It is important to note that when comparing results between material re-encoded at a different frame rate, then the number of failed frames will differ, and the results may differ as a result of the extra / missing frames that were introduced during the conversion process.

The following items are phenomena commonly seen in the results along with explanations for the behaviour.

1) A flash occurred but the normal flash risk trace didnt appear The main flash risk trace (dark green line) may not appear if flashing is less than 20cd/m2 in contrast or if the flash frequency is significantly within guideline limits. Remember that 2 opposing transitions make up a single flash.

The example below shows that two transitions have been detected by the diagnostic trace (light green line) but that the main risk trace has not yet appeared because the flash frequency up to this point is not considered to be significant.



(1) Showing a flash but no flash risk trace


2) The flash risk trace (dark green line) appeared close to the pass-fail limit for a long sequence of images but didnt enter the fail zone The system has detected flashing of above 3Hz and 20cd/m2 in amplitude; but the flash area is less than 25% of the screen area.  If the criteria for failure are not all met then the system will generate a line based on how close the material is to failure.  If the line is hovering just below the failure line then it might only need a tiny change in size of the flashing area to push it into failure.  Such a tiny change could be introduced when converting between formats, frame rates or codecs if this is not done carefully using professional grade codecs.


The example below shows that the flash risk has remained close to the failure line for long enough to generate an extended flash failure. This occurs whenever more than 80% of the frames in the last five seconds generated flash risk warnings of 0.3 or 0.4 (i.e. close to failure).



(2) Long sequence of flashing that eventually led to an Extended Flash Failure



3) The flash risk trace (dark green line) persists for several frames after a flash occurred This is perfectly normal, and arises because of the way the system has to detect flashing frequencies over the most recent second.  You do not need to worry about the persistence of the flash risk traces, you need to principally examine the second before the line moves into failure, or where the diagnostics trace shows that the transition count is still rising.  Once you have corrected all the causes of the line first moving into failure then the material at that point will pass the test.  Note, however that lots of flashing close together will generate a much longer compound failure: the best thing to do is deal with the flashes one at a time until the material passes the test.



(3) Flash graph persisting after the flash.



4) The diagnostic transition count increased where no obvious flash had occurred The diagnostic trace represents the number of transitions seen by the most active 25% of the screen over the most recent second. Therefore continuous image activity (e.g. localised flashing and rapid movement within the scene caused by camera pan or zoom etc) can steadily increase the number of transitions that individual pixels have seen, and when at least 25% of those have seen an extra transition will the diagnostic count increase. This can be quite tricky material to fix, and may only be possible by reducing the brightness of the image or cut down on the whole area.


(4) Transition count (light green trace) increased without an obvious transition



5) A flash clearly occurred but the diagnostic trace didnt increase The diagnostic trace shows the number of transitions seen in the last second of material, and transitions older than that will be discarded. This means that the diagnostic count may not always coincide with an obvious flash.  For example, a visible transition in an image may not lead to a higher transition count if the pixels that see the transition are not part of the most active 25%.




(5) A flash clearly occurred but the diagnostic trace hasnt increased.

Alternatively, an incoming transition may coincide with an outgoing transition from one exactly second earlier. When this occurs, the new version 3 analyser inserts a squiggle (see picture with inset) to indicate that the diagnostic trace has simultaneously gained and lost a transition over the most recent second between video frames.



6) The spatial pattern trace remains in the pass zone even though the detected spatial pattern exceeds contrast and screen area limits A number of limits have to be exceeded before a detected spatial pattern can generate a failure. The Advanced Information tab in the example below shows that a spatial pattern has been detected with 50 cd/m2 contrast (limit 20 cd/m2) and covers 46% screen area (limit 40%). However, in this example, the system has not generated a failure because motion, caused by the camera pan and zoom, makes the detected spatial pattern exempt from failure under Ofcom rules.







7) The diagnostic trace decreased within a few frames after it had increased. Shouldnt it have taken one second for the transition to flush out?  -- Not necessarily.  It is true that the diagnostic trace monitors transitions over the most recent second but the most active pixels that determine this diagnostic count are, in most cases, constantly changing. The diagnostic trace will only follow this one-second pattern if the most active pixels are changing together in phase.

The example below shows the diagnostic trace (light green) increasing from zero to one for a period of only two frames (around the vertical amber current frame line) before returning to zero.


8) The system passes a stationary pattern that looks as though it should have failed Some patterns that are clearly bar-like in one direction may also possess some local structure in the orthogonal direction.  Alternatively, a pattern may not be sufficiently regular or may not have sufficient contrast throughout. Any such structure may cause the system to see fewer than 6 light-dark bars or may separate a provocative pattern into two or more regions. Either of these mechanisms can save a provocative pattern that would otherwise have failed.

The spatial pattern in the example below passes because of text and foreground objects which break up the bar-like pattern into smaller irregular regions.






9) The analyser generates spatial warnings but its not obvious where the pattern is! Occasionally the spatial trace may appear when there is no obvious spatial pattern present in the video stream. The example below shows a picture of the sea generating spatial warnings caused by waves in perspective creating faint, repeating structure. Other candidates for generating unexpected spatial responses are: landscape in perspective, net curtains and reams of paper. However, it is highly unlikely that any of these scenes would actually lead to a spatial pattern failure.





10) Scene Changes (Japanese NAB Analysis only) - When analysing under Japanese NAB guidelines, it is possible for the flash risk trace to go into failure while the diagnostic trace is still in the caution zone (see image). T his can occur if one of the transitions in the most recent second is classified as a “scene change” (see bottom entry of the Advanced Information tab) where 80% of the image has seen a significant luminance transition of 20IRE units or more. When this occurs, the maximum allowable number of transitions is reduced from 6 down to 3 and, in this example, failure took place when the 4th transition was detected.