Clover Systems DVX-4, Руководство по эксплуатации

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DVX Instruction Manual 25
Validating Audio Discs
Audio discs do not have any sectors; they are a continuous stream of audio data. Since there are no sector headers with
addresses, the only way for the drive can locate to a particular spot is by using the subcode. As there are no standards
for locating on an audio discs, different models of drives will interpret the start of a track in different ways. Therefore,
a signature made on one model drive may not match one made on a different model drive. To use this feature on audio
discs, it is best to use the same model drive for creation and validation. In the case where the two drives are not
identical, DVX will attempt to synchronize the two drives. If successful, the offset in bytes, between the two drives
will appear in the Copy Offset edit box and the test will proceed. If DVX is unable to synchronize the two drives and
error message will appear, and the test will not be performed
Clover Signature vs. MD5
Another data validation scheme known as MD5 is commonly used to verify files. The Clover Signature uses an
algorithm devised by the National Institute of Standards and Technology, called SHA-1. SHA-1 is more secure and can
accommodate larger files (up to 2
64
bits) compared to MD5. MD5 produces a 128 bit result; SHA-1 produces a 160 bit
result. Additionally, MD5 is not secure; with MD5, it is possible to change the data and still generate the same result.
FORMAT TESTS
ISO9660
Test | Check Format | ISO9660 will check the logical formatting of a DVD or CD data disc. It checks the System
Identifiers, Volume Identifiers, Directory Identifiers, and Path Tables for correct formatting and illegal characters.
Level 1 is strict ISO9660 format, which only allows 8 + 3 file names, and limited characters. Level 2 tests to Joliet
Extension format, which allows long file names and Unicode characters.
Subcode Skew (CD Only)
Test | Check Format | Subcode starts the Subcode Skew test. The Q-channel subcode on a disc contains time code,
called Atime (short for Absolute Time), which is used for rapid locating of specific places. This time code should be
identical to the sector address contained in the sector header of a data disc. Any discrepancy between these two values
is called Subcode Skew. This type of error can be caused by the recording system. Skew up to ± 7 sectors is allowed,
but any skew will adversely affect search times. Positive skew means that Atime is higher than the header address.
Negative skew means the Atime is less than the header address.
DVD discs do not have subcode, so this test is not available.
Post Gap (CD Only)
Test | Check Format | Postgap starts the Postgap test. The last track on a disc must be followed by a Postgap. The
Postgap contains all zeros (or digital silence). A minimum of 150 sectors is required.
DISPLAYING DISC DATA
View | Data | Disc displays sector data on a disc. The current LBA (Logical Block Address) is displayed. You can
type a new LBA into the edit box, or use the PREV and NEXT buttons to select a different sector. Press ENTER after
typing a new LBA. Use the scroll bar to scroll through the data. All 2352 bytes in a CD sector, or 2048 user bytes in a
DVD sector, are displayed in hexadecimal on the left side, and as ASCII text on the right side of the window. You can
print the sector data by pressing
File | Print in the main menu, or using the print icon in the toolbar.
Audio discs do not have sectors, so this window displays the section of the data stream corresponding to a subcode
frame. In this case, the logical block number you enter is the frame number from the beginning of the first track.
CD Data disc sector data is displayed as “raw” data; that is, with the sector header and parity bytes included. The first
12 bytes of every sector are a SYNC pattern that is used to locate the start of the header. It is identical on every sector
header, and consists of zero, followed by 10 FF hex bytes, followed by zero. The next four bytes are the sector header.
This consists of 3 bytes of address, and one mode byte. The address bytes are encodes in MSF format: Minutes,
Seconds, and Frames. Note that these bytes are encoded as Binary Coded Decimal, not binary. In this example, the
header address is 0 minutes, 2 seconds, 16 frames, which is logical block 16.
The mode byte can be 0, 1, or 2, and determines the size of the user data:
Mode 0: 2048 user data bytes which are all zero, + 288 auxiliary bytes which are all zero.