Meeting Topic: Managing Wireless In Real Life Situations
Moderator Name: Sy Potma
Speaker Name: Chris Brooks and Ike Zimbel
Other business or activities at the meeting:
Sy opened the meeting. He thanked Ryerson for use of facilities and took an informal survey of Toronto AES website visitors. The international convention was mentioned. Future Toronto section meetings were announced.
Robert DiVito's tech talk highlighted recent DAW updates.
Sy introduced the speakers.
Meeting Location: Eaton Lecture Theatre, Rogers Communication Centre, Ryerson University
Chris Brookes started the talk which was a modified version of what he teaches.
He began by reviewing the radio spectrum (in reality the electromagnetic spectrum). He then briefly discussed the subject of wavelength as a lead-in to intermodulation.
Easy formula to calculate wavelength: Wavelength = 300/(f in MHz) metres.
This application is good for determining antennae length, as the ideal amount is approximately ¼ the result. When two antennae are used for diversity, in order to be effective they have to be at least ¼ of a wavelength apart. Diversity performance increases the farther the antennae are apart up to one wavelength — after that there is no advantage.
Discussing changes in the RF noise floor: any interference you don't want to receive generally is referred to as noise. Sources of interference include TV transmitters both analog and digital; digital equipment: LED & LCD screen dimmers; and other wireless systems. Secondary sources are unknown radio transmitters such as wireless in ear monitor systems; portable studio transmitter links, GSM devices, and hi-capacity power equipment.
The 'big one' is inside interference — within the system itself otherwise known as intermodulation distortion (IMD).
Each system must operate on a unique frequency. Minimum frequency separation depends on the selectivity of the receiver: 0.4 MHz for better systems up to 1.5MHz apart for lesser systems. 400 kHz is the minimum separation one would want on the same stage between two transmitters.
Transmitter IMD products: inherent non-linearity of wireless circuitry occurs between 2 or more transmitters. Generated in transmitters and /or receivers. IMD product strength is proportional to square of transmitter power; inversely proportional to square of transmitter separation.
Mr. Brookes then performed a demonstration with two transmitters to illustrate his point. He displayed a chart of IMD mathematical calculations for two frequencies. The ones of concern were these: 2f1 — f2 and 2f2 -- f1, referred to as 3rd order intermod. 5th order (in most cases) and higher products are not of a concern because they are weaker. 5th order can be a problem if you get too close to the receiver, 15 feet being the recommended minimum.
IMD was not talked about in early days of wireless.
There is also something called 3 transmitter intermodulation distortion. This creates 12 frequencies — 3 wanted, 9 NOT. There are no 4 transmitter IMD products, and IMD products themselves don't create own IMD!
This problem can be worked out at the manufacturer's end, or with an intermod program
such Wireless Workbench which is free. There are other more sophisticated programs as well.
Diversity reception was the next topic. Diversity systems have two antennae to deal with multipath interference — the re-introduction of transmitted wave reflected off a surface back into the receiver. This delayed addition introduces phase distortion and, if it's 180 degrees out of phase, a complete cancellation referred to as a dropout. This can be the disappearance of a signal or momentary rise of the noise floor.
Diversity systems 'look' for signal drops and switch to the 2nd antenna to avoid this. The basic systems are built with 2 receivers in the case with a switch comparator. Shure has a system called Predictive Diversity which looks for very sudden signal drops. Another system is called Maximum Ratio Combining Audio Diversity (MARCAD) which uses a comparative audio combiner.
Squelch was the final topic. All FM receivers make a 'shhhh' all the time. Squelch is a noise gate at the audio o/p. It mutes the noise when the transmitter is turned off. Noise sensitive squelch is a later innovation which mutes the audio if it drops below a certain S/N ratio. Finally tone key squelch (or tone coded squelch) where the transmitter, additionally, sends out a unique tone which the squelch circuit looks for. The value being that the circuit can identify its own transmitter. For systems that are on 24/7, stray RF signals are eliminated. It also eliminates pops at the output when shutting off the transmitter.
Questions were taken from the audience.
After the break, Ike Zimbel 'entertained' the attendees with his real life experiences.
'Rule No. 1' — The number of wireless channels on a show are never what you're told they will be!
The topic of scanning was next. Scanning is not the answer to wireless problems. Scanning is a tool but a time sensitive one. It is entirely dependant on what is happening when you do the scan, meaning it depends on what channels are on or not when the scanning is done.
Scanning does not address intermod issues. What a scan can do is give an overall picture of the RF environment at the time of scan.
The summary of his next illustration showed it's possible to run 26 channels of very mixed fixed & agile frequency systems for 126 tapings over a or 6 month period with no dropouts or disruptions of any kind. This, in a studio, with reports of being a very tough RF environment.
This was possible by using a computer program to figure out intermod, which Ike demonstrated. Ike accepts only 'clean' frequencies — those without any intermod products - because you can't predict what even 7th order products can do.
Mr. Zimbel's next example — the World's Aid Conference of 2006 opening night event - described a situation where he could not find a clean frequency for all involved requiring him to 'manage' frequencies instead of coordinating them meaning he had to make sure that only the wireless channels that needed to be on were indeed the only ones on.
The next example involved 'One Night Live' of 2008 illustrating the need for a more effective and advanced intermod program because of over 60 active channels on the show. The program was the IAS Intermod Calculation Program from Professional Wireless Systems. He also used the Aaronia AG Spectran spectrum analyzer. The analyzer can highlight problems that the intermod program will otherwise declare is an acceptable frequency.
His last topic was entitled 'When things go wrong'. If something's going wrong chances are it's probably an intermod problem. If that is sorted out, then it's problem that requires an attitude that's free of the opinion that wireless is 'some spooky, weird' stuff.
The most common problem: if it works here but not there, it's most likely an antenna or cable problem.
Mr Zimbel then took questions from the audience.
Blair Francey thanked all the presenters.
Written By: Karl Machat
