Zoom H2 & drift correction

JohnnyB wrote on 5/5/2008, 12:01 PM
First a quick introduction - from a refugee from Ulead Media Studio Pro 8 which is not being developed further, I decided to transition (gradually!) to vegas pro 8. I love both products, and hope to learn more here. I particularly like the Vegas audio features.

I thought I'd report on the use of the Zoom H2 portable recorder, which I've recently used in some productions. I was shocked at the level of time drift that the H2 showed against my Canon HV20 - doubted my sanity until I checked out various internet forums. Apparently this is unit dependent also.

Anyway, to report on some results of tests I did on both H2 wav 48k/16bit and MP3 at 320k, the drift on my unit for both was around 12 PAL frames per hour (very noticable both from lipsynch and echo perspectives). Correction with the ctrl-trim in vegas worked a treat. Most importantly, the drift appears to be at a consistent rate, at least on my unit, so this works over the whole time period.

Now I'm scratching my head to get the Vegas settings for using the 4-mic recording from the H2 to simulate a 5.1 surround sound recording. Will report back.

Regards JohnnyB

Comments

ChristoC wrote on 5/5/2008, 3:02 PM
That drift is not unusual - both units are relatively cheap, and I would surmise contain crystal clocks built to a price and which are obviously free-running. Therefore there will always be drift, and I imagine the drift rate will never be consistent from day-to-day as temperature varies. Even with high-end professional gear some drift is evident over such a long time, unless the gear is synchronized from the same source.

>> Vegas settings for using the 4-mic recording from the H2 to simulate a 5.1 surround sound

Set |File|Properties|AudioTab - Master Bus Mode to 5.1 surround
then you will find you have surround panning and you can point the H2 recordings wherever you want.
musicvid10 wrote on 5/5/2008, 6:41 PM
There seems to be a lot of assumptions about this -- the truth is any two devices with their own clocks will drift apart unless they are connected by a wire (timecode sync).

There are different approaches to getting them lined for mastering:

*One is to use time stretch / squeeze on the least important audio track. I've generally stopped using this approach because I don't like the quantization / aliasing noise that is introduced in small amounts.

*The other one I use is to chop the shorter track into 10-minute chunks on the timeline and introduce 1- or 2- millisecond gaps in the splits until waveform spikes (like a drum hit) at the beginning and end of the program line up exactly. There are very few instances where the gap is noticeable in the mix, and if it is, you can just fill it with a ms or 2 from the end of the previous clip.

Hope this helps.
Geoff_Wood wrote on 5/6/2008, 1:25 AM
12 frames per HOUR significant ?!!!

Jeepers , wouldn't you getting more difference in lip-synch by moving your listening postion a couple of feet ?

For anything tighter that that, you need devices that have a clock input.

geoff
ChristoC wrote on 5/6/2008, 3:20 AM
>> Jeepers , wouldn't you getting more difference in lip-synch by moving your listening postion a couple of feet ?

PAL:
1fr=40ms
12fr=480ms
approx speed of sound = 1foot /ms
therefore 12 fr is like moving position by approx 480feet !!!!!
JohnnyB wrote on 5/6/2008, 5:15 AM
Thanks a lot for the feedback, I appreciate it.

I figure that there is not much harm in the (very minor) time correction on the basis that, effectively that was what the unit was doing anyway. I guess my naivity was based on that I wouldn't expect even a cheap digital watch to be this much out! The H2 audio quality is much better than the built-in cam mic/preamps.

Timecode synchronisation is obviously the answer, but for an ad hoc, portable, budget solution, this appears to be effective.

Regards JohnnyB
reberclark wrote on 5/6/2008, 8:09 PM
Hi all,

I have an H4 and have not used it for sync yet. Is there a problem with it as well as the H2? All info appreciated.
musicvid10 wrote on 5/6/2008, 8:22 PM
**I have an H4 and have not used it for sync yet. Is there a problem with it as well as the H2? All info appreciated.**

There is not a "problem" with the H4 or the H2. I repeat -- ANY TWO CLOCKS WILL DRIFT UNLESS THEY ARE SYNCED. It doesn't matter if they are cheap, expensive, or in two identical pieces of equipment. Unless there is something referencing the timecode between two devices, there is nothing to keep them together.

The most accurate and expensive clocks in the world, the cesium time clocks, are adjusted and averaged constantly by radio signals to keep them together, or guess what? They would drift apart!

The OP's difference is by a factor of 1.33x10^-4 (or .000133), which may not seem significant, but a 1/2 second bias over one hour of material will certainly be heard.

Take an example: If you take two identical rafts and set them adrift at exactly the same moment side-by-side on a calm ocean, will they end up at exactly the same place at the same time?
reberclark wrote on 5/6/2008, 9:56 PM
I guess if I'd read the entire thread I wouldn't have asked ! :-) Sorry! Thanks for the very good info!
Geoff_Wood wrote on 5/7/2008, 8:29 PM
Yeah, but the 12 frames drift was over a hour ! Which must equate to a very small fraction of that in a live streaming sense.

geoff
Geoff_Wood wrote on 5/7/2008, 8:33 PM
Another quick thought. Line the audio and video up, edit them both to a specific audio+visual event near the end, and timestrech the audio to re-align. Or did somebody already suggest that ?

geoff
JohnnyB wrote on 5/14/2008, 2:22 PM
Yep, I did use timestreching and it appears to work well at this level of correction at any rate.

I've seen on other forums that the H2 was fairly prone to this drift (in comparison with other ssd recorders).

Fraid I still don't get how even 2 very cheap digital watches would get that far out of synch over an hour (I'm not going out to get a caesium clock!). Still, that's the way it is, and this approach appears good enough to work in these circumstances.

Regards Johnnyb