Question: How Big a Problem Is Interference for Wireless Mics?

D

Denniscasey

Well-known member
Due to my inexperience I had not considered interference from other sources when considering an inexpensive wireless mic for interviewing race car drivers at racetracks. I appreciated earlier advice and video reviews of various mics from my earlier post about this. However, this is not something I do a lot of, so continued looking for "inexpensive" (around $200) options such as Azden WLX-Pro or Audio Technica PRO 88W-830.

Today I was talking to B&H Photo about these mics and person said a problem at public events like race car races could be interference from other PA and broadcasting going on. The cheap wireless set ups offered only two selectable VHF channels so no real option to tune out interference if I encountered it on these two channels. He suggested the Azden 105LT which offered 92 selectable UHF channels at reasonable price.

I have no experience in this area so would appreciate feedback from forum members regarding reality of encountering interference??

Is the only feasible use for cheap wireless systems interviews away from public events (less interference) and when at public events one had better pay to get systems with many different available frequencies because it is likely one will get interference??

The other option suggested from my earlier question was a separate recording device like Tascam DR 40 to record interview and then sync up later in post and that may be my only option at a cheap price??
 
Here in the UK, we have a small band of channels available to people who don't have a license - and you can just about squeeze in 4 or 5 channels. The trouble is everyone with radio kit who doesn't use it enough to buy a license (or even know then need one) is being forced into this tiny space. We have been using channel 69 and 70 for a few years now, but ch69 is now populated by 4G mobile phones. Not everywhere yet, but growing. Lots of people who don't know better are still in 69, but interference is going to mean they'll probably google it and discover ch 70 should be available and there will be chaos. It will just be unreliable, working fine one day and failing the next. On Broadway and in our West End - there are dozens of theatre shows each trying to use radio mics, and coordinating this takes a lot of patience and skill. Theatre people are adept at identifying co-channel interference and learning how to minimise issues. Video people do just seem to expect radios to work all the time - it's going to get worse!
 
You could rent a Sennheiser G3 wireless in most places for about $25-$40. Those can still get hit with interference, but less than the cheapies and you have more frequencies that you can use.
 
Hey Dennis .. I found that the newer azdens are not diversity and have drop outs. Try and find a 100ULT receiver and 10BT transmitter ... plenty of channels and they punch thru anything..great and cheap ..I bought one on eBay for $160.00
 
Practically all battery operated systems are not diversity, and even those that are are not immune from dropout. I've never found any brand that follows the usual power output rules punches though anything! RF links work best with the wind behind them, and downhill - or at least many people think they do! Co-channel interference, intermodulation and weak signals apart - they are great. In fact, many older fixed channel units are still out there working in almost empty bands giving solid use. Much, sadly is down to location, band occupancy and luck!
 
The advantage of using a separate recording device is you never have to worry about RF interference. I have used the Zoom H2 in the groom's pocket to record the vows at a wedding. Syncing the audio is a bit of work, but the results were very good.

Now, a word of warning: The vows are a short segment of audio and I didn't have to worry about drift between the camera's AV and the Zoom mic. I have NOT done longer tests to see if the sampling of the audio in the recorder is crystal controlled or just a basic RC timer. The first (crystal) is VERY accurate and drift will not be an issue. However, if they used an RC timer, the sampling rate is NOT exact and the camera's AV will appear to drift out of sync on long clips.

To understand, if your camera is sampling audio at 48,000 times per second, but the separate recorder is slightly off say, 47,900 samples per second. After 1 second, the difference is only 100 samples or 0.002 seconds. Keep recording for 100 seconds and the error will add up to 0.2 seconds of difference.

Pity they don't say in the specifications for these mics if they use a crystal for timing (very accurate) or a less accurate RC timer.

Because timing is so critical in video cameras, they have to use a crystal for timing.


Bob Diaz
 
Depends on a few more details. How big are the races, some have all the drivers and crews talking via their own wireless systems apart from the race
and news outfits. At the biggest races there could be more than 100 wireless systems operating.

And how and where are you running the interviews? Are you asking the questions trackside while someone else runs the camera?, or are you running everything? Are you in the press room? How long is the average interview and how much time do you get? The answers to those questions determine what audio system
and what level of quality you need.

If this is a local club meet, and you're by yourself, you could check out running a lav and a cable to the camera.

Cheers.
 
Thanks to all for sharing their field experience and technical input. So far I just don't have the experience to even think of many of the points that were brought up and I appreciate learning about them on the forum instead of after I make an uninformed purchase. At the moment given my budget, all the good and valid cautions that have been mentioned when trying to be too "economical" with equipment, and my learning curve may dictate using Allan's suggestion of using a lav and a cable. The events I attend are vintage road racing events ( i.e. Road America in Elkhart Lake, WI) where the cars are collectables from the 1950's and 1960's worth a lot of money but the owner/drivers are generally very friendly and like to talk about their cars. So my informal "interviews" are generally standing by the car with the driver discussing the car and its history so the cable would not be ideal, but workable. As I explore interviewing technique (for example I am thinking I may walk around the car and get closeups of engine, brakes, suspension as driver explains them and I can monitor his comments in camera's headphone output) I expect I will need to graduate beyond cable, but maybe best to start there and learn and then move on. Again, thanks for the help. Dennis
 
Andrius Simutis said:
You could rent a Sennheiser G3 wireless in most places for about $25-$40. Those can still get hit with interference, but less than the cheapies and you have more frequencies that you can use.
Click to expand...

This would be a good way to test out equipment and find issues as I actually use it. Thank you
 
ChrisHarding said:
Hey Dennis .. I found that the newer azdens are not diversity and have drop outs. Try and find a 100ULT receiver and 10BT transmitter ... plenty of channels and they punch thru anything..great and cheap ..I bought one on eBay for $160.00
Click to expand...

Chris, thank you for sharing experience again and eBay is something I need to look at more for used equipment.
 
BobDiaz said:
The advantage of using a separate recording device is you never have to worry about RF interference. I have used the Zoom H2 in the groom's pocket to record the vows at a wedding. Syncing the audio is a bit of work, but the results were very good.

Now, a word of warning: The vows are a short segment of audio and I didn't have to worry about drift between the camera's AV and the Zoom mic. I have NOT done longer tests to see if the sampling of the audio in the recorder is crystal controlled or just a basic RC timer. The first (crystal) is VERY accurate and drift will not be an issue. However, if they used an RC timer, the sampling rate is NOT exact and the camera's AV will appear to drift out of sync on long clips.

To understand, if your camera is sampling audio at 48,000 times per second, but the separate recorder is slightly off say, 47,900 samples per second. After 1 second, the difference is only 100 samples or 0.002 seconds. Keep recording for 100 seconds and the error will add up to 0.2 seconds of difference.

Pity they don't say in the specifications for these mics if they use a crystal for timing (very accurate) or a less accurate RC timer.

Because timing is so critical in video cameras, they have to use a crystal for timing.


Bob Diaz
Click to expand...

Bob, I would not have thought about that technical insight and can see where it could become an issue the longer the duration of recording.
 
Dennis, given your explanation, walk before you run. Where are your interviews going to be used? DVD club sales?, club website?, Utube?, your website??

When you say 'they like to talk about their cars' sounds like you need to get a vintage car expert to help you, someone who can ask pointed questions then discuss the answers with the owners and drivers. eg: after they talk about the motor, later you can shoot detailed cutaways to lay over it.

So before you decide on the sound gear, check around, there may be a club expert available to do this.

Then you could use an interview mic on a cable, while you concentrate on the camera work .. that's a full time job in itself.

Cheers.
 
I thought that non-PLL systems were ancient history. To be honest a PLL device is still not absolute because the lock system is very stable, but locked to the crystal - and there will always be deviation from the absolute - hence why most digital kit now used in the studio will have the ability to lock to a common source - the theory being that this source could well be 47.9999KHz, not 48K - but at least everything is wrong together.

Lip sync is rubbish nowadays anyway. You might spend ages getting it just right and the client plays it on a 'clever' TV that has a built in video code/decode delay. I've just been working on a LED screen project for a live show, and the client wanted some images synchronised to the audio click (for the band). The screen company could tell me their processing delay - 4 frames, but warned me that as the media would be sent via embedded audio on a video line to them, they had no idea of the in to out delay before it reached them. So any attempt at accurate sync seemed doomed to failure.

If I have separate audio for my theatre shows - it's usually around 60 minutes worth . I'd expect to probably need one or two re-sync cuts in that kind of length. Easy to do, but I'm lucky in that there is usually a drummer visible in the wide shots, and they're great for doing re-sync
 
Allan Black said:
Dennis, given your explanation, walk before you run. Where are your interviews going to be used? DVD club sales?, club website?, Utube?, your website??

When you say 'they like to talk about their cars' sounds like you need to get a vintage car expert to help you, someone who can ask pointed questions then discuss the answers with the owners and drivers. eg: after they talk about the motor, later you can shoot detailed cutaways to lay over it.

So before you decide on the sound gear, check around, there may be a club expert available to do this.

Then you could use an interview mic on a cable, while you concentrate on the camera work .. that's a full time job in itself.

Cheers.
Click to expand...

Allan, Walking before running is wise (and can save a lot of money), but I confess to having several skinned knees from trying to run too soon. I film vintage events for my enjoyment and have started giving DVD's/Blu-rays (my output is just to Blu-ray for myself and friends) to car owners/drivers who "share" these great old and valuable (i.e. Scarabs, Maserati Birdcage, etc) cars with us--great sounds and lots of action on the tracks.

Vintage sports/racing cars are an avocation of mine so I generally know enough to let the owners' know I have an appreciation of their cars. I am basically a one man show (releasing my wife from her feigned enthusiasm/accompaniment a few years ago) but your advice to do the interview and then do detailed cutaways separately is very helpful---usually it does not require a lot of question to get owners to tell you all about the car and its features so not really "interviews" in the classic sense. Cheers, Dennis
 
paulears said:
I thought that non-PLL systems were ancient history. To be honest a PLL device is still not absolute because the lock system is very stable, but locked to the crystal - and there will always be deviation from the absolute - hence why most digital kit now used in the studio will have the ability to lock to a common source - the theory being that this source could well be 47.9999KHz, not 48K - but at least everything is wrong together.

Lip sync is rubbish nowadays anyway. You might spend ages getting it just right and the client plays it on a 'clever' TV that has a built in video code/decode delay. I've just been working on a LED screen project for a live show, and the client wanted some images synchronised to the audio click (for the band). The screen company could tell me their processing delay - 4 frames, but warned me that as the media would be sent via embedded audio on a video line to them, they had no idea of the in to out delay before it reached them. So any attempt at accurate sync seemed doomed to failure.

If I have separate audio for my theatre shows - it's usually around 60 minutes worth . I'd expect to probably need one or two re-sync cuts in that kind of length. Easy to do, but I'm lucky in that there is usually a drummer visible in the wide shots, and they're great for doing re-sync
Click to expand...

For me this insight into the issues /challenges you face is very helpful as I continue to learn and try to straighten out my learning curve a bit. Regards, Dennis
 
Crystal timing is very accurate, in general it's 6 place accuracy or better (that's 1/100,000 error). Two recordings for 60 minutes = 3600 seconds. Assuming the first is bang on, the second could be off by 3,600 seconds / 100,000 = 0.036 seconds = about 1/28 of a second. It's a tad off, but not enough to matter.

When I've recorded with multiple cameras (all crystal controlled), audio sync was never a problem. The maximum I've done is 30 minutes.

I'm sorry, but I'ver been too busy to try the same test at home with my HMC40 and the Zoom H2 audio recorder. I have no idea if it's crystal controlled timing or not.


Bob Diaz
 
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