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Last update:  2003/04/24

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How a Backer works

A Backer ``draws'' the data being saved by turning each bit of the data stream, 0 or 1, into a horizontal black or white dash in its video output.  In high density mode there are 80 of these dashes across each line of video and in low density mode there are 32.  Each video line also begins with 8 additional dashes in the pattern 01000101, equivalent to the byte 0x45.  These additional bits are added by the Backer hardware during data output and are automatically stripped from the signal during data retrieval.

Here is an example of one line of video from the Backer's low density mode.


Beneath the black and white dashes is shown the bit pattern they are equivalent to and below that is shown how the bits are divided into bytes.  The bits are written and read in left-to-right order (the same direction, of course, as a television's scanning).  That's all there is to a Backer:  put it in ``transmit'' mode and send it the bytes you want written to tape;  put it in ``receive'' mode and read bytes from tape.

Of course, turning this into a useful data storage and retrieval system requires a lot more work on the software side.  The first problem to overcome is that Backer's do not retrieve a fixed number of bytes from each video field, rather a Backer may skip over the first and/or last few lines.  On the one hand this means that one should not use these lines for data storage and on the other hand means the read-out software needs to be able to identify, asynchronously, the start and end of each video field.  Once a solution to this framing problem has been acheived, the next obstacle to data storage and retrieval is tape noise.  A VCR and VHS tape is not a data-grade recording medium by any stretch of the imagination so a fairly robust forward error-correcting code must be applied to the data stream to allow its accurate recovery.  Finally, during playback Backers rely on the data stream being ``self clocking'' meaning that the Backer circuitry uses the black-white transitions in the video signal to find the bits.  If a long stretch of the line has no transitions in it, the Backer circuitry can loose track of where it is and stop reading out bytes.  The solution is to introduce a run-length limiting bit modulation technique to ensure a minimum transition frequency.  In practice, however, it proves effective to simply mix the data being written to tape with noise from a simple random number generator and this has the advantage of maximizing tape data capacity.  The software included in the linbacker driver package is capable of performing all of the encoding and decoding tasks above.

What you see on your television

Stable 3.x Versions

These images are for NTSC video.  Most televisions can't display some number of the top and bottom lines of video so they won't actually be visible on your set.  These pictures were obtained by piping the output of bkrencode into a program I've written for analyzing tape data streams and then doing a screen capture with the GIMP.

This first image is of a data sector (one video field) and it's not too helpful unless you know what you're looking at.  The same output can be generated on your television by typing

cat /dev/zero > /dev/backer/0/nhs

The version 3.x format is not as much of an improvement over 2.x as 2.x was over 1.x but it is not without its merits.  The error control code is now used to only correct errors in actual data, it is not wasted correcting errors in unused portions of the sector.  Also, the key bytes are spread more uniformly throughout the sector.  This last improvement combined with a new, more robust, algorithm for identifying the key bytes means that it is practically impossible for noise to cause a sector to be lost --- the software can accurately identify sectors even on tapes which have been badly physically damaged.

This image is more informative.  This is a sector from the beginning of record (BOR) mark and it allows you to make out the structure of the format.  First, the vertical lines down the left side of the image are not part of the tape data.  They are produced by the Backer hardware which prefixes each line of video with the byte 0x45.  This presumably provides the means by which the hardware's bit clock is synchronized on playback.  The top few lines contain the sector leader which is just filled with 0xe2 and is used to pad out the first few, unusable, lines of video.  Likewise the bottom few lines contain the sector trailer which also pads out unusable lines but with 0x33.  The key bytes can be seen scattered diagonally through the sector and are used to identify the location of sectors in the data stream.  The vertical bars filling the bulk of the image are the filler bytes (0x33) used in BOR and EOR sectors and would contain data if this were a normal data sector (see above).  The parity bytes for the error control code are at the bottom, filling several lines immediately above the trailer.  Finally the sector header can be made out as four bytes, here all 0, appearing just above the parity bytes towards the right side of the video frame.

Here's what a low density data sector looks like. You can get output like this by typing

cat /dev/zero > /dev/backer/0/nls

And here's a sector from the low density BOR mark.


Version 3.2

The image at right shows bkrmonitor after a 545 million byte recording that had been made in EP mode was recovered from tape.  As you can see, the raw recording could not be recovered from tape completely error-free as there were 108 uncorrectable data blocks.  However, this recording was made using the new bkrenhanced utility which had no trouble filling in the lost sectors as can be seen from it's output below.

bkrenhanced: DECODING at 92.2% efficiency
bkrenhanced: Number of lost sectors detected: 108 of 321045
bkrenhanced: Number of errors in worst block: 3 (20 allowed)

This shows that 108 sectors of the 321045 data sectors in the recording needed to be corrected which is a mean error rate of about 3*10-4.  We also see that no sector group required more than 3 of its 255 sectors to be replaced.  bkrenhanced's error control code can replace up to 20 sectors in each group so there appears to be plenty of headroom as one would expect given the mean error rate and the code rate.

bkrmonitor screen shot