Transcript
Pro Sound Web Live Chat With James Stoffo
April 11, 2001

(Moderated by Dave Dermont)

Moderator: Welcome to our chat with James Stoffo.

Moderator: James, can you start with some basic background information about yourself.

James Stoffo: Hello everyone. I began dealing with RF as an electronics tech in the US submarine service. My job was to analyze RADAR and communications. After I surfaced for the last time, I went to work for Vega Wireless Systems as an applications engineer.

James (continued): I learned quite a bit by seeing professional installations on Broadway, theme parks and broadcast operation. I was supposed to be the “guy from the factory”
that helped pick frequencies and antenna systems. Fortunately, I had some very good teachers.

James (continued): Now, I work as a show RF technician on special events, and install wireless systems throughout the country. These include wireless microphones, intercoms, in ear monitors and IFB systems. All are subject to the same rules and laws of physics.

Moderator: How do you get information on the RF realm when going into a new city?

James: There are many ways to obtain this sort of information. If you do not have a radio spectrum analyzer, you can contact the local SBE (Society of Broadcast Engineers) coordinator (www.SBE.org). The main concern is TV transmitters, so you can even consult the local paper for TV channels local to a city.

James (continued): And of course, there is the radio scanner. You can purchase a good one
for about $300 at a local ham radio retailer. I recommend this.

Gary Stanfill: When you do big shows, how do you handle people showing up with their
own wireless systems on random frequencies?

James: We establish a media check-in point where we have a spectrum analyzer or two,
a frequency coordination program, a list of ALL of the previously cleared frequencies and a handful of different colored ribbons for antennas.

James (continued): At the checkpoint we scan each transmitter that a person would like to use. If it meets specification and is no less than 300 kHz from a known good frequency,
we allow use. If not, we ask that the receiver antenna be disconnected. This flags the RF crew that the system will not be used.

James (continued): During the event, we monitor OUR frequencies. If there is a renegade frequency, we "DF" (direction find) it, real time. If I find an unauthorized transmitter...I get very mad! We also have RF "war games" - every transmitter is "lit" to find out if there may be intermod problems.

Moderator: How good of a job do scanners built into wireless microphone receivers do at finding a frequency?

James: The scanner built in to a wireless receiver will only scan the frequency band of that particular system. This will give you a 24 MHz picture of the RF within that band, but it will not include other RF sources, which will affect the system. Many RF problems are caused by harmonics and RF distortion that come from outside of the actual wireless microphone receiver band.

Moderator: How will DTV and public safety, as well as other high-power RF sources affect wireless operation? (Both in the near and distant future.)

James: DTV has already begun to affect wireless microphone systems. The answer is that DTV and public safety will render wireless microphones unusable. The trick is to ensure that none of YOUR wireless are within the RF band of active DTV stations At least we know what the DTV stations are and which RF bands they will affect.

James (continued): By the way, some of these DTV allotments may change due to new reallocation by the FCC. Public safety will probably take out more wireless microphones in one swoop than any of these other broadcasts, because the public safety frequencies will all energize at one time, over a large portion of the spectrum. Whereas DTV is energizing one station at a time over a period of six years.

Gary Stanfill: Isn't it true that a receiver can only monitor one frequency at a time, but a spectrum analyzer can look at hundreds?

James: My first choice in the field is a good radio spectrum analyzer, but that could cost many thousands of dollars. The scanner within the wireless receiver has many limitations. If a signal is energized on 525.5MHz, but the receiver scanner happens to be looking at 525.9MHz, scanning upward, it may miss the potential interference.

Gary Stanfill: Well, considering the problems with DTV, public safety, medical telemetry, video assists, won't a lot of working pros have to spend the money on an analyzer to be effective?

James: If wireless microphones, intercoms and in-ear monitors are within your realm of responsibility, I recommend that you purchase a spectrum analyzer. I would say that within two years you will not be able to survive a show without one.

Arthur: Can you give some pros and cons of using passive vs. active antenna splitting systems for multi-channel wireless systems?

James: The wonderful thing about a passive antenna splitter is that there are no active RF circuits to overload, tune or mix RF into nasty, noisy problems. The problem is that there is no gain, only loss. I wouldn't use a passive splitter on more than four wireless receivers
and only then if they are close to the receiver antenna. And I would use a passive gain antenna such as a Yagi or Log periodic.

James: An active splitter, on the other hand, has gain to make up for the split loss
but there are active RF circuits involved. This increases the potential for intermodulation distortion, which may result in those horrible noise bursts that turn our stomach.

Moderator: Which antennas are best for certain applications?

James: Recently I become a big fan of helically polarized antennas. These would be used for long-range applications. Next on the list would be Yagi antennas, which are very directional and have a narrow RF bandwidth as well as up to 10 dBd of gain. Last but not least are the Log Periodic antennas. These have some forward gain, usually about 6 dBd
and are very wide band up to 500 MHz.

James (continued): I generally do not use omnidirectional antennas for wireless microphones - you can use the directional properties of a Yagi or LPDA (Log Periodic)
to reject potential interference. An omni antenna is best used on a wireless intercom system
a situation where you never know where the portable packs might be.

Gary Stanfill: What has been your experience with amplified antennas and RF line amps (preamps)?

James: I try to stay away from using RF amplified antennas, but sometimes there is the need to run long coaxial cables to center stage. I would rather use a passive gain antenna, but if that is not enough, I will use a helical antenna with about 30 feet of RG-8 before inserting the RF line amp. This decreases the chance of overload in the amp but provides the gain needed to make it through the rest of the coax run.

Keith: I seem to recall reading a while back about a proprietary antenna splitter system you've developed. Is this the case, and if so, can you give us an overview?

James: We develop all kinds of things to fit a particular application. The latest is to use a wideband antenna, like a helical or log, and split the signal by RF band at the antenna so that only the RF bands that we want are amplified.

James (continued): This reduces the chance of intermod. When dealing with multiple RF devices in close proximity, the name of the game is less power from the transmitter, more selectivity and sensitivity in the receiver antenna system.

Keith: I know you use high-end systems, primarily, but is there an "entry level" (low cost) wireless mic system you're comfortable recommending?

James: To my knowledge the best bang for the buck would be Sennheiser's Evolution Series. They have multiple frequencies, sound good and are $500.00 (US)!

Gary Stanfill: What are the main differences between doing industrial/corporate shows and doing big events?

James: The wild card seems to be the ENG wireless user. There is less of a chance of running into uncoordinated wireless at a corporate industrial than there is at a big event.
If people would like to see it on TV, there will be news media there. And news media LOVE wireless microphones, even if they only need to work six feet away from the camera. But they still pump out 50 to 250 milliwatts of RF and they still create interference on wireless receivers 500 feet away.

James (continued): At an industrial event, the main source for interference seems to be self generated - lighting dimmers, digitally controlled devices, etc.

Keith: From an RF standpoint, what's the absolute worst event you've ever worked, and why?

James: If I tell you, I won't get booked next year! They all have their ups and downs - the ones that are the most difficult are also the most rewarding. But the Super Bowl halftime rehearsal in San Diego will always be at the top of the list.

Todd Herrbach: Hi James. How many wireless systems were in use on game day at this year's Super Bowl? How many were used during the halftime show? Any stories to share from that show?

James: Hello Todd. This year was interesting because MTV produced the show. Every wireless microphone used during the halftime show was live this year. There were 12 RF microphones, 12 in-ear monitors, and about 30 drops of RF PL. These had a backdrop of
52 “coach com” frequencies, about 65 RF microphone and intercom systems for the broadcast, several ref mics and NFL films microphones, and something like 275 ENG crews (that we knew about). Also many DTV and LPTV transmitters lit in Tampa.

Gary Stanfill: How many wireless systems are "typically" used for a big industrial show by a Fortune 250 company?

James: It’s getting pretty ridiculous. The last industrial that I was asked to coordinate
had 95 wireless microphones and intercom frequencies operational at the same time.

Keith: So, project the next decade... Will wireless as we know it still exist? Or, what will the most significant changes be?

James: Up until the year 2006 we will have to contend with DTV energizing daily. That will keep us on our toes and may cause many problems for people not in the know.
After the year 2006 it gets much worse. That is when the Public safety begins to energize
and if the FCC reallocation plan takes place. We will lose over 100 MHz of spectrum
currently used by wireless microphones.

Moderator: Are there any advantages to sending a digitally encoded RF signal?

James: A digitally encoded RF signal will prevent a noise burst, but if there is a stronger carrier on the same frequency it will quiet the receiver, no matter who is singing on the transmitter.

Keith: So, when you're not looking for offending RF, just what are you listening to on that "super scanner" rig?

James: Cell phones… Just kidding! Usually, I am looking for TV stations and monitoring my own RF systems. The RF environment is a dynamic one. In addition to RF problems, I am listening for problems with microphones like broken connectors, improperly worn lavs, etc.

Arthur: What about spread-spectrum technology? Do you think it can be developed to work with wireless mics?

James: I have listened to spread-spectrum wireless intercoms, and at the current time
I don't think that the production community is willing to sacrifice audio quality in exchange for RF performance. I do believe that this will change, and that if given a choice between a wired com pack or a wireless with a 25-millisecond delay and squashed audio, they will take the wireless.

James (continued): I don't think that there will be a usable spread-spectrum wireless microphone for quite a long time, but I sure hope that someone is working on it.

MarkA: Is the 5 GHz ISM band viable for wireless mics?

James: The problem with moving too high up in the spectrum is that the signal becomes very line of “sight-ish”. Either more power or some very serious antenna systems will be required. In addition, type and placement of the antenna systems will be critical.

James (continued): It also becomes more complicated to design and maintain these devices at higher frequencies. For example, in our shop we work on all sorts of RF devices but our spectrum analyzers do not go over 3 GHz. I don't even want to think about the cost of test equipment and accessories for systems operating at 5 GHz.

MarkA: But are not the 500 to 700 MHz bands also very line of sight, and human bodies absorb quite a bit if part of the path loss...

James: Picture a typical stage. Obstructions include metal props, chicken wire, lights, cameras, etc. 500 to 700 MHz signals will get around these with proper antennas. Also cable loss is not great, less than 4 dB per 100 feet.

James (continued): At 5 Ghz one would have to use a line amp on an antenna at the other side of a stage. At this point in time the highest frequency wireless microphone device, to my knowledge, is 2.4 GHz. And THAT band is not the best choice, because there are more device types authorized to use that band than the current wireless microphone band of 500 to 700 MHz, and 170 to 216 MHz.

Gary Stanfill: Have you run into "video assist" transmitters on any of your jobs?

James: AAAHHHHHH! VVVVVideo Asssssssist! Video assist devices are responsible for more of the gray hair on my head than any other RF source. They are illegally used at most major events. They transmit more power than the average wireless microphone, take up as much RF bandwidth as a TV transmitter, and move around so quickly that they are nearly impossible to locate. Even with a spiral antenna.

Arthur: What is a video assist transmitter so that I can stay clear!

James: A transmitter that is placed at the video output of a camera so that someone can remotely monitor what the camera is seeing in real time. The problem is that the user does not know that they need to coordinate, and if they tried, the RF coordinator would not allow use. This is because the device has such a long history of causing interference on other wireless devices.

James (continued): They are up to 6 MHz wide in RF band and operate in the television spectrum, right where wireless microphones operate. Oh, by the way, did I mention that the FCC would like to make it legal to use these devices, at .5 watt, from 698 to 746 MHz?
Right on top of what little RF band we have left. This is a current reallocation plan.

Tom B: Are there any non-RF solutions on the horizon to help with mics?

James: Only one, I’m afraid - a wire! I have heard of an IR system, but it’s not ready for the professional market yet.

chat.boy: This concludes this evening’s chat. On behalf of ProSoundWeb, I'd like to thank James for sharing his time and knowledge.

James: Thank you all, and good night.

chat.boy: Be sure and join us on April 17th for Dale Alexander, April 23rd for Dave Lee of Rhino Acoustics and April 30th, Dave Rat, Alternative Sound legend. All chats kick off at 9:30 pm Eastern.

chat.boy: Visit http://prosoundweb.com for more info, and join us in the Live Sound Chat for "After Chat".