How does Cambridge Research Systems interpret the meanings of terms like 'flash' and 'stripe' that are used in the Broadcast Guidelines?

"These terms were OK when broadcast material was being manually checked by experts like Professor Graham Harding (a painstaking and time-consuming job); however computer-based testing requires a greater explanation of acceptable limits and precise definitions of how and when a guideline is violated. Read Cambridge Research Systems' Explanatory Notes to find out more."

The Harding FPA generates a number of graphical traces. What exactly do they all mean ?

"The HardingFPA Analyser graphical interface displays the results of analysis in two ways. The results of luminance flash, red flash and spatial pattern analysis indicating a level of risk are displayed as bold green, red and blue traces respectively (Risk Trace). An additional selectable option shows a diagnostic trace which identifies where a luminance transition or red transition has occurred (Diagnostic Trace). This trace is feint green or red and is square in profile. Each upward mark or step represents a transition gained between two frames and contributes to the total present within one moving second. A downward mark or step indicates that a transition has been lost from the moving second."

Tell me more about the flash risk traces

"The Analyser identifies a single transition of at least 20cd/m2 at pixel level after compensation for motion and noise and then searches for an opposing transition where a similar transition level exists. An internal count is maintained of transitions seen by each pixel over the preceding second. When pixels are detected which have seen 7 or more alternating transitions within the previous second (which consists of 25 frames PAL or 30 Frames NTSC), full analysis is then carried out and only these pixels contribute to the Risk Trace. This full analysis takes place at pixel level over the most recent 8 frames and this enables the Analyser to identify all violating frequencies.

The Analyser identifies the transitions contributing to single, double or triple flashes within a second but only carries out a full analysis when a seventh transition or continued flashing above 3Hz is detected. In order for a failure of the guidelines to occur the flash area must exceed 25% of the image otherwise only a warning (Pass with Caution) will be indicated."

What is the difference between a flash warning and a flash failure?

"A flash warning occurs when a flash risk trace appears but remains below the horizontal black pass/fail line. A flash failure occurs when the flash risk trace moves above the pass/fail line.

The Risk Trace will appear when guideline criteria have been exceeded - warnings will appear when frequency and amplitude have been exceeded and failures will appear when frequency, amplitude and area criteria are exceeded."

The Risk Trace appeared in the warning zone (Pass with Caution) for a long sequence of images but did not enter the fail zone. Why?

" The system has detected flashing above 20cd/m2 amplitude and above 3Hz frequency but the permitted flash area of 25% was not exceeded. (NB But see also Extended Analysis/Failure). "

The Risk Trace persists for several frames after a flash occurred. Is this right ?

"This is an unavoidable artifact of in-depth analysis over the most recent 8 frames/images in order to identify all violating frequencies. The system response may persist for up to 8 frames/images while the most recent transition propagates through the necessary filters. Thus it is important to identify and correct the images which contribute to the failure rather than those frames/images which have persisted in the carryover. These contributory frames can easily be identified by timecode."

Tell me more about the Flash Diagnostic traces

"The Diagnostic Trace for luminance and red is intended to provide additional feedback on the amount of flash activity present in the sequence and if the permitted number of transitions have been exceeded. A Diagnostic Trace represents the minimum number of transitions that the most active 25% of image pixels have seen over the last second. Thus a sequence analysed with 4 upward transition marks/steps (the count of 4 is also indicated at the top of the interface screen) indicates that the most active 25% of all image pixels have seen 4 or more opposing transitions over the last second i.e. 2 complete flashes. In reality some of these pixels may have seen considerably more than 4 transitions but insufficient to exceed 25% of the image.

The Diagnostic Trace is a useful guide to contributory flash activity which can cause a violation of guidelines since all flash activity above 0.5Hz is indicated. Thus advance indication is given to enable removal of a flash or flashes to achieve compliance.

The Diagnostic Trace should be used with care since the image pixels which comprise the most active 25% are constantly changing. Thus pixels that cause the diagnostic step count to change from 4 to 5 may not be the same pixels that subsequently cause a transgression of guidelines 10 frames later. Conversely, a visible transition in an image may not necessarily lead to a higher transition count if the pixels that experience the transition are not part of the most active 25%."

The Diagnostic Trace made an upwards step where no flash was obviously observed

" Persistent image activity (e.g. rapid motion, fast cuts, scene changes etc.) can steadily increase the number of transitions that pixels have seen over the last second. This results in sudden changes in the minimum number of transitions seen by the most active 25% of image pixels. These changes will usually but not necessarily coincide with obvious transitions in the current image. "

A flash clearly occurred but the number of Diagnostic Trace steps did not increase. Can this be right ?

" Since the diagnostic trace constantly modifies (updates) the number of pixel transitions seen over the most recent second, it is possible for the system to gain a transition from the current image and lose a transition from exactly one second previously. Accordingly the user will observe a transition clearly occur in the current image but the Diagnostic Trace step count will remain the same. An alternative reason why a visible transition does not increase the step count is because pixels that see the transition are not part of the most active 25% of all pixels. "

A flash or flashes occurred (as indicated by the Diagnostic Trace) but the Risk Trace did not appear

" This can occur if the flash is less than 20cd/m2 in amplitude or if there were no more than 6 opposing transitions in the last second."

The Diagnostic Trace step count shows 7 or more transitions but the sequence is not failing. Can this be right?

" In most cases, the appearance of the Flash Risk Trace will be accompanied by a Flash Diagnostic Trace step count of 7 or more. However since the Risk Trace and Diagnostic Trace operate over different time intervals there may be rare exceptions. Thus for example, a sequence of 10 alternating transitions followed by a sequence of static images will cause the transition step count to remain at 10 for a total of 15 frames PAL or 20 frames NTSC until discarded over the moving second. Since the main flash risk analysis is carried out over the most recent 8 frames, the Risk Trace can only persist for 8 frames after the last transition. Thus, in this example the Diagnostic Flash step count would persist at 7 steps or more for several frames after the Risk Trace returns to zero. "

The Diagnostic Trace step count decreases within a few frames of an increase and not after one moving second has passed. Can this be right?

" Since the most active pixels that determine the step count are constantly changing, the one second pattern will occur only if the most active pixels are changing in phase."

Tell me more about the spatial pattern risk trace.

"The HardingFPA Analyser examines material for pattern structures that contain pairs of alternating light-dark bars of at least 20cd/m2 contrast. The Analyser will ignore chequerboard patterns but will detect arrays of repeated elements which are bar-like in structure. When a bar-like pattern is detected the Analyser will compute the number of pairs and the area over which the pattern exists. If the Analyser detects 6 or more stationary pairs which persist for more than 0.5 seconds and the area occupied is greater than 40% then a failed sequence will be indicated. If these criteria are only partially met then a warning (Pass with Caution) will be issued.

The Analyser will probably issue a large number of warnings due to the abundance of bar-like structure in the environment or in man-made imagery (animation etc.) Mostly these pattern structures will be insufficiently regular, may drift or contain insufficient pairs of light-dark bars for complete failure. Additionally all other guideline criteria must be fulfilled before failure occurs - area of image and pattern contrast.

Finally patterns which change direction, oscillate, flash or reverse in contrast are analysed both as a pattern and as a flashing stimulus. Such pattern stimuli might therefore indicate a Flash Risk Failure when the pattern occupies more than 25% of screen area."

The Spatial Pattern Risk Trace persists at level 0.4 (warning) although the pattern is constantly changing.

" A pattern must remain stationary for at least 0.5 seconds in order to fail even though all other criteria have been met. "

The Spatial Pattern Risk Trace response to a spatial pattern initially increases but persists even after the pattern stimulus has disappeared

"In-depth analysis is averaged over the most recent 5 frames to prevent erratic response to video noise or sudden changes in pattern area or contrast. This averaging causes the initial system response to increase with pattern duration and persist for a short while after the pattern is no longer present."

A stationary pattern, seemingly containing bar-like patterns does not fail. Can this be right ?

" Some patterns that appear to be clearly bar-like in one direction may also contain some local structure in the orthogonal direction. Patterns may be insufficiently regular or contain insufficient contrast throughout. In such instances the number of bars may be effectively reduced or the pattern is caused to separate into two or more regions each of which are less than the 40% permitted area."

The Spatial Pattern Risk Trace suddenly changes from Warning (Pass with Caution) to Failed with little or no apparent change in the image.

"A Spatial Pattern might not satisfy all the criteria in the guidelines but is running very close. Then a very small change in the image due to perhaps lighting, camera position or occlusion might be sufficient to cause failure."

Tell me more about the Extended Analysis trace

"The Analyser, as previously explained, continuously monitors flash activity including activity which is close to failure. If such flashing activity persists for more than 5 seconds a separate warning of failure is indicated as in Figure 2. The black Extended Failure Trace of constant height appears if such flash activity is detected in at least 80% of the most recent 5 seconds. This is equivalent to 100 in 125 frames PAL and 120 in 150 frames NTSC."

The Luminance Flash Risk Trace or the Red Flash Risk Trace has appeared for more than 5 seconds but the Extended Failure Trace has not appeared. Can this be right ?

"The Extended Failure Trace will be activated by Flash Risk activity only. The Diagnostic Trace will appear but the step count will still only be relevant to a moving second whereas the Extended Failure Trace reacts over a longer period and is activated by an accumulation of flash activity of a certain amplitude i.e. Flash Risk Level 0.1 to 0.4. If Flash Risk Activity exceeds these levels and causes an actual failure then the Extended Failure mechanism is deactivated. The Diagnostic Trace (which incidentally can be turned on or off as required by key F5) will still provide useful information about the location of transitions throughout a sequence."

The Extended Failure Trace appeared before a full 5 seconds of Flash Risk activity is indicated

"This trace is activated when at least 80% of the preceding 5 seconds reach the relevant Flash Risk Level. This could happen after a full 4 seconds of activity and thus the trace will appear. "

The Extended Failure Trace is erratically intermittent

"This mechanism is deactivated by either full failure or lack of warnings (Flash Risk Level 0.1 to 0.4). Thus if there are not at least 100 frames PAL or 120 frames NTSC of either type present in the previous 5 seconds then the trace will be intermittent and possibly erratic depending on the nature of the flashing. This ensures that the Analyser does not issue an extended failure for up to a full second of clean material after the last provocative image or flash."