Sennheiser G2 Wireless fatal flaw?

Steven Davis · May 27th, 2008, 01:23 PM

Quote:

Originally Posted by Ty Ford

Wrong frequency block, cell phones, Nextel Phones, iPhones.

Regards,

Ty Ford

Ty, you crack me up, to the point, that's funny but you're right, could be any of those things. I have documented my trials with my wireless issues and ended up (after trying lectros on the same frequency) getting a set of sennheiser g2 100s on a frequency block that was contrary to sennheisers chart. My issues are much better, but you still have to watch out for cell phones etc. One suggestion, if any of your wedding party has a cell phone, tell them to turn it off or just don't carry it down the isle.

Paul R Johnson · May 27th, 2008, 03:57 PM

I've been running theatrical multi-channel radio systems for years and you're making some assumptions based on common sense, but they are wrong!

The kind of problem heard on the link is very, very common. It isn't interference from cellphones, or powerful radio systems or anything like that. It's classic intermodulation interference, normally caused by the choice of frequencies you're using. It's really nasty, difficult to track down, but quite easy to solve.

Firstly, you have decent equipment - the Sennheisers are simple to programme, and will cheerfully operate on a wide range of frequencies, some of which are legal to use freely, some require (here in the UK) a licence issued by JFMG.

The problem is that due to the design of the receivers (not usually a transmitter problem) - when you use more than one odd things can happen. The maths is a bit complex and you need to know some internal technical details of the receivers, but if any of the frequencies in use are mathematically related to each other, and/or the internal frequencies generated as part of the receiver circuitry, then phantom signals get generated, and these have the nasty habit of wiping out even quite strong signals on the input - as in why even being very close in, isn't strong enough and you get drop-outs, as in the recording. You also get that characteristic rumbling sound as audio frequencies mix internally. A Google for RF intermodulation will give you some of the technical details if you're interested.

The solution is to go to the manufacturers web site and they will have lists of frequencies that have been tested to be intermod free.

Only last week I was presented with a rack of 10 Trantec UHF receivers that had exactly this problem. A quick trip to the Trantec site showed lists of frequencies for 6, 10 and 12 channel set-ups. Changing to their suggested frequency plan totally cured the system. The ones picked originally by the supplier were the four 'free' channels, plus some from the regulated ones. They just weren't any good with each other.

Some radio mic channels will stand the chance of being interfered with from TV transmitters in certain areas of the country, and digital TV, squeezed into the gaps is very radio mic unfriendly, but this is unlikely, and sounds a bit like a buzz saw - not a rumble. Signal strength when it falls below the idea is white noise - so varying levels of hiss signify loss of level. Just beware that the intermod products may make level an issue even close in. I like to think of 10m as being a long way. Over that, and signal dropouts are likely. A receiver on the back of the camera, like the popular 112 Sennheisers isn't offering any diversity reception, so if it fizzes once, it will do it again.

Find working frequency combinations, make notes of what channels are useful. Scanning for empty channels really isn't necessary. It will tell you if another channel is in use by somebody else - common in theatrical venues that could be next door to each other, but it's a crude system. In crowded signal areas such as Broadway and the West End, there are much better and accurate devices available to do this. Some of the Sennheisers have a pilot tone system, so scanning for empty channels may not reveal the other user - because their tone may be different or missing and you could 'land' on a already occupied channel and not notice. Also weak signals may not trigger the scan 'move on' feature.

Other tips for maximising signal strength.

Mens inside pockets are handy - but make sure the aerial is not squished up, or pressed hard against the body - 15 stones of person is a nice absorber of RF energy, so a quick turn that places the body between aerial and aerial will be bad news. If the body is sweaty, even worse - salt makes absorption worse.

Radio mic systems are so unreliable that you do need to be careful. I think that operating on the four free channels is a bit risky - the local pub or karaoke will almost certainly be using them. Even if you have to pay a license fee, it's well worth it.

Solid 100% radio systems are possible - but it's the second one in operation that starts the problems rolling.

In my world, it's very common to have somebody whose job is simply baby sitting the things. One set of batteries is still pretty common per show, despite the manufacturers specs suggesting two or three should be possible. Rechargeables are still not common, no longer because of battery life, just because the possibility of re-inserting a nearly flat, un-recharged one is still too high.

Somebody will be listening to them, ready to swap packs, mics and batteries as soon as the clicks, bangs and hisses are heard.

Dean Sensui · May 27th, 2008, 06:22 PM

BTW, the noise I was referring to was the one that Graham Risdon posted.

It's a familiar RF noise picked up right before a Nextel phone starts to ring.

May 27th, 2008, 06:22 PM	#48
Dean Sensui Inner Circle Join Date: Oct 2001 Location: Honolulu, HI Posts: 2,054	BTW, the noise I was referring to was the one that Graham Risdon posted. It's a familiar RF noise picked up right before a Nextel phone starts to ring. __________________ Dean Sensui Exec Producer, Hawaii Goes Fishing

May 27th, 2008, 03:57 PM	#47
Paul R Johnson Inner Circle Join Date: Nov 2005 Location: Lowestoft - UK Posts: 4,015	I've been running theatrical multi-channel radio systems for years and you're making some assumptions based on common sense, but they are wrong! The kind of problem heard on the link is very, very common. It isn't interference from cellphones, or powerful radio systems or anything like that. It's classic intermodulation interference, normally caused by the choice of frequencies you're using. It's really nasty, difficult to track down, but quite easy to solve. Firstly, you have decent equipment - the Sennheisers are simple to programme, and will cheerfully operate on a wide range of frequencies, some of which are legal to use freely, some require (here in the UK) a licence issued by JFMG. The problem is that due to the design of the receivers (not usually a transmitter problem) - when you use more than one odd things can happen. The maths is a bit complex and you need to know some internal technical details of the receivers, but if any of the frequencies in use are mathematically related to each other, and/or the internal frequencies generated as part of the receiver circuitry, then phantom signals get generated, and these have the nasty habit of wiping out even quite strong signals on the input - as in why even being very close in, isn't strong enough and you get drop-outs, as in the recording. You also get that characteristic rumbling sound as audio frequencies mix internally. A Google for RF intermodulation will give you some of the technical details if you're interested. The solution is to go to the manufacturers web site and they will have lists of frequencies that have been tested to be intermod free. Only last week I was presented with a rack of 10 Trantec UHF receivers that had exactly this problem. A quick trip to the Trantec site showed lists of frequencies for 6, 10 and 12 channel set-ups. Changing to their suggested frequency plan totally cured the system. The ones picked originally by the supplier were the four 'free' channels, plus some from the regulated ones. They just weren't any good with each other. Some radio mic channels will stand the chance of being interfered with from TV transmitters in certain areas of the country, and digital TV, squeezed into the gaps is very radio mic unfriendly, but this is unlikely, and sounds a bit like a buzz saw - not a rumble. Signal strength when it falls below the idea is white noise - so varying levels of hiss signify loss of level. Just beware that the intermod products may make level an issue even close in. I like to think of 10m as being a long way. Over that, and signal dropouts are likely. A receiver on the back of the camera, like the popular 112 Sennheisers isn't offering any diversity reception, so if it fizzes once, it will do it again. Find working frequency combinations, make notes of what channels are useful. Scanning for empty channels really isn't necessary. It will tell you if another channel is in use by somebody else - common in theatrical venues that could be next door to each other, but it's a crude system. In crowded signal areas such as Broadway and the West End, there are much better and accurate devices available to do this. Some of the Sennheisers have a pilot tone system, so scanning for empty channels may not reveal the other user - because their tone may be different or missing and you could 'land' on a already occupied channel and not notice. Also weak signals may not trigger the scan 'move on' feature. Other tips for maximising signal strength. Mens inside pockets are handy - but make sure the aerial is not squished up, or pressed hard against the body - 15 stones of person is a nice absorber of RF energy, so a quick turn that places the body between aerial and aerial will be bad news. If the body is sweaty, even worse - salt makes absorption worse. Radio mic systems are so unreliable that you do need to be careful. I think that operating on the four free channels is a bit risky - the local pub or karaoke will almost certainly be using them. Even if you have to pay a license fee, it's well worth it. Solid 100% radio systems are possible - but it's the second one in operation that starts the problems rolling. In my world, it's very common to have somebody whose job is simply baby sitting the things. One set of batteries is still pretty common per show, despite the manufacturers specs suggesting two or three should be possible. Rechargeables are still not common, no longer because of battery life, just because the possibility of re-inserting a nearly flat, un-recharged one is still too high. Somebody will be listening to them, ready to swap packs, mics and batteries as soon as the clicks, bangs and hisses are heard.