"venetian" "banding" "RED X" error

Shawn Murphy

Shawn Murphy

Well-known member
QUESTION: What is the technical explanation for the dirty head venetian/banding effect on playback? I mean, where is the data for the other footage that is interspersed with the footage I just shot (obviously not on the tape where I just recorded because when I put the tape in a clean machine it plays back fine). Weird and I'm really curious!


I ask because today while on my first formal film shoot ever (and having been asked to be the DP at literally the last minute), half way through the shoot I get the RED X error (and honestly I kinda wanted to pretend it wasn't happening and not tell anyone!). Well, as I didn't have my manual with me and didn't know what it meant, I started to religiously do a playback after every usable take to make sure the footage was there and good, but what started happening was I was getting the "venetian blind" effect on playback, where the footage I just shot occupied every other row and my color bars or other footage occupied the other rows, Yikes, I freaked! Luckily we had a second DVX and when I put the tape in the other camera it played back fine and the footage was good (what a relief). So, I figured the playback head was dirty and when we had a break I ran to the store and bought one of the "10 sec" cleaning tapes, well, it seemed to do the trick and going forward I'll probably clean the heads manually as per Barry's thread:

http://www.dvxuser.com/V3/showthread.php?t=6889


*I've got about 75 hours on my machine (using it as my capture/transfer has got to stop!)


Thx.
 
Maybe Spec-Comm will give us the right answer, but it's my understanding that yes, something gets stuck on the playback head. Fortunately it didn't affect the actual recording! I've had this happen when I didn't have the cleaning tape with me, and it is an absolute heart-attack-in-a-box!

Learn from our mistakes, fellow DVXUsers, and keep that cleaning tape with your camera! :)
 
Thanks Barry, it just seems strange that the data that might be getting stuck is so detailed, i.e. you can see the footage very clearly in each alternating row of the "blind" effect. I'll drop spec-comm a note to see if they have any of the details.


mikkowilson said:
...And the manual - and Barry's book.

- Mikko
Click to expand...


I don't follow, are you saying that either of those documents explain the specific technical reasons for the condition? I do have Barry's book, and the manual, though admittedly the manual has been looked at far less than Barry's book!
 
Hi Shawn
I had this happen while doing the Sci-Fi short.
The X didn't appear until at least 10 min of shooting .Like yours i had the venetian blind effect as well . I did lose that first 10 min though.
I ran the cleaner thru it and all was ok .
A few days later while filming some B shots it happened again .UUGGG!!!
Luckly my dealer is located near where I was shooting . Luck was with me as tape footage was still good .Upon inspecting dust had seemed to have crepted in . Strange as I always keep the camera in a cover plus the soft case .
My cleaning tape is always in the bag in case of the above reasons .
I have 122 hrs on mine and yes it is enough to give one heart problems !!!!

Ian
 
A DV video drum holds two videoheads. The drum rotates at a very high rotational speed (9000rpm), and unlike a VHS recorder, where a whole field stands for one (azimuth) trace on the tape, DV writes 10 (for PAL 12) traces per field, alternatively made up by both heads. So a field is made up by 10 "bands" per frame. If one of the two heads get partly stuck, only one head is resulting in a sliced data output and the other headsignal doesn't get sliced and the datastream is being filled up by what was in the buffer before (e.g. colorbars or nothing)). This is the case when the tape is being played back on the "dirty" player (2 times attenuation: the recorded level is lower, and so is the playback signal level) In many cases the recorded signal from the dirty head is still strong enough so that a clean drum (no attenuation in playback) can still slice and slice the weaker recorded track signal and playback the tape correctly. It is advisable to re-record such a tape because the signal can get under the detection treshold after some time and the footage will be lost.
 
Hello Shawn,

You’ve already established the fact that the "Venetian Blind" effect is caused by a clogged playback head.

The DV/DVCPRO 25 head spins at 9000 RPM, 5X faster than analog VCR’s (i.e. VHS, BetaCam, MII, etc…) The reason that I bring up analog is because in analog, you can scan one field with one pass of the head, so if you get a head clog, the entire screen is affected. 5X faster = 5 tracks/field for one head, 10 tracks/frame for both playback heads. When one head is clogged, 5 out of 10 tracks are getting scanned. Thereby giving you 5 good “bars” of video and 5 bad “bars”, causing your famous "Venetian Blind" effect.

Remember, when you see lines in the video don't lose your head ... well I guess you already have! :)

I hope this helps.

Best regards,

Andrew Hoffman
Senior Operations Manager
Specialized Communications Corp.
20940 Twin Springs Dr.
Smithsburg, MD 21783-1510 USA
800-359-1858
Fax: 301-790-0173
andrewh@spec-comm.com
www.spec-comm.com

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Andre and Andrew (hmm, one letter difference but both with great responses!)

Thanks for your answers and the level of technical detail (some of us just can't survive on high level explanations!) ;-)

One last minor question: when you say buffer, "..the datastream is being filled up by what was in the buffer before..", do you mean an actual memory buffer as in RAM?
 
CCD's are being rapidly readout right after each exposure period. The analog voltages are then amplified and digitized. The digital data is buffered in an intermediate memory, which is rather a FIFO type (first in first out) than a RAM (random acsess memory). This memory structure is being read out for further processing until new data is being made available (next field or frame or multiple frame/field times) and refreches the FIFO.
 
I'd like to jump in on this old thread. Forgive me if my question's been answered elsewhere -- still searching the topic.

You say that the venetian blind effect is caused by a dirty head. Is there any other cause. My DVX100a is practically new -- less than 10 hours on it. I was shooting with a brand new tape -- one of the tapes that came as part of the DVX100a rebate program. The very first footage I shot on that tape had the venetian blind effect. Everything after was fine.

I keep the camera in a hard case, and the tape was in its shrinkwrap right up to that morning when I put it into the camera. So I doubt my heads are dirty at all. Can anything else cause the blinding effect?
 
Dust is dust. It floats around us all the time. You were likely in the wrong place at the right time when you changed a tape. It can happen to anyone.
 
When you see the red X, use the cleaning tape imediately. Do not pass go, do not collect $200. If you replay the video to see what's wrong, you risk getting what ever is on the heads back on the tape and ruining whatever you recorded, or even worse, ruining your heads. I speak from experience. Clean the heads and start on a new tape imediately.

The worst thing you can do as a professional is to hope the problem just goes away and not say any thing.
 
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