Zoom H4n and drift

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DAVE M
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Joined: May 17 1999

a friend is looking for a good quality device for audio.

His work is mainly classical and similar music events with multi camera recording.

Large gigs have a separate sound recordist but for the lower budget ones he uses a pair of mics and leads al the way back to the camera. This can take ages rigging the leads around balconies.

He's after a Zoom type unit to be rigged independently but is aware that some units can drift.

Any problems with the H4n? he's using FCS to edit.

thanks

rogs
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Joined: Jun 16 2006

There's no way of predicting in advance whether drift will be a problem with any free running audio recorder. The crystals controlling the internal sample rate clocks will all have a tolerance, so that you may be lucky and have one that runs at exactly the same as the camera crystal clock - or you may not.
Which is why some people report perfect sync with a specific camera/ recorder combination, and others don't!
And it can be that the camera clock is the one that is slightly adrift from the quoted figure. In that case, it would still be the possibly 'perfect' external audio track that would have to be changed to resync. The camera track is the one in sync with the picture, so has to be the master -even if it's the one recorded at the wrong frequency!

For most short takes, it's usually not likely to be a problem, but when the individual takes are longer - as with classical music (or wedding services?) the problem can be annoying.

Some time ago, a friend had an audio 'sync' problem, and I posted a way of syncing two devices here: http://www.mfbb.net/myvideoproblems/myvideoproblems-about25.html
Might offer one or two pointers.

Obviously written for a specific software (freeware) solution, but easily adaptable.

Dave R Smith
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Joined: May 10 2005
DAVE M wrote:
a friend is looking for a good quality device for audio.
...but is aware that some units can drift.

Interesting link Rogs. Another one here:
http://www.studiodaily.com/main/technique/craft/f/audio/Keeping-Audio-In-Sync-Wordclock-Solutions_11315.html

Both reference accuracy at 50 ppm, alternatively expressed as .005%.

The studiodaily link references accuracy of the clock at it is given frequency as the problem and not, as I expected, directly linked to video using different frequency to the music business.

I was expecting a statement that recording both audio and video at same frequency will mitigate differences, but such a statement seems conspicuous by its absence.

Also, the logic applied to calculating the accuracy of 50 ppm, does not reference it being consistantly, say a delay, or in advance, so should therefore be expressed as accuracy of + or - 50 ppm.

Maybe there's an unstated factor, meaning the +/- aspect is n/a?.

Otherwise, with sods law applying to the 2 recording devices, the 5.4. frames per hour drift becomes 10.8 frames per hour.

With pop music tracks up to 5 minutes, that's possibly 1 frame out and just acceptable, but for classical and other longer pieces it can be a problem.

The last para follows my own philosophy, though you are not always able to pre-test:
Remember, no matter what your workflow is, the most important element is to test it. For some productions, any audio drift can be dealt with by simply varispeeding the audio to match the picture. Other productions may find that solution intolerable, and require frame accurate audio and timecode.

rogs
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Joined: Jun 16 2006
Dave R Smith wrote:
The studiodaily link references accuracy of the clock at it is given frequency as the problem and not, as I expected, directly linked to video using different frequency to the music business.

Nearly all PCM audio for video is sampled at 48KHz, whereas the music CD uses 44.1KHz sampling. Strangely though, a lot of multirack audio mastering is carried out at 48KHz!

Not that it makes any diference. If you have an audio track sampled at 44.1 KHz , and resample it at 48KHz the track length doesn't change. It stays exactly the same length. (At least it should, with a decent audio editor!)

The problem only occurs when, as you suggest, the '50ppm' crystal tolerances all add up in the wrong direction!

The very cheap audio recorders - those that use ceramic resonators - can be notoriously 'off', but commercial crystal oscillators are really quite good. I recently built and fitted an electronic controller to bring an ancient 'master clock' on the local bowls pavilion back to life , and a 4.194MHz crystal , selected at random, is accurate to within about a second a day - so not bad! If the '50ppm' error on that crystal had been all in the wrong direction, it could have been 4 seconds a day!