What characterstics of the F75/F75 LTD are transferrable to the F70?

Mr. Dankowski --

Thanks so much for this site. Your stuff on detectors/detecting is the best there is, period. Me being a scientist, I really appreciate how you do things and trust your info, as a result, far more than anyone I know. Your articles are absolutely tremendous, and the forums -- and your input there -- contain a WEALTH of information for someone who enjoys getting into the technicalities. While a novice detectorist, I like to approach all new things from a scientific, factual perspective while trying to learn, and that is EXACTLY what you offer. Simply AWESOME stuff. It is MUCH appreciated.

Two questions, if I may.

1. I just ordered an F70 (would LOVE to get an F75 LTD but finances dictate that that will have to wait a bit!) I am loving the wealth of info about the F75 LTD and comparisons with the F70. Is there any info on your site this detailed about the F70? Do you have any input for me about the F70? Is it going to perform similarly, depth-wise, to the F75 (not the LTD) such that I can relate your comments about the F75 to the F70? I have seen a few comments from you in threads that suggest alot of the info carries over to the F70, but would like to know any details you could offer.

2. Can you describe for me why "disc" affects performance on the F75 LTD so much? I saw you talk in the "Rcpt Ack of F75 Ltd prototype" thread about disc. at 5, and then a "world class" performance improvement if you bring disc. down to 4, and then continued improvements as you drop disc further and further. I don't understand this. Sensitivity increasing giving better performance, I understand. But disc? I don't see why discriminating out iron affects performance so negatively; I am lost here on how this works. Can you describe what goes on in the detector when you increase disc that kills performance so much? Is this specific to the F75 and the F75 LTD, or is this true with all machines (including the F70?) I'm also confused about how disc can be used to help with EMI mitigation...

Thanks so much for this site -- I LOVE your information and your approach, holding things constant while trying to change only one variable at a time. That's GOOD STUFF that you don't
find ANYWHERE else.

Steve



Edited 1 time(s). Last edit at 02/05/2011 11:54PM by steveg.

I've used them both and I saw no difference in depth between the F75-lite and the F70. I think you'll benefit from starting with the F70 because it's ability to control rcv and tx gain make it a tad more usable than my original F75 around EMI. The LTD was better (somehow) in EMI, but I thought the TID was poor. You'll like the F70 and same holds true less discrim is better performance.

Thanks much Shambler for the info. Additional input is welcome.

Shambler, please forgive this next question, as it will reveal how much I have yet to learn, but...

I have been trying to wrap my brain around the terms "sensitivity," "threshold," and "gain," as far as understanding exactly what they mean with respect to detector function -- i.e., what is being changed in a detector when you adjust these things. In your post, you mention "ability to control rcv and tx gain." I am assuming this to be shorthand for "receive and transmit gain?" Taking this a step further, I am thinking about these terms, receive gain and transmit gain, and trying to fit this into the mental picture of what I have thus far understood with respect to the terms sensitivity, threshold, and gain. Let me see if I get this -- "sensitivity" control on the F70 is what you are calling "transmit gain," yes? And so, "sensitivity" will control, if I understand correctly, the strength of the electromagnetic field sent into the dirt by the coil, yes? Then, "threshold" control on the F70 is what you are calling "receive gain", yes? And so, "threshold" will control, let me see how I can say this properly -- the "degree of disruption" of the transmitted electromagnetic field returned to the coil by a given metal target in the ground, such that "higher receive gain" would allow you to sense smaller "disruptions" (i.e. smaller or lower-conductivity targets), assuming a properly ground-balanced machine. Have I understood and stated this correctly? If not, can you please correct me? However, if so, then your "transmit and receive gain" way of discussing this actually clears up my mental picture much better than the way I've had it explained to me in the past.

Thanks!

Steve

That's the way I understand it, so I THINK you have it correct.

Here's some decent info:

[www.fisherf75.com]

Really good stuff at that link. Thanks, Shambler!

Steve

I would say none with LTD in boost mode.
In down town hunting my LTD runs smoother in boost mode than DE
I can almost always run in Senc in the 80s in any mode with no problem..

Thanks gman.

Yeah, I realize that an LTD in boost mode will be a different animal from the F70. I am just trying to figure out how much of the wealth of info I am reading on this forum regarding the LTD might also apply to the F70 (processes and controls specific to the 75 LTD notwithstanding).

Steve

I thought that I read on one of the other post from Tom that the "transmit" signal is a fixed strength signal. Am I correct on this? I am under the impression that "Sensitivity" determines the "receive gain" and the "Threshold" is a gain control that determines at what level the receive signal is reproduced as an audible result over the headphone/speaker.

I am sure if this is not correct info then someone will help us out on this. I also have been enjoying a new F70 and I have been doing quite well with it.

Michael

I am SO short on time.......but here goes.

First.......The F70 and 1st Gen original F75 are identical in performance. If you read the threads COMPILATION #1 and COMPILATION #2...... the F75 data is exactly the F70 data. (There are a few catches.....IRT the F70's 'fast' and 'slow' (deep) microprocessor speed). Use the slow mode for fairly clean areas.......and fast for trashy areas.

Transmit power ....... the moment the unit is turned on...... is a 'constant'.
Sensitivity...... refers only to receiver-gain........ie... how small of a signal will be reported.
Threshold...... is like squelch, whereby the base carrier (audio 'hum') can be adjusted. No 'hum'....and it takes a stronger/larger signal to break squelch....and produce a audio response.

Thanks for the info, Mr. Dankowski.

Might I ask for a couple of additional quick clarifications? One is regarding slow versus dE (default). I get the "trashy" vs. "clean" and which to use (slow in clean ground, dE when trashy), but why? What is it that the microprocessor is doing when running "slow" that makes it "deeper?" And, does the "slow" imply that I need to adjust my sweep to also be "slower" than normal, or no?

Also, on sensitivity and threshold...I see that transmit power is not adjustable. OK that's clear. I'm trying to think through the implications of "receiver gain" (sensitivity w.r.t. the F70) and "squelch" (threshold w.r.t. the F70). My initial read suggests that these both have a similar affect -- they allow you to adjust how "small" of a target you will "hear" reported. When you have time, I'm interested in knowing some real world scenarios which would tell me how to adjust them. For example, "if low EMI and neutral ground, then running sens. and thresh. high does 'X' ", or "if you want to accomplish 'Y' in the field, this type of sens. and thresh. adjustments would be best." Again, just trying to understand the implications of adjusting these two things on my detector's performance.

Finally, I have read compilations #1 and #2, but will go back and re-read with the understanding that anything that says "F75" is essentially the same for the F70 (with a few caveats). Thanks!

Oh, and on my "question 2" in the first post of this thread, I may have discovered the answer in reading other stuff on this site -- regarding the "world class improvement" (Mr. Dankowski's words) in the 75 LTD when going from disc. level 5 to 4. Is this due to changes that happen in processor speed at different disc. levels? As I understand it, -- 0-4 disc is "slower" microprocessing (deeper), 5-19 is "faster," and the greater than 19 is "slower" (deeper) again? I guess there are implications/relationships here in terms of selecting "slow" process on the F70 vs. the dE (default) process?

Thanks,
Steve
Steve



Edited 1 time(s). Last edit at 02/13/2011 12:30PM by steveg.

The 'slow' option on the F70 .... is the precursor (a variation thereof).......of a form of 'boost process' on the F75 LTD/Special Edition. (((( Funny how that worked out! )))) It simply means the microprocessor is a bit slower to respond ..... BECAUSE... it is analyzing more 'snapshots' of the ground; subsequently, allowing the detector to 'see' things (targets) with more clarity/resolution (and deeper). You want a faster (lessor amount of snapshots) in high trash areas....so as to enhance adjacent target separation characteristics. In target-rich environments....you will never be able to ascertain max depth anyway.............................so...................by virtue of forfeiting some depth.....to gain better adjacent target separation characteristics.......is a (unfelt) cost.

Replace the word 'small' (IRT target).....with 'weak' target(s). If you have the conditions/ability (due to low EMI and low Grnd minerals)..... by all means.......run Sens as high as possible........Disc as low as possible (assuming utilization of multi-tones)....... and a 'barely audible' threshold. WEAK targets will sound more 'resolved'.......to the point of greater human-audible clarification (aka "Dig Me").

Whilst dropping Disc to '4' (and below)...... a base-line threshold drop (looks/sounds like a Sens boost) becomes employed.

Mr. Dankowski --

Perfect. I have it clear now.

I presume your last sentence is referring to the F75 LTD only, or is this true also on the F70? Either way, I do now see that the 4 and below disc. is NOT a slower microprocessor speed as I had assumed, but a "threshold" drop. Thanks for that clarification.

THANKS!

Steve

I'm gonna love reading when these two get together on a post.

TerraDigger --



And worse, I started my college education in Electrical Engineering before switching -- not surprising I guess that I have an annoyingly long-winded nature!

Through my life, I have exasperated literally countless numbers of folks (teachers, professors, co-workers, oh yeah, wife ) with my "million-and-one questions" personality. I ALWAYS longed for a teacher with a like-minded nature; Mr. Dankowski seems to be one of those types -- loves information, has an exceptionally large amount of it in his brain, and seems to like to share it! A rare individual indeed, in my experience! I love to talk with people like this; my challenge is to not "wear out my welcome," because I would otherwise tend to continue these conversations for hours on end!

Steve