Accumulate, Average, And Fade Explained

[Steve Herschbach]

The Accumulate, Average, and Fade controls exist on several White's models, having first been introduced on the White's Eagle Spectrum. The functions govern how the SignaGraph™ and SpectraGraph™ displays show targets and so are unique to White's detectors. These display controls are also used on the DFX and V models. This note appended to the Engineers Guide to the Spectrum XLT offers a basic explanation of what these controls are doing. The display controls are also covered on pages 45-46 of the XLT Users Guide (excerpted below).

Spectrum XLT Engineering Note
Using Spectrum XLT Modes: Accumulate, Average, and Fade
by Mark Rowan

Although the SignaGraph™ display format has been well received, there seems to be some confusion regarding the option (average accumulate, and fade) and how to use them. Perhaps a more detailed explanation is called for.

The default (preset) condition is Accumulate/Average/Fade, the fade rate being fairly slow. With each sweep over the target, several attempts are made to assign it a V.D.I. number. Each reading (There may be only one or two, or as many as six or eight readings per sweep) is reported to the user as a vertical bar on the SignaGraph™. (NOTE: If all eight readings are the same, the user will only see one bar).

In Accumulate mode, these bars are not "cleared out or "blanked" on subsequent sweeps, but continue to "build up" in the display so that the user sees the entire history of multiple passes over the target. Squeezing the trigger will clear the display if it gets too cluttered, or if the user chooses to move to a different target with those of the second one. However, continually having to squeeze the trigger can be a nuisance -- this is where Fade comes in. If a certain period of time elapses with no new target responses, the vertical bars will be shortened by one increment. Eventually, they will disappear from the display. Notice, however, that if a new response comes along quickly enough, the Fade timer is reset (this is necessary to prevent readings from beginning to fade before the user has even had time to see them). Thus, if the Fade rate is slow and the user is sweeping the loop fairly quickly, no fading will occur until he stops swinging the loop or moves away from the target If he is working a trashy area or has his sensitivity cranked up to the noise threshold, the Fade out may be disabled entirely. This is the justification for the Fade Rate control. If the fade rate is increased, the user can find a level at which Fading will reliably occur, but which is not too fast for his personal taste or his style of hunting.

If the Accumulate control is turned off the instrument is in the "single sweep" mode. Each sweep of the loop causes the display to be cleared, and only those readings made during the current pass over the target to be shown. The primary advantage of this is that it reduces the chance for readings from two different targets to be confused with each other. If the Fade rate is set to 0, fading is disabled and display bars will remain indefinitely.

Average mode is entirely different than either the Accumulate or "single-sweep" mode of operation. When Averaging is turned off (the default in all preset programs have Averaging on) the vertical height of the display bar is an indication of the strength of the signal when that reading was taken. With Averaging, however, the height of the bar represents a running total of the number of readings that have occurred at that point on the display.

For example, if you pass the loop several times over a gold ring and get readings of 1, 10, 40, 10, 10, -20, and 10, the display will show a tall bar at the place corresponding to 10 on the display, and very short bars at the positions corresponding to -10, 0, and 40. If the count exceeds the vertical resolution of the display, the bar remains at its maximum height and all other bars in the display are reduced by one increment, eventually disappearing from the display. The usefulness of this is that if the error in the V.D.I. readings is random (such as that caused by electromagnetic interference or irregular loop motion), it will eventually average out and the display will "lock on", showing a single prominent segment at the correct point on the display. Unfortunately, there is a catch. Iron and foil targets which mimic coins will also appear to "lock on"; the smearing one would normally expect will be suppressed somewhat. However, in the hands of a skilled user this should be a very useful feature.

Mark Rowan was a Senior Engineer for White's Electronics, Inc. Mark holds degrees in General Science, and Electronics Engineering Technology, and is a graduate of the University of Oregon. His background includes satellite communications and RF test and measurement instrumentation.