Each model of X-TERRA has the discrimination capabilites to categorize targets according to the phase shift induced by target properties. For the sake of discussion, let's refer to the most ferrous target providing the lowest display number. And the most non-ferrous target as having the highest display number. And instead of constantly referencing phase shift, I'll use the words ferrous/non-ferrous. When you consider the 305 having "4-digit" notch segments, the 505 having "3-digit" notch segments and the 705 having "2-digit" notch segments, it is obvious the width of the notches vary from "wide" to "more narrow" when comparing the three models. So even though the ferrous/non-ferrous properties of a target remain constant, each detector will categorize that target into a "notch" that can be represented by a different number than the other two. As example, if I found an IH penny with my 705 that gave a consistent TID of 34, what number would the 305 or 505 provide? Since 34 is not divisible by either 3 (representing 3-digit notch segments of the 505) or divisible by 4 (representing the 4-digit notch segments of the 305), that IH penny cannot give a TID of 34 on those two models. The TID for each model will be assigned to the notch segment that includes those specific ferrous/non-ferrous properties. In the case of the 505, I'd be willing to bet that it would provide a TID of 33. I say that because dropping down from 34 to 33 is only one increment. For it to jump to a 36 (the next available notch segment on the 505), it would have to jump up two increments. In the case of the 303, it could go either way. It might drop down two increments to a 32, or go up two increments to a 36. So then the question becomes..... what if the actual TID of that IH cent was just on the low side of 35? The reason it came in on the 705 as a 34 is because the 705 does not have a notch segment number 35. Since there is no 35 on any of the three models of X-TERRAs, again, it will display a number relating to the notch segment that most closely identifies with it's ferrous/non-ferrous properties. In the case of the 705, it went to a TID of 34 even though the actual value was on the low side of 35. Being on the low side of 35 is still closer to 34 than it would be to 36. (only even numbered options on the 705) Not to make it more confusing, but what if that same IH cent that came in as a 34 on the 705 actually had the ferrous/non-ferrous properties that were determined to be just barely above a 33? It would still read a 34 on the 705 because 34 is closer to the "high side" of 33 than 32 is. And it would read a 33 on the 505 because that is also the most closely related notch segment available, based on the targets ferrous/non-ferrous properties. And, it will read a 32 on the 305 because, being on the high side of 33, its property values are "closer" to 32 than 36. And remember, on the 305, there are no other notch segment choices between 32 and 36.
As to maximum and minimum........I believe that the most conductive target will TID with the highest number available on each detector. And I believe that the most ferrous target will TID with the lowest ferrous number of each detector. In comparing all three models in the All Metal mode, I've always found that a target reading -9 on the 505 will read -8 on the 705 and will read -4 on the 305 . But that does not mean that all three models have the same "ferrous/non-ferrous" properties programmed into their upper or lower notch assignments. In other words, if the 305 indicates a ferrous value of -4, it isn't necessarily going to be a -9 on the 505 or a -8 on the 705. As we've discussed, the 4-digit notch segments are wider. As such, they encompass property values that would represent more than one notch segment on the other two models. A target that indicates -4 on the 305 could be a -8, -6, -4 or even a -2 on the 705. In other words, the 305 only has one choice in categorizing ferrous targets. The 705 has four different "ferrous notches" it can "chose" from. And on the 505, it could be a -9, -6 or a -3 because it has three different "ferrous notches" to chose from. But regardless of which model you have, that detector will base a targets placement on the actual ferrous/non-ferrous value as determined by the electronics. With all that said, I don't believe that all three detectors have the same discrimination notch capabilities, as far as Minimum - Maximum (ferrous/non-ferrous) values are concerned. While Field Testing the X-TERRAs, I found one exception to the school of thought that the "top end" and the "bottom end" of the ferrous / non-ferrous scale is the same for all three models. I discovered this using the -9 notch segment on the 50 during my initial Field Test. Using the 50 in a Pattern mode with zero discrimination, I was able to detect a specific piece of ferrite that registered a -9 on the TID. When I bought my 505, I got the same results. On the 30, 305, 70 and 705, using the same Pattern Mode with zero discrimination, I am not able to detect that same piece of ferrite. I am, however, able to detect that same small piece of ferrite when using All Metal mode in all X-TERRA models. To me, this suggests two things....... one is that the -9 of the 50 and 505 goes "lower" on ferrous scale than the -4 of the 30 or 305 OR the -8 of the 70 and 705. And two, setting up a Pattern with zero discrimination is not the same as running in All Metal. I believe that the discriminating characteristics of a Pattern mode actually limit the acceptible value of targets, based on the target's ferrous/non-ferrous properties and the parameters established in each notch segment. By using All Metal, even the most ferrous of objects can be detected by all three models. I believe that if you set your X-TERRA with a zero discrimination Pattern, even though you are not rejecting anything, there are properties that are not being accepted. Simply stated, I believe that Pattern modes are groupings of discrimination (notches) that are capable of rejecting any target whose properties fall within the characteristics assigned to them via software. However, by accepting the notch segments, I believe you are only accepting those targets whose values are defined (by software) to represent a target within each notch segment. If a target has properties that fall outside of those "notch definition" parameters, it will be ignored. Just as my one piece of ferrite is ignored by using zero discrimination on the 30, 305, 70 and 705.
As to Noise Cancel......as I understand it, the operating frequency is established by the software, as instructed by the microchip in the coil. However, frequency shifting (Noise Cancel filtering) occurs in an effort to minimize the harmonics of nearby electrical sources. The shift is minimal, compared to changing coils in that it may represent a small fraction of a kHz. Just enough to break the harmonic influence that could induce noise into the circuitry. Keep in mind that the detector acts as a transmitter and a receiver. The electronics in the housing transmit signals through the coil, into the ground. When those eddy currents are induced into a metallic object, that object generates it's own eddy currents of a lesser value and of opposite polarity. Those eddy currents are then recognized by the receive winding of the coil, and that information is passed to the electronics in the control housing. But just as the coil acts as a receiver for the target's' eddy currents, it also represents an antennae to outside electrical interference. VFLEX minimizes this interference by passing the signals from the coil to the control housing digitally. Other makes of detectors pass the information along the coil cable in an analog format, which allows for an even greater influx of EMF, RFI etc. My recommendation for performing a Noise Cancel is to hold the coil parallel to the ground, at waist height. By holding the coil parallel to the surface of the ground, you minimize the antennae effect of the coil. And by holding it waist high, you minimize the risk of having a detectible target under the coil at the time of your NC.
An experiment on Noise Cancel...something you can try at home..... if you have access to fluorescent light fixtures, stand under one of them, turn on your detector and max out the Sensitivity. If it chatters, change noise cancel channels and you'll likely notice a distinct change in the stability. If it doesn't chatter when you turn it on and max out the Sensitivity, go through the channels manually and you will see that there is a difference. JMHO HH Randy
. Otherwise, the 305 would be a nice quiet machine since the nuts, nails,and other trash that ring at -6 and lower on the 505 and 705 would not be detected (the 305 only goes to -4). In reality, they are all detected on the 305 but as -4. 
Oh my
HH TomB