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Readability Testing of Recorded Media

January 10, 2012

The question that is difficult to answer – even today – is after a disc is recorded, will it be readable by someone else? More specifically, after a disc has been recorded on Drive A can it be read on Drive B?

This is the sort of thing that can get pretty important. If you are a graphic artist preparing materials for a client on a deadline sending them an unreadable disc is not going to win you any friends. Similarly, if you are trying to send a disc containing pictures from a wedding to a family member they aren’t going to think much of your computer skills if they can’t read the disc. So there are personal and professional reasons why this question needs to be answered.

Nearly all software for writing discs includes a read-back test. This is where the recording that was just made is checked to make sure it can be read. Sometimes this check is done by sampling the disc, sometimes by just reading the entire disc and sometimes by comparing the contents of the disc to the original data that was being recorded. In truth, there is very little difference between reading the data and both reading and comparing the data. If the correct data was being sent to the writer, the correct data was being put on the disc – the real question is if the data was written in a readable fashion.

The problem that the read-back test does not address is the differences in quality and capability between different drives. The professional creating the disc may have a very good drive whereas the client receiving the disc may not have a drive of the same quality. This raises the question how does one test a disc for readability on a drive that you do not have?

The way most testing of discs works is by reading the disc with the default settings of the drive and without doing anything further. This gives a yes/no answer to the question if the disc can be read on that drive but says nothing about how well other drives may be able to read the disc. The tool CD Speed (or DVD Speed and also known by other names as well) comes the closest as it will actually display a graph of the time required to read different areas of the disc. Spikes in this graph are useful as they indicate problems where it took longer to read a particular area.

Both of our products CD/DVD Diagnostic and CD/DVD Inspector can do readability testing and take this testing to a significantly greater degree. Two additional steps are taken beyond simply reading the disc:

  1. The drive is configured to not perform any additional error recovery when reading the disc. When the drive supports this it removes any additional recovery capabilities the drive may have and puts the testing at more of a baseline that is comparable to all drives. Unfortunately, most drives do not support the commands used to change the error recovery configuration.
  2. Additionally, the time required to read each sector is taken and compared with nearby sectors to determine points at which data is returned more slowly or more quickly. Either of these conditions indicates that some type of error recovery has occurred and this is counted as a recovered error.
  3. The drive is also configured to return indications that error recovery was required and succeeded. Almost no drives support this today.

The results of the testing are displayed in graphic form as below:

Readability Test Results

This graphic can also be printed out to document the condition of a disc.

It is important to understand that this test is “uncalibrated”. There is no standard involved in the test against which things can be measured. Both the drive and the media are considered to be unknowns with this. This is quite different than other disc testing where the drive is a calibrated known and can be tested against specially constructed test discs. So in reality, this is testing both the disc and the drive together.

What about special drive commands that return more information about the drive? At this time there are some drives that implement proprietary commands to disclose more information about the results of reading a disc. So far, the drive manufacturers have not been willing to discuss these commands and have taken legal steps to shut down companies that have reverse-engineered proprietary software to discover these commands. At this time we use only commands that work on all drives in our software for some really excellent legal reasons.

Note: The readability test is normally not shown by CD/DVD Inspector but turning off the option “Forensic Use” and restarting CD/DVD Inspector will display it.

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