Shure SLX IM

[Mattangles]

Hi Guys,

 

Just a quick post to try and get an understanding to a situation that im having.

 

I have been given 16 shure slx recievers and transmitters to run on a show. Now I understand that the system can only run a maximum of 12 systems before IM etc becomes a big isssue, but why? I have used 16 before (Because I had too) and I very carefully selected channels to ensure a decent interval between frequencies (around 400mhz I think) and ran the frequencies through a planner to see if there where any obvious IM problems according to the software. The receivers were running on the S4 band which I think is 839.000 to 865.000Mhz. Using 16 channels on other systems before I havent had any issues and ive followed the same procedures. Why is 12 the limit.

 

Like I say its curiosity and the craving to understand more than anything.

 

Cheers,

 

Matt.

,

[Simon Lewis]

Matt,

 

One limiting factor will be selectivity - how good is the receiver front end at accepting the wanted frequency and rejecting unwanted adjacent frequencies.

Another is (as you've mentioned) intermodulation - which can be a problem with just two systems, let alone 12 or 16. It isn't just about separation, but

The manufacturer is likely to state an upper limit to indicate what can be reasonably achieved. This is more helpful that just saying that the radio can tune to hundreds of frequencies - we need to know how many can reasonably coexist across the model's tuning range.

Getting systems to work can also be problematic from the licensing viewpoint. Whereas a channel 69 licence gives you ~ 14 frequencies to work with, and you have a subblock of spectrum in 70, you would need to buy fixed site specific frequencies below channel 69. This can limit the frequencies that you have to work with...

 

Simon

[Al M]

If you download "Shure Wireless Workbench" from the Shure website (it's free), you can use 'synthesis mode' to enter exactly which model of Shure mic you are using, and which TV channels you want to use them in.

 

It will then tell you how many frequencies you can fit in without intermodulation.

[Mattangles]

Thanks Simon,

 

That makes alot of sense, also the fact that the quality of the kit can result in the receiver not always successfully rejecting unwanted frequencies. Would the power of the transmitter make much difference. I think the slx transmitters are 50mw. Would a higher powered transmitter cause more issues or not, just out of intertest.

 

Matt.

[Simon Lewis]

Thanks Simon,

 

That makes alot of sense, also the fact that the quality of the kit can result in the receiver not always successfully rejecting unwanted frequencies. Would the power of the transmitter make much difference. I think the slx transmitters are 50mw. Would a higher powered transmitter cause more issues or not, just out of intertest.

 

Matt.

 

 

I missed out a bit (hit return before I should have done)... Intermodulation is quite complex but you can read articles here or just Google... The RF experts can better predict the effect of higher RF output...

[Rob_Beech]

The problem here may not be as it appears.

 

As Simon mentions we get a certain number of frequencies in Channel 69 and some in part of Channel 70. However, what you are stating is that you are using the full available frequency range of the system to achieve this number of systems. There is little wonder you can achieve more than the recommended number of simultaneous systems. It's a fair sized gap but not one you are allowed to use in full.

 

The list of frequencies you are allowed to use will be found on your copy of your Channel 69 license. Using anything other than these (particularly outside of this) is against the law.

 

 

Rob