8.5 Signature Analysis

In a digital routing system, reliability is synonymous with data integrity. A data integrity testing system doesn't care what the video waveform is, or indeed that it is a video waveform at all. A data integrity system considers the digital TV field as a block of binary data and simply checks whether that data was received with bit accuracy or not. The message is to forget the pictures and worry about the data.

Signature analysis is a data integrity testing technique using large quantities of data and tests down to extremely low bit error rates. As a digital interface having no bit errors is transparent, signature analysis is a useful way of verifying the transparency of a channel following installation or maintenance. Signature analysis requires a stationary test signal to be applied at the transmitting end of the interface. Stationary means that in the case of component video every frame contains identical data. In the case of composite video the data repeats every four fields (NTSC) or eight fields (PAL). Digitally generated colour bars are suitable but other patterns work equally well. The received data are then processed to generate a value known as a signature. In typical equipment the signature will be a four-digit hexadecimal number. If the transmitted data are always the same, the received signature should always be the same. Any change in the signature indicates that an error has occurred.

The signature generation process divides the incoming bitstream by a polynomial, as was described in Chapter 3. The remainder of the division expresses an entire frame in component or an entire colour sequence in composite as a single signature word. A change of a single bit is enough to change the signature. Any number of bit errors up to the number of bits in the word will guarantee to change the signature, whereas larger numbers of errors are detected with a high degree of probability. In the presence of high error rates the occasional misdetection is irrelevant as the goal of the testing is to determine whether or not remedial action is necessary.

Signature analysis is similar in principle to EDH except that the signature is not transmitted with the data. This avoids complexity at the transmitting end and the need to reserve word positions in the data. As the transmitted data is not a codeword, the remainder will not necessarily be non-zero but could have any value. An error is indicated when the received signature changes, not by its absolute value.

Signature analysis can be used in two ways shown in Figure 8.8. In absolute analysis, shown in (a), the test signal comes from a special generator that displays the signature of the data. The data are fed down the channel under test and then into the signature analyser. The signature displayed on the analyser is compared with the signature on the generator. If the two remain identical for an extended period, then no errors are occurring.

Figure 8.8: In absolute signature analysis (a) the signature at the generator is known and can be compared with the received signature. In relative analysis (b) the signature is unknown but if the generator is known to be static the signature should be constant. Thus changes in the received signature indicate errors.

It is also possible to use a relative detection method, shown in (b). In relative signature analysis, any stationary generator can be used. The correct signature is not known, but if errors occur, the displayed signature will not be stable but will change. Thus in relative signature analysis the goal is not that the signature is correct but that it should not change for an extended period. Relative signature analysis cannot detect permanent errors, such as a stuck bit in a parallel interface, as the same signature will always be obtained and so its use is restricted to systems having no hard faults.

In case of doubt, the test pattern signature can easily be obtained by connecting the generator directly to the signature analyser as well as to the path under test.

Signature analysers can be designed to work on specific parts of the transmission only. If the interface is carrying programme material, the signature will vary from frame to frame. However, the ancillary data slots can still be used for signature analysis.

In component systems, the signature analyser may be set to operate on only one selected component. Some machines can be set to operate only on selected bits in the sample, making stuck bits in parallel systems very easy to find.

As signature analysis works in the data domain, it cannot be used to test a channel in which the received data are not necessarily the same as the transmitted data. There are a number of cases in which this could occur. If the digital signal is returned to the analog domain and then converted back to digital, noise in the analog domain will cause data differences. In DVTRs, uncorrectable errors due to dropouts result in concealments and changed data values. Thus signature analysis can be used to detect concealment. Note that concealment errors are much less visible than general transmission errors by a factor of about 1000:1, i.e. 1000 concealment errors are about the same level of visibility as one transmission error.

Systems using compression are not transparent and signature analysis is of no use if a compression codec is included in the test channel. In order to test the channel the generator must be connected after the compressor and before the decoder.