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Old June 23rd, 2008, 05:39 AM   #1
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Sync question for you.

Posting this here after some advice on the Vegas forum.

Here is one for you audio experts.

I recorded some Audio using an Olympus DS-30 Digital Voice Recorder. It was at it's highest quality setting recording in wma format at 44.1 kHz / 128 kbps.
My problem arose when I then tried to sync this up to my XH-A1 HD Footage.
The sync very quickly drifted out.
I'm a Vegas user.
Only solution I could get was to go to the clip properties for the Olympus track and do the following:-
Go to Time stretch/pitch shift
Method Classic
Set the new length value to approx 99.76% of the original length. (too tired to work it out exactly..sorry!)
You then have to go back and re-sync the start of the clip.
After that you go to the end of the clip and just adjust the new length a fraction at a time until you are in sync.
Took a few attempts having to adjust/re-sync the start and check but it's now spot on.


So…….anybody got an explanation for what goes or a more elegant non hit or a miss solution?

Up until now, I have been using mini disc recorder up until now which worked great apart from the slow upload times.
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Old June 23rd, 2008, 06:43 AM   #2
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I'm the same as you Alastair - I've used Minidisc for years and they happily sync up on my timeline even for hour-long takes in RC churches. So I thought it was time I moved into the digital age and bought myself a Zoom H2, but whatever format I record in (wav or MP3) and at whatever quality setting, sync is soon lost. More post fiddling needed.

tom.
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Old June 23rd, 2008, 08:15 AM   #3
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It is my understanding, from everything I have read hear, that drifting is common. How much drift will always vary due to the time clocks in each recording unit.

As for the ds-30, I've also read that it is best to resample the audio from the 44.1 Khz wma format to a 16-bit 48Khz wav. I also understand this will help with the drift.

Then I came across this forum, http://www.mfbb.net/myvideoproblems/...s-about25.html. It talks about synchronising external audio with video audio. I haven't tried it yet, but I'll be doing my first wedding this weekend using the ds-30 and I hope that discussion will help me.
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Old June 23rd, 2008, 09:14 AM   #4
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Same here. I've been using an HHB minidisc and it has stayed in sync for very long takes.

For what it's worth I am now also using a Fostex FR2 LE (flash memory recorder) with similar results.

I have read that an audio recorder with timecode is de rigeur in some quarters but this system seems to work OK for me.
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Old June 23rd, 2008, 11:13 AM   #5
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First thing is to render a Sample Rate Conversion from 44.1 to 48K before importing to work with picture.
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Old June 23rd, 2008, 11:14 AM   #6
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In a digital video workflow, with wav files produced by the audio recorder, timecode DOES NOT in itself prevent sync drift. What it does is establish a 'line-up' point similar to a slate that lets you align the video and audio files in your editor. But that's only one point ... if the files are playing back as slightly different rates (caused by the video and audio sample clock not being identical) they will drift out of sync when you move away from that alignment frame. What is necessary to prevent drift is to slave the camera sync and the audio sample clock to a common timebase. This can be done in a variety of ways - one, for example, is to use an audio recorder such as the Tascam HD-P2 or the new Sound Devices 788t that accepts a composite video or video blackburst signal from the camera and slaves its clock to it. Multi-camera shoots and shoots where you need the camera and recorder to be untethered use tools like Lock-it Boxes that are highly accurate clocks that are tuned to each other and attach to both the cameras and the recorder, supplying genlock to the camera and word-clock to hte audio recorder (obviously this requires you to be using pro cameras that accept genlock sync input - a rarity in prosumer cameras).
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Old June 23rd, 2008, 11:49 AM   #7
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Quote:
Originally Posted by Brooks Harrington View Post
First thing is to render a Sample Rate Conversion from 44.1 to 48K before importing to work with picture.

This one keeps cropping up however, If I do this, then place both the wav and wma file together on the timeline, they align perfectly, which means the sync issue will still happen.

I still don't get how a mini-disc IS in sync yet a digital recorder isn't. Heard that ipods also don't drift.
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Old June 23rd, 2008, 11:51 AM   #8
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Quote:
Originally Posted by Alastair Brown View Post
This one keeps cropping up however, If I do this, then place both the wav and wma file together on the timeline, they align perfectly, which means the sync issue will still happen.

I still don't get how a mini-disc IS in sync yet a digital recorder isn't. Heard that ipods also don't drift.
I think it's more a matter of degree than anthing else.
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Old June 23rd, 2008, 12:38 PM   #9
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In Vegas are you using a 48K sample rate for your timeline? I'm assuming you are.
To put a 44.1k file in a 48k timeline is asking for trouble.
Proceed in a logical manor.
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Old June 23rd, 2008, 01:20 PM   #10
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I was going to ask the same.

If you're recording to sync up with video 48khz would be the way to go.

Someone recently said (can't remeber who) that 44.1khz won't sync up with video properly because some things don't match (I'm sorry, can't remeber exactly what).
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Old June 23rd, 2008, 02:52 PM   #11
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Quote:
Originally Posted by Brooks Harrington View Post
In Vegas are you using a 48K sample rate for your timeline? I'm assuming you are.
To put a 44.1k file in a 48k timeline is asking for trouble.
Proceed in a logical manor.
Tried all sorts of combinations and still the same. I think the article Dave Anderson posted has come closest to explaining whats going on. SUKS, but at least it's a reason!

http://www.mfbb.net/myvideoproblems/...s-about25.html

Last edited by Alastair Brown; June 23rd, 2008 at 03:50 PM.
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Old June 23rd, 2008, 03:39 PM   #12
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The link to Anderson's explanation is broken.

The reason is quite simple: most consumer/music market audio devices aren't designed to tightly meet the timebase standard; that is, they run on internal clocks that in many cases aren't running at precisely the right rate. This means that instead of recording 44,100 samples in a second, the number of samples will be off. The NLE assumes that 44,100, or 48,000 samples equals one second on the timeline.

A little error of a few percentage points isn't an issue unless you're trying to sync to another device.

An hour-long take with my HHB Minidisc has always been right on the money, my Zoom H4 not so much.

So, you make adjustments or buy better equipment. Your garden-variety dictation device, like the Olympus, was never designed for sync. This is rarely a problem in short takes, but always shows up in event coverage where you have hour-long takes.
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Old June 23rd, 2008, 03:50 PM   #13
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My mini-desc is the same. Never out, never a problem. So whats difference? They are still recording zeros and ones?

Link sorted.
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Old June 23rd, 2008, 04:52 PM   #14
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Here's the easy explanation - the internal workings of the recording device have a "clock" - much like the things that regulate the speed of an old analog tape deck.

These little "clock" chips come off a production line somewhere in China most likely, produced with some tolerancing no doubt, but unknown Quality Control. All it takes is a slight variation (or a minor design flaw), and the clocks in your devices run slightly slower or faster.

Remember the old saying, "man who has two watches never knows what time it is". Your cam has one "watch", your sound recorder another - add to the mix different sample rates, and now you have most of the problem sussed out.

At least it's not like old analog gear that could speed up and slow down WHILE recording or playing back... the "drift" should be consistant, and easier to correct for.
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Old June 24th, 2008, 08:15 AM   #15
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Just as 2 clocks are never the same time, off by fractions of a second, 2 time-clocks in recording devices are never the same.

To minimize the drift, and if you don't have access to a common time-sync device, you should try and use the same model camera or at least from the same manufacturer.

I have 2 of the same model cams and I have experienced a drift after 2 hours of continous video. The drift was 1 frame, but still a drift.

It all goes back to the chip that keeps time in the recording unit. No 2 chips will ever be 100% in sync. They should be close and I would even say at short lengths it also should not be a problem. But after an hour I would understand a slight drift and would not mind correcting for it if the price was good audio.
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