Are these dropouts?

[teeg]

I have enquired previously about dropout compensation, but I am unsure as to whether I am actually seeing dropouts in my videos.
I decided to take three excerpts from three frames. This is taken from the original VHS tape (good condition, very rarely played TDK Super Avilyn EHG I believe) recorded in 1986 on a National Portapak camera that I digitized to MiniDV in 2002 through a Panasonic passthrough. This was without TBC and with no colour correction. I will recapture in the near future and plan to purchase a TBC.
As you can see, recently, I have made a feeble attempt to colour correct using some plugins in VirtualDub.



I have been thinking and this is what leads me to believe that (hopefully) these aren't dropouts. The lines/defects were never visible when playing back the original VHS tape on several televisions. There also appears to be detail and information in those blue-lined spots.

Could this be caused by an overly sensitive A/D conversion in the passthrough causing some sort of error? Anybody have any ideas?

[edDV]

Random tape generated dropouts will affect single lines of a field. If it extends locally (in vertical) to other fields it is probably something else.

[teeg]

These lines seem more like a discolouration, rather than a complete line drop. What do you mean by, if it extends locally to other fields?

EDIT: By isolating the fields, it seems that some of these lines are found in both fields in some cases, as well as being only isolated to one field.

[edDV]

A VCR dropout occurs when the tape emulsion fails. It is seen on one field and can last for a few nanoseonds to more than a line. Here is an example of VHS tape dropouts (lower right).



Digital tape dropouts result in pixel errors. This usually indicates dirty heads.



Data transfer dropouts can cause corrupt pixels, lines, fields or even frames

[teeg]

If these are dropouts, then would sourcing an old Umatic vtr with dropout compensation from ebay or wherever solve the problem by looping the video in and out of the machine?


Thanks

[edDV]

If these are dropouts, then would sourcing an old Umatic vtr with dropout compensation from ebay or wherever solve the problem by looping the video in and out of the machine?

Thanks

So you need to transfer U-Matic tapes only?

Time Base Correctors from that time operated closed loop with a "steering" sync signal to the input and a few lines of memory for correction. The RF signal from the head was sent to the TBC. A dropout was sensed as a loss of RF signal from the tape. When that happened, video from the previous line was copied to the current line to mask the hole in the waveform.

You could capture from U-Matic without a timebase corrector if you had to. Back then only TV stations and duplication centers worried about dropouts.

[Wile_E]

edDV,

So dropouts appear as pure white spots? But that area you point out, is in the head-switching area that creates noise. How can you tell if they are tape dropouts or just interference? Almost all the VHS tapes I have, were recorded over-the-air. All over-the-air broadcasts, always have some time of interference in them. Speckles/pops/etc... Is there a way to tell if these are truly dropouts? I've used Avisynth plugin DeSpot on them, and have had great success. But this takes more time. Maybe a TBC would be the way to go to record all these old tapes?

[edDV]

edDV,

So dropouts appear as pure white spots? But that area you point out, is in the head-switching area that creates noise. How can you tell if they are tape dropouts or just interference? Almost all the VHS tapes I have, were recorded over-the-air. All over-the-air broadcasts, always have some time of interference in them. Speckles/pops/etc... Is there a way to tell if these are truly dropouts? I've used Avisynth plugin DeSpot on them, and have had great success. But this takes more time. Maybe a TBC would be the way to go to record all these old tapes?

Hardware dropout compensators in early TBCs fixed only the tape based dropouts detected by loss of RF. This was a major problem in the early days of U-Matic, Betacam, VHS and 8mm for acceptable broadcast quality.

A filter based dropout compensator would need to apply some kind of image analysis to distinguish a dropout from picture info. For example think of fireworks. How would you detect a dropout?

One way is to look for pixel motion that occurs in only one field, but maybe a flash bulb would trigger that. So then you alter the definition to say motion that occurs only on one line and then figure the false triggers from that and so on.

PS: If you are looking for examples of tape dropouts, you can usually find alot of them at the beginning or end of a VHS or 8mm tape. The color of the dropout depends on how the downstream equiment reacts to a signal discontinuity. Usually they show as monochrome on my equipment. The picture above came from a ADVC-100.

[teeg]

Nevermind then, I had this idea of passing a signal from a VHS vcr to a umatic/betacam machine with DOC and using that facility which may have been cheaper than sourcing a TBC with it. All conversions are for VHS and 8mm.

Is it possible then that my TV is doing it's own dropout compensation if these defects aren't seen when playing the original tapes?

[edDV]

Nevermind then, I had this idea of passing a signal from a VHS vcr to a umatic/betacam machine with DOC and using that facility which may have been cheaper than sourcing a TBC with it. All conversions are for VHS and 8mm.

Is it possible then that my TV is doing it's own dropout compensation if these defects aren't seen when playing the original tapes?

Well I'll tell ya I had a hard time capturing a dropout from recent VHS tapes. My main dropout battle is with older late 80's early 90's Hi8 tapes that are otherwise very good quality.

[teeg]

Oh well, I'll see how I go.
How do you normally 'battle' against dropouts (rotoscoping?)?


Thanks.

[edDV]

Oh well, I'll see how I go.
How do you normally 'battle' against dropouts (rotoscoping?)?
Thanks.

I'm open to suggestions. Rotoscoping (hand painting pixels) is the extreme solution. I have some great Hi8 video of winter Kyoto, Japan that I shot there in the late 80's. The problem with dropouts is the eye is drawn to motion. A dropout flash is detected by the eye as motion and stands out.