Due to the physical property of the magnetic material and the nature of digital data on a Mini DV tape, recovering the footage you recorded over is almost impossible.
MiniDV tapes have a sliding write protect switch. Once you?ve finished filming slide the switch to protect your recording. Once you?ve inadvertently recorded over a precious moment you were planning to keep for ever, you?ll truly understand what this feature is for.
A technical explanation why:
When a tape is being recorded on, or played back, it moves at almost 18.9 mm a second past a drum rotating at 9000 rpm. The drum contains recording/playback heads and its angled so the heads can read and write data tracks diagonally across the tape, a process called helical scanning.
An NTSC camera will lay down 10 tracks for one frame of NTSC video, and a PAL camera 12, all in a space the width of a human hair. These tracks contain the video, audio, and data the camcorder uses to manage and keep track of everything.
The recorded video (or data) is essentially a continuos stream of 1's and 0's. An electrical voltage corresponding to those 1's and 0's gets stored into the magnetic material on the Mini DV tape. When you play the video back, the magnetic patterns reproduces an equivalent electrical signal turning all of those 1's and 0's into moving pictures.
Since it is digital, any voltage variation above a certain high value is recognized as "1" and any voltage variation below a minimum value is recognized as a "0". Once the digital values are recognized, they are "regenerated" (ie. you get a value of say 5V instead of your actual 500uV from playback head and you get a value of say 0.2V instead of the actual 50uV from playback head).
What happens when you overwrite?
When you record over old footage, older values are overwritten with the new data (new magnetization). There could also be an erase head which tends to cleanup the tape before recording (this is sometimes not added in to reduce the cost of the camera since digital data does not need a clean demagnetized tape for recording).
So, what you will see now as new voltage from the actual playback head could be say 475uV on places where the previous data was a "0" and close to 500uV when the old data is "1", when the overwritten data is "1". You could similarly figure out the voltage level for previous data with value of "0" overwritten by a "0" or "1".
But the problem here is that the output circuit from the player "regenerates" the value if it is above or below a certain threshold say whatever voltage above 400uV is output as 5V and whatever value less than 100uV is output as 0.2V. The situation becomes even more complex as you overwrite the data multiple times(luckily this is not your case).
So, inorder to get hold of the old data, you need to get access to the raw voltage level from the playback head, then have a programmable circuit threshold to identify these overlaps to generate/seperate the old data. This amounts to having required electronics skills to do the same. You should be able to do it cheap, if you could find someone proficient in working with the internal circuit of these devices (a repair shop owner who likes to help you).
You could also do it yourself at much lesser cost than what you would pay for a recovery service otherwise, if you have a bit of electronics knowledge and don't mind investing on the needed test equipments.
Before any decision is made, first identify if your "camcorder" had an erase head. This could help decide if you could do this yourself, or need a professional recovery service.
Last update: 08:16 PM Friday, January 5, 2007