ to get an idea of how these products form and so forth.

[DavidDaMonkey]

Ok, so when I run the Sennheiser frequency finder for my area and I find that, for example, channel 44 is vacant, I then look up on wiki (Television channel frequencies - Wikipedia, the free encyclopedia) and find that channel 44 corresponds to 3 different frequencies (651.25, 651.31 ,655.75). From this, I assume I can put mics on two of those (since the first two are pretty close together I would only use one of them). Am I doing this correctly?

I'm still confused about the scanning thing. I've gone through all 8 banks, and only one bank had 4 open frequencies total (all except bank #2 returned zero free channels). Does this mean I can only reliably use 4 mics? I've used over 20 mics in here before without issue so I really feel like I'm missing something.

[fredthe]

I'm still confused about the scanning thing. I've gone through all 8 banks, and only one bank had 4 open frequencies total (all except bank #2 returned zero free channels). Does this mean I can only reliably use 4 mics? I've used over 20 mics in here before without issue so I really feel like I'm missing something.

When did you last use 20 Mics? Remember that a lot changed with the DTV transition.

-Fred

[mbenonis]

Ok, so when I run the Sennheiser frequency finder for my area and I find that, for example, channel 44 is vacant, I then look up on wiki (Television channel frequencies - Wikipedia, the free encyclopedia) and find that channel 44 corresponds to 3 different frequencies (651.25, 651.31 ,655.75). From this, I assume I can put mics on two of those (since the first two are pretty close together I would only use one of them). Am I doing this correctly?

The frequencies that you found on Wikipedia correspond to the NTSC Luma (brightness) and Audio carriers of an NTSC (analog) television signal (and they threw in the ATSC pilot signal just for fun, even though it doesn't occur with NTSC signals). A television signal is not narrowband around these frequencies, but rather spans an entire 6 MHz block (analog, not so evenly, but it does fill it). DTV actually fills this 6 MHz block equally throughout (which is actually good news for us, because the signal energy is spread out!).

So, if Channel 44 is vacant for you, I would say you can probably fit upwards of 6 or 7 mics in that 6 MHz block (ranging from 650 to 656 MHz, spaced ~800 kHz apart) with a bit of planning and decent mic receivers (i.e., not Shure PGX stuff).

BTW, before someone jumps in and says that analog TV is gone, it isn't. Low power, translators, and Class A stations can all continue to broadcast in analog for a few more years.

I'm still confused about the scanning thing. I've gone through all 8 banks, and only one bank had 4 open frequencies total (all except bank #2 returned zero free channels). Does this mean I can only reliably use 4 mics? I've used over 20 mics in here before without issue so I really feel like I'm missing something.

When you scan, you don't want to use the banks. Rather, scan the TV frequencies that you know are clear. For instance, scan up from 650 to 656 MHz. This will tell you if that channel is totally clear. Be sure you do the scanning with your receivers and antennas in their final locations, btw, as this will affect your results.

[TimmyP1955]

You also need Sennheiser SFM to tell you what frequency spacing to use. I don't remember if it has a place to insert the frequencies that are unavailable owing to TV or other wireless systems - our building rejects everything from outside, so I did not bother to look at this when I set up our wireless.

Sennheiser Worldwide: Microphones, Headphones and Wireless Systems

[Anonymous067]

... - our building rejects everything from outside, so I did not bother to look at this when I set up our wireless.

Care to elaborate?? :]

[mbenonis]

You also need Sennheiser SFM to tell you what frequency spacing to use. I don't remember if it has a place to insert the frequencies that are unavailable owing to TV or other wireless systems - our building rejects everything from outside, so I did not bother to look at this when I set up our wireless.

Sennheiser Worldwide: Microphones, Headphones and Wireless Systems

I'm assuming you say this because your cell phone probably doesn't work inside...but that doesn't mean that TV stations or other RF will be blocked (remember, they use 10's of dB more power than your local cell tower does, even accounting for distance and antenna gain).

[Chris15]

Actually it's quite possible for a venue to block most things, The gig I've just come home from is one of them. Only about 5 channels manage to make it into the arena. Apparently one of the venues in Melbourne basically completely blocks outside RF...