Sennheiser 300 series radio microphones - frequencies

[Lamplighter]

When your using multi radio mics you need to keep 500mhz spacing between bands to get noise free operation, anything below this gets you into the intermodulation zone.

I think this needs looking at more closely. The UHF radio mic band uses frequencies around 800 MHz so 500 MHz seperation would be somewhat limiting. The actual frequencies to use are determined by the design of the transmitter and receiver, so vary between manufacturer and even model chosen. Noise free operation with some channel separations as low as 100KHz is achievable if the channels are also intermod free. The first requirement is to consult the manufacurers literature re separation and then check your licence and finally do your sums.

Brian

[boatman]

When your using multi radio mics you need to keep 500mhz spacing between bands to get noise free operation, anything below this gets you into the intermodulation zone.

 

I don't think 500 milli-Hertz separation is going to be much help. Maybe you meant to write '500MHz', which is impossible as the available bandwidth for PMSE is considerably smaller than that.

 

Please clarify your statement.

[Bobbsy]

When your using multi radio mics you need to keep 500mhz spacing between bands to get noise free operation, anything below this gets you into the intermodulation zone.

 

No.

 

As lamplighter says, different manufacturers and models have different filter characteristics which affects how close together you can place channels. By the way, I assume the OP mean 500kHz, since "mhz" would be millihertz and far too small a separation while 500 MHz is more than the whole UHF TV band! However, most decent radio mics can work at rather better than 500kHz spacing.

 

However, this is where it gets more complicated. When you generate a specific radio frequency you will also be creating "harmonics" which are spurious frequencies with specific relationships to the original. These harmonics can also cause intermod problems and are why you can never give a simple, constant spacing which always holds true.

 

If you read farther back in this topic, you'll see that Sennheiser provide some software called SIFM which will either let you check planned frequencies for intermod issue or even work out a frequency scheme automatically. You can download SIFM (free) HERE.

 

Note that, for the reasons given by lamplighter, a programme like this can't be perfect when you mix manufacturers since the characteristics aren't always the same.

 

Bobh

[Lamplighter]

Not hand-held radio mic's, body-pack transmitters, need 20 UHF along with 8 VHF for a musical.

 

I may be off the mark but 28 radio mics is a huge number to aim for unless you are at Broadway or West End producion levels. Firstly you will need a suitable memory desk and secondly some very careful planning to keep the number of active mics under strict control. The intermod possibilities with 28 packs are enormous, and will require very carefull plotting. An experienced assistant or two backstage is also likely to be required.It is not neccessary to mic every member of cast, and often it is easier to move packs around the minor parts. I would advocate that each user has their own microphone and are properly trained how to connect and disconnect the mic with eyes shut! Whichever way you chose to go come back and let us know, I hope it works out for you

Brian

[Bobbsy]

In the past I HAVE done 28 RF mics on a non memory desk (gotta love digital now though!) and managed it okay, though my "beer to wind down" quota at the end of each show went up a bit! I hasten to add that this was a production decision against my advice. I had an easily workable "swap" scheme that would have halved the necessary mics. Indeed, in some of the chorus numbers I didn't bother bringing up the mics for some people and got better results working that way.

 

However, do be aware that the cost of licences and batteries (let alone renting the mics themselves) may come as a shock to your producers. I know that on the show I mention above, even though I kept the producers and the director informed throughout the process what they were letting themselves in for, when they got the final total in the end there was consternation.

 

Also, at the risk of sounding like a broken record, I notice one poster still suggesting constant .3 or .2 MHz spacings and I say again NO! Use SIFM or similar to get a workable scheme or you'll be destroyed by intermod.

 

Similarly, are you sure you can get 8 VHF frequencies? It's a few years since I've used any VHF but am pretty sure 8 would be pushing it, at least if you stick to legal frequencies.

 

Bob

[dunk_1984]

I notice one poster still suggesting constant .3 or .2 MHz spacings

 

Was that aimed at me? I know I generated some .2's and .3's, but that was just to see what you could squeeze out of the range.

 

Anyhow just pdf'd the .4 and .5 spacing specs:

 

830-866 @ 400

830-866 @ 500

[Bobbsy]

Okay, I see what you're doing.

 

Within SIFM you can specify the minimum spacings you wish to use, but when it calculates a frequency plan it doesn't simply stick to that and, where intermod products on harmonics would be a problem, increases the spacing. Your .pdfs show this...the actual spacing is often wider than your minimum to avoid the harmonics.

 

As generated by SIFM these schemes should be okay. What I was meaning is that you can't just pick a generous spacing and assume you can use all the frequencies that far apart...intermod problems from harmonics will get you every time.

 

Bob