Intermod calcs for mixed radiomics

[gibbothegreat]

Hi, Have a show coming up where the company are bringing their own 12-way system of Trantec 4.16 radiomics (8 on Ch38, 4 on Ch70) and wish to use another 3 systems. I have sources from which I can hire Evolutions which can be set to Ch39 or 40, however, the difficulty I have is that, on both of the intermod calculators I've tried (Audio Technica and Audio Ltd), the original set of 8 Trantec frequenceis are already throwing up interference products before adding any extras. Does anyone have any tips for how to work out the best additional frequencies under these circumstances?

[paulears]

The problem is that the manufacturer specific calculators also take into account the deviation from the centre frequency that their system is designed for. So as this has an impact on system performance. There is always a trade off - higher deviation may well increase quality and improve signal to noise, but it will impact on the number of available channels. Computers have no problems with the maths, when it gets complex - which it does when you have more than three channels on the go. The maths isn't harder, there's just more of it - so the basic maths to identify the products on a straight carrier wave is messed up when the presence of audio widens the active spectrum it uses. So you need to test the operating frequency plus or minus the maximum deviation. If you try a few of the online calculators, you can add or subtract a bit from the centre frequency and see what results change - often wiping out a 'space' you could have used. This is why different manufacturers have different results. Sennheiser tweaked their spec a little on the G3, enabling a few extra channels compared to the previous G2. Cheaper receivers are also often poorly filtered - so the front end is wide open to all sorts of off channel activity.

 

Mixing brands puts you in a tricky position because any results you get from a computer will not necessarily work with other kit.

[Wilflet]

Have you tried Shure Wireless Workbench?

If memory serves theres an option to turn off certain orders of intermod errors- so if theyve got a frequency list that they want to stick to that they are confident works in practice but the software is a little iffy about you can work around it.

[gibbothegreat]

Thanks both, will give the Shure program a go tomorrow. Mixing brands also unavoidable, because the Trantecs come with fixed frequency options which only include Ch38 in that option.

[Lamplighter]

The problem will not be solved by changing calculation program. There is inherently 3rd order intermodulation between the Trantec channel 38 and channel 70 preset frequencies. It may be that these can be lived with in practice but be prepared to find sudden unexplained noises. If you want try adding additional frequencies you could try calculating just the channel 38 frequencies and adding the proposed new ones one by one. If it helps the s4.16 appears to require a minimum separation of 425kHz and on my intermod program, a deviation window of 49kHz.. Don't try to find clear frequencies using 5th order calculations, just 3rd order, 3 transmitter filters. PM me if you have some frequencies that you would like me to try.

Brian