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Using a tone higher than human hearing to sync cameras

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andy lewis
Using a tone higher than human hearing to sync cameras
on Mar 2, 2012 at 1:46:32 pm

I'm currently using Pluraleyes to sync footage from 2 5d mark iis and audio from a lav recorded on a Zoom H4N. This generally works pretty well but not always. Of course, when it doesn't work well it's always with the kind of raw material that is a nightmare to sync manually.

My question is this: Do these devices (recording PCM 16/24bit 48mhz) record sounds higher than human hearing? If so I am thinking that it would be possible to have a device in the room when filming, that sends out super-aural (?) beeps every second or so.

Presumably, if this was possible someone would already be selling the beeper, right? Or would this work, but alienate anyone in the audience with young enough ears to hear a dog whistle?


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Richard Crowley
Re: Using a tone higher than human hearing to sync cameras
on Mar 2, 2012 at 2:13:55 pm

It would need to be very loud to ensure that all the mics could pick it up reliably. Because sound waves fall off more rapidly with distance the higher the frequency.

To be that loud would be to make it possibly dangerous, especially to children and small animals.

Remember that your ears can be damaged by sound that you can't perceive ("hear"). Just as your eyes can be damaged by light waves you can't perceive ("see").

Remember also that except for expensive test equipment, not many microphones even extend all the way up to the theoretical top end of the range (typically 20KHz) without significant roll-off. Because there is little or no real content of interest up that high, and it is usually noise that must be filtered out, anyway.


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Bill Davis
Re: Using a tone higher than human hearing to sync cameras
on Mar 2, 2012 at 5:28:17 pm

And heaven help you if you're ever called on to use such a system at the Westminister Kennel Club Show.

"Before speaking out ask yourself whether your words are true, whether they are respectful and whether they are needed in our civil discussions."-Justice O'Connor


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Peter Groom
Re: Using a tone higher than human hearing to sync cameras
on Mar 2, 2012 at 5:32:08 pm

I really cant decide if this suggestion is brilliance or madness!
Peter

Post Production Dubbing Mixer


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Eric Toline
Re: Using a tone higher than human hearing to sync cameras
on Mar 2, 2012 at 6:20:25 pm

Well a long time ago there was a system known as Fairchild Picsync that put a 14khz tone on the field recorder tape and it was used to sync the playback recorder to the mag dubber that made the audio mag track for editing. If your under 60yo you have no concept of what I'm talking about.

Eric


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Richard Crowley
Re: Using a tone higher than human hearing to sync cameras
on Mar 2, 2012 at 7:56:10 pm

There is nothing as impressive today in production sound as a bank of a dozen mag dubbers all rolling along in servo sync. Studios used to have on-premise 3-phase generators to send special power out to all the cameras and the sound recorders to maintain perfect sync.

I still use the response "Speed!" when confirming that we are underway and recording properly. Of course, back then, it really meant that all the equipment had revved up to sync speed.


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John Livings
Re: Using a tone higher than human hearing to sync cameras
on Mar 3, 2012 at 3:30:55 am

I would not assume that 48 kHz between any 2 pieces of equipment actually means 48,000 +- "0".

If the clip is longer, the need to "nudge" things may increase.

Regards, John


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Jordan Wolf
Re: Using a tone higher than human hearing to sync cameras
on Mar 3, 2012 at 10:40:46 pm

"My question is this: Do these devices (recording PCM 16/24bit 48mhz) record sounds higher than human hearing?"

Andy,

According to the Nyquist Theorem, the sample rate should be at least 2x the highest frequency you wish to record. Therefore, anything recorded at a 48kHz sample rate should have a high frequency cutoff around 24kHz...how true this is in reality, I couldn't tell you because I haven't measured it.

Your idea is used all the time by radio-frequency microphones/in-ear monitor units as well as to keep TC. It's called a carrier or pilot tone (19kHz is fairly common).

See here for an explanation from Wikipedia - from my knowledge, it's a pretty good overview.

Wolf
<><


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Andrew Rendell
Re: Using a tone higher than human hearing to sync cameras
on Mar 3, 2012 at 11:38:43 pm

To avoid aliasing audio signals have a very steep low pass filter (a "brick wall" filter) at 20kHz before being digitized, and practically nothing over that gets through.

IIRC, Apogee established their reputation with brick wall filters back in the '80s.


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Bouke Vahl
Re: Using a tone higher than human hearing to sync cameras
on Mar 8, 2012 at 7:55:28 am

Common way of syncing is to use Timecode. Also an audio signal that can travel by wireless or wire.(at a frequency of about 2Khz, very well human audible, and (for me) almost human decodable...
(very well doable in software / hardware of course...)
Pluraleyes works way different. Even if you broadcast a beep every second, it would not know what to do with it as there are too many sync points. (It probably would work if you randomize the broadcast interval)
But as stated by others, it seems a bad idea...
So invest in transmitters to get timecode across, or in Lockit / Clockit or other Ambient design syncing mechanisms. They cost money, but well worth it.

Bouke

http://www.videotoolshed.com/
smart tools for video pros


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