audio for multi camera shoot

[Dale Paterson]

Hi Paul,

You are right:

I think sync is often overstated, and really complicated timecode sync isn't required. Lip sync is the obvious aim, and getting it right isn't that difficult.

The problem comes in when you are not aware of the possible pitfalls (or that they even exist) of using external audio devices for audio capture or using multiple cameras for long shoots.

Picture this: my first (paid) multi-camera shoot (for eventual broadcast), notebook, PA system and mixing desk supplied, set up and tested, six wireless mics, lighting all set up, ready to go. The event ended up being just over about two and a half hours long. Imagine my surprise after getting back to my studio, downloading all of the footage from each camera tape, methodically synching the beginning of each camera tape / track with its right place on the master audio track that came from the desk, only to see on the video tracks toward the end that the last speaker / presenter had already packed up his things and gone home (and was probably sleeping) by the time his speech on the master audio track ended (I exaggerate of course but you get the picture).

Personally I think it should just work BUT although I did get a bit frustrated and add a couple of years onto my life at the time I did get a certain sense of satisfaction out of testing and finding out why this clock thing is an issue at all and that's the only way I learn!

Once again - it was not my intention to hijack this thread. I just think that it needs to be mentioned that if you think that you can go out and buy three identical cameras and an external audio recording device and expect them to all be in synch with each other over a period of time once pulled up into your NLE you will be sadly dissapointed BUT there is a workable although not elegant solution to the problem which has been detailed by everyone above.

Regards,

Dale.

[Douglas Spotted Eagle]

Dale, if you're interested in working in a mid-high end broadcast environment, genlock is critical. Just because the small format cams don't offer it, don't fool yourself into thinking it's not necessary; it most certainly is.
clocks are clocks. There is no way to make two clocks run at exactly the same frequency for any significant length of time and have it be affordable. You can get devices that measure the difference between the clocks and constantly adjust the median as a result of those measurements, but that's about it. It's a problem that has been part of the world since the beginning, and is likely going to be a problem for some time to come.

[Dale Paterson]

Sorry Douglas, as always you are right.

What I should have said is that you cannot expect to go and buy three (consumer or prosumer) cameras and some or the other (consumer or prosumer) audio capture device etc. etc. and expect them all to operate in synch.

However (and this is not meant to be a joke although the question may come across as such) BUT how do we know that the time of day is actually the time of day??? I mean just think of how many clock crystals there must be in the world all suffering from the same problem and yet, somehow, world time seems to be able to stay in synch for years and yet my Sony's can't stay in synch for longer than a couple of minutes!!! I'd settle for two or three hours!!!

Dale.

[Douglas Spotted Eagle]

Dale, the answer to your question regarding time of day is:
the Atomic Clock in Boulder, CO. By order of law, we know what time it is. :-) Amazing...politicians can't balance a budget but they can create a world order of time.

On a significantly more serious note, if your cams are literally out of sync after a couple minutes, then you have something incredibly wrong with either your cameras or your workflow or both. Even mixing Canon, Sony, and Grass Valley cams on a timeline, I have no issues with sync whatsoever over lengthy periods of time. Shooting both Z1 and V1U camcorders for a 24 hour event, all 24 V1 tapes and Firestore from the Z1 are of exact timing once the tape-changeout and hard drive changeout is extracted from the work sequence.

[Bob Grant]

If you've got cameras with external TC in you can use GPS to lock but that only avoids the problem of running cables.
I have to agree with DSE, I've shot long multicam with a Sony and a JVC camera and not had any significant sync drift and when I do it's a piece of cake to compensate in Vegas. If you're getting out more than a few frames in an hour something is seriously wrong.
I record all location audio into our Edirol R4, can't undertand why people will spend zillions on cameras and baulk at spending half as much on good audio gear. Being able to record your audio at 24 bit can be a real plus, having a few spare tracks to record the room from a few extra mics of your own can also make a huge difference to the final sound of the show. Feeds from desks are almost always dry and mono, having a bit of stereo 'feel' in the final mix helps put the sound back into the space of the venue.

[Ron Evans]

I have little problem in syncing the following cameras from three cameramen, PD150, two FX-1 and a GS400 and occasionally a TRV50. In a two and half hour show there is no real sync issues with the FX1's or the PD150, occasionally the GS400 and TRV50 may be out a frame in this time. Unfortunatley for these two consumer cams it is not consistent. It may go forward a frame in the first hour and then back a frame!!!!. I am sure this is well within the spec of 1/3 a frame but the drift is not always consistent, depends on when they are started after a tape change? Video sync has never really been a problem over these times. As mentioned earlier I sync/mix audio in Vegas though most of my editing is with Edius Pro3. Most of the time I don't use the audio from these two consumer cams anyway but it is there if needed. For interest, the PD150 and one of the FX-1' have shotgun mics and the other FX-1 one uses a AT825 single point stereo. Main audio comes from one of the shotguns with the stereo feed providing some "air". I too would love to find a nice easy way of getting sync free audio but have resigned myself to working as I do considing the equipment we are using. I seem to spend a lot more time on the audio these days than editing the video!!! Camera outputs are better, NLE's are more capable and faster but getting clean audio to match the quality of the video is another thing alltogether!!!

Ron Evans

[Marcus Marchesseault]

"I seem to spend a lot more time on the audio these days than editing the video!!"

Spot has often said that audio is half or more of a motion picture. Perhaps he will need to append that in the near future to say that it is always MORE than half the production.

BTW, the atomic clock is adjusted every year to account for changes in the Earth's orbit. I remember that 2004 had a bigger adjustment due to the effect of the Tsunami in the Indian Ocean.

If one of your cameras has a clock faster than the other, just put it on a fast plane and time dilation will help correct the difference.

Seriously, I'm happy that I mostly work on productions where only a few minutes at a time need to match. I don't know how I would match up 24 hours of footage from multiple cameras. I doubt I could even keep the tapes organized!

[Douglas Spotted Eagle]

Originally Posted by Marcus Marchesseault

Spot has often said that audio is half or more of a motion picture. Perhaps he will need to append that in the near future to say that it is always MORE than half the production.
!

Actually, Spot has a copyright on the phrase "Audio is 70% of what the audience sees..." I do believe audio is much more than half the image. It warms my heart that folks actually heard that. :-)

[Dale Paterson]

Hello again,

For the record, if I remember correctly when I was doing all of my tests in April this year, there is a one or two frame difference per hour between my three cameras i.e. they don't go out of synch after a couple of minutes but you can just start hearing that 'flanging' from the audio tracks at about thirty minutes or so and onwards.

And just when I was ready to (once again) put this subject to rest somebody goes and says something to draw me in again:

Bob. Please could you explain this further for me:

If you've got cameras with external TC in you can use GPS to lock but that only avoids the problem of running cables.

Regards,

Dale.

[Dale Paterson]

Hi,

Sorry, but I am really interested to know what Bob Grant meant by using GPS with cameras that have external timecode capabilties (see previous message).

Regards,

Dale.

[Steve House]

Originally Posted by Dale Paterson

Hi,

Sorry, but I am really interested to know what Bob Grant meant by using GPS with cameras that have external timecode capabilties (see previous message).

Regards,

Dale.

GPS works by very precise time-of-day signals. With appropriate GPS receivers a time hack can be derived that fed into a camera would allow all the cameras in a multicamera shoot to have exactly the same clock setting with their clocks locked to the GPS signal. I seem to recall seeing something about Ambient selling equipment that can do that but I don't recall any detail

[Dale Paterson]

Steve, thanks for that.

Excuse my ignorance but does GPS not depend on line-of-site? You know - like a satellite phone? I mean the equipment (receivers or at least their antennae) has to be able to see the satellite(s)? As you can maybe tell I don't know much about (or have a portable) GPS!!!

Edit (Sorry - I forgot to Google before posting the above question):

This comes from http://electronics.howstuffworks.com/gps1.htm:

In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond. To make a satellite positioning system using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in the receiver itself. But atomic clocks cost somewhere between $50,000 and $100,000, which makes them a just a bit too expensive for everyday consumer use.

The Global Positioning System has a clever, effective solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets. In a nutshell, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy. In other words, there is only one value for the "current time" that the receiver can use. The correct time value will cause all of the signals that the receiver is receiving to align at a single point in space. That time value is the time value held by the atomic clocks in all of the satellites. So the receiver sets its clock to that time value, and it then has the same time value that all the atomic clocks in all of the satellites have. The GPS receiver gets atomic clock accuracy "for free".

This is very exciting BUT correct me if I'm wrong but (obviously) you need cameras than can accept and lock to an external time code (in) and have a clear view of the sky? Cost wise are you not back to square one i.e. professional equipment with Genlock?

Regards,

Dale.

[Steve House]

Originally Posted by Dale Paterson

...

This is very exciting BUT correct me if I'm wrong but (obviously) you need cameras than can accept and lock to an external time code (in) and have a clear view of the sky? Cost wise are you not back to square one i.e. professional equipment with Genlock?

Regards,

Dale.

Yep ... and I can't imagine why it would be needed for any except very special situations and it certainly would be a budget solution. Genlock is something that I think a lot of people are confused on anyway - its main purpose is NOT to sync audio and video or to lock the clocks of two cameras together so they show the same values for Time of Day, though that might be a fringe benefit. When two cameras in a multicam shoot are both recording to their own tapes which will then be cut together in post, genlock is irrelevant. It's needed when you have multiple cameras and need to live switch them in real time. As you know, the video image is formed by scanning a raster. If you cut from one camera to the other, the position of the scanning point should be in the exact same position in the frame in both cameras otherwise there'll be a jump due to the momentary loss of sync in the resulting image. For example, if the scan jumps from being at 30 pixels in on line 22 of the frame on camera 1 to be at 400 pixels over on line 300 on the frame from camera 2, a noticable glitch will occur. Genlock locks together the horizontal and vertical sync sweeps of both cameras to the same clock signal, insuring this doesn't happen. Of course, it's also needed in on-line post editing, insuring the frames of the multiple playback VCRs are similarly synced together. In a broadcast studio it's vital, providing a house clock signal that all video sources sweep in lockstep to. But for typical DV multicam shoots where the source material is in the form of separate files that are cut together in post in an NLE, it's not needed.