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Darkness Before The Dawn? Is The UHF Spectrum Going To Ease Up After June 12?
Will it be like the heady days of the 1990s? Certainly not. But the good news is that things should lighten up a bit
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One of the more common phone calls I’ve been receiving lately is from people looking for advice on which frequency bands to choose for their new wireless systems.

Maybe it’s all of the tours going out, or maybe people are finally getting the message about the spectrum changes and they want to “do the right thing”.

I’ve certainly noticed that quite a few systems proliferating the market in the last 10 years fall within the “700 MHz band” (between 698-806 MHz). I’d say that for many manufacturers, close to a third of what they’ve sold has been in this part of the spectrum. Sennheiser Evolution “C” range, Lectrosonics blocks 28 and 29, Audio Technica “E” range, for instance, all fall within the 700 MHz band. Some manufacturers have offered nothing but products in this range.

So here’s how these kinds of calls usually go:

Sound Guy: “So, we’re gearing up for our tour, and we already have vocal systems from Shure on J5, IEMs from Sennheiser in their B range, and a few other backline systems on a variety of ranges. We need to get some additional vocal wireless channels. What frequency band should we be looking at?”

Mike Wireless: “Well, you’ll need to squeeze in between what you already have since you’re covering everything from 518 to 650 MHz. Most manufacturers don’t offer much below 518 MHz. And some of the bands starting at 470 MHz can be problematic in large metro areas due to the public safety bands in that range.”

Sound Guy: “Wow, so what are we going to do?”

Mike Wireless: “You might want to consider consolidating all the backline stuff into one frequency band, and then look at something in the 490-518 range, then maybe something in the 650-698 range. That’s about all that’s left.”

SoundGuy: “OK, we’ll start there.”

Here’s the bottom line: with the analog TV stations still broadcasting, and DTV transmissions starting up or moving down below 700 MHz, the available UHF spectrum is more crowded than it’s ever been. Is there a light at the end of the tunnel? Maybe.

Come June 12, full-power analog TV broadcasts should cease, thus opening up some of the spectrum between 470 to 698 MHz.

Will it be like the heady days of the 1990s? Certainly not, because DTV signals take up a full 6 MHz where analog TV signals did not. And there are more channels overall now than there were 10 years ago.

But the good news is that things should lighten up a bit, and logically, we shouldn’t expect a whole slew of DTV channels to be added any time soon. The summer touring season should get a break, then.

That is, until the TVBD (TV Band Devices) start showing up. It’s difficult to predict how those might impact our use of wireless mics, but the main problem is that the signals will be transient rather than steady, so we may have to employ different techniques for scanning and coordinating wireless microphone frequencies.

Signing off for now…
Mike Wireless

Mike Wireless is the nom de plume of a long-time RF geek devoted to better entertainment wireless system practices the world over.

Previous posts by Mike Wireless:
Change The Only Constant In Marketplace For Wireless Spectrum


Comments (3) Most recent displayed first
Posted by Henry Cohen  on  06/02/09  at  07:56 PM
"in principal that it might be possible to slide a wireless mic or two in between two adjacent stations. However, in practice I’ve found this difficult, even when inside buildings and many miles away from the DTV towers. I’d be interested in further input from you on the subject, however, because these are the kinds of situations people will encounter."

Almost the same considerations as when 'slotting' in a mic channel between the chroma and audio carriers of an NTSC TV signal are needed when determining the viability of using the channel edge between adjacent DTV signals: Overall environmental RF noise floor across the occupied channel bandwidth and RF link budget of the wireless mic transmitter/receiver.

A few additional points are to know where the DTV transmitters are in relation to the stage or performance area and to try to use the gain and null characteristics of the typical LPDA antenna to advantage; keep active system gain to a minimum in order to keep from increasing the overall RF system noise floor with each successive gain stage and possibly saturating the multi-coupler or receiver front ends. This means no active antennas, using passive or unity gain multi-couplers with high IP3 and high quality, low loss coax of proper size. Lastly, pay attention to RF gain structure; insure there is sufficient gain above the environmental RF noise floor to account for the variable free space and obstructed path loss from the transmitter to the receiver.

Posted by PSW Editor K Clark For Mike Wireless  on  06/02/09  at  05:03 PM
Henry,

Thanks for weighing in on this important subject - I appreciate the additional details about the June 12 date and the implications of the changes we have in front of us. I was certainly generalizing and these details you've given may help users in some locations and in specific situations.

Regarding the bandwidth of DTV broadcasts, I agree with you in principal that it might be possible to slide a wireless mic or two in between two adjacent stations. However, in practice I've found this difficult, even when inside buildings and many miles away from the DTV towers. I'd be interested in further input from you on the subject, however, because these are the kinds of situations people will encounter.

Mike Wireless

Posted by Henry Cohen  on  06/02/09  at  01:03 PM
"Come June 12, full-power analog TV broadcasts should cease . . ."

Not quite. The DTV Border Fix Act of 2008 which the Senate passed last year and the house can still approve would permit any full power analog TV transmitter within 50 miles of the Mexican border to remain in operation until 2014. Further, several full power analog TV stations in major markets will remain on the air as "night light" stations to broadcast emergency announcements (Short-term Analog Flash and Emergency Readiness - “SAFER” - Act) for up to 30 days after June 12th.

On a side but related note, the FCC seems to be permitting TV stations to increase translator/booster power, and even add boosters where necessary, in order to fill in contour coverage area where the digital signal propagation falls short.

". . . because DTV signals take up a full 6 MHz where analog TV signals did not."

Also not quite true. The fundamental energy mask is only 5.38MHz wide centered within the 6MHz channel. Even with two adjacent DTV stations, the common channel edge will be over 70dB down from the aggregate channelized power, meaning it will be a very real place to put one, maybe even two two wireless mic/com/IEM/IFB frequencies providing DTV signal field strengths aren't too strong within the venue.

Henry Cohen

Production Radio Rentals

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