Frequency Shift Test

[ColonelDan]

Approximately one year ago, Savannah Harps posted the video below wherein he reported that frequency shift affected VDI consistency.

I just saw this video and decided to run an informal test to see for myself what the impact was on VDI of manually changing frequency shift values. 

I used a modified Beach Sensitive program with the Deus II version 1.1.  Max Freq was 40kHz. I did a frequency scan at the start of the test to reduce the effect of minimal EMI in the test area.  I manually shifted the frequency from 0 - 6 using common targets found on the beach---pull tab, clad quarter, 925 ring, 10K ring.  Each target was buried in 6 inches of beach sand.  My results are shown in the table below.

Freq Shift	TID
From 40kHz	Pull Tab	Clad Qtr	925 Ring	10K Ring
0	62	93	89-90	75
1	61-62	92-94	89-90	75-76
2	61	92-94	89-90	74-75
3	61-62	92-93	90-91	76-77
4	61-62	92-93	90-91	76-77
5	61-62	92	90	77
6	59-62	92-93	91-93	76-79

  As shown, there was a slight change in a few of the VDI numbers that Savannah Harps noticed as I ran through the available frequency shift options but nothing that would change my ability to ID what was under my coil.  VDI values remained in the expected range as far as I could see.

Your opinion and results may differ depending on the environment and test conditions.

Just a curiosity to fill  an uneventful morning....😆

[CPT_GhostLight]

Great report, Colonel! 😎

I imagine Harps was using V0.63 or V0.71 (Big Numbers) back then and it looks like that is one of the things that has been addressed in V1.1.

That's why I always carry a nickel and dime to test the numbers that day after Ground Balance and Frequency Scan (if needed) wherever I hunt. Of course the TIDs will vary a bit in the ground, but that gets me in the ballpark. 😉

[ColonelDan]

CPT,  You bring up a good point regarding test targets.  Below is the "Test Kit"  I take to the beach.  Each target can slide along that rope so I can separate them for individual testing or place them close to the other one(s) for separation/unmasking testing.  These examples/samples pretty much cover what we're most likely to find on our tourist beaches.

Clad quarter, pull tab, 10K ring

Bottle cap, 925 ring, tent stake

Glad you brought that up.  You just may have helped some fellow detectorists...especially new ones to the game...by giving them a good idea!  

Good job CPT

[CPT_GhostLight]

22 hours ago, ColonelDan said:

CPT,  You bring up a good point regarding test targets.  Below is the "Test Kit"  I take to the beach.  Each target can slide along that rope so I can separate them for individual testing or place them close to the other one(s) for separation/unmasking testing.  These examples/samples pretty much cover what we're most likely to find on our tourist beaches.

Clad quarter, pull tab, 10K ring

Bottle cap, 925 ring, tent stake

Glad you brought that up.  You just may have helped some fellow detectorists...especially new ones to the game...by giving them a good idea!  

Good job CPT

 

 

 

Thanks, Colonel, that was taught to me by Richard, one of my mentors and detecting buddies in my detecting club who always carries a dime and a nickel to test before hunting. 

That's a great idea for a portable test rig and it looks like you got all the bases covered. 😎

[CLH]

I had nonmetallic shoes and kept a nickel on one shoelace so I could check that it wasn't being notched out, and a pull tab on the other so I could notch it out. In the old days with my old machines gold and nickels fell in the same range and I could notch out the tabs, but still found gold rings, not too many other choices for settings. Those days are gone as the machines get more finely tuned and I have gained another 30 years experience.

[midalake]

On 7/13/2023 at 1:19 PM, ColonelDan said:

These examples/samples pretty much cover what we're most likely to find on our tourist beaches.

I admire people who have good target lock. The reality is in Black Sand conditions there is wild swings in target ID. It would be VERY unheard of to get any 10k ring in my location in the 70's. In my last beach hunts, Pulltabs would come in 52-64 with the D2. [disappointing] 
But testing in your location is very important! 

[UKD2User]

I've noticed that coins can give different VDIs in different circumstances. I always use autoscan on (so my D2 will do an automatic frequency scan every time I change search mode - or, if I ever chose to (I don't use this feature), adjust max freq for a chosen search mode).

To date, I've assumed that different VDI from the same coin type has been caused by:

1) The state of corrosion of the coin;

2) The specifics of the particular hole (compaction/moisture/nearby junk/mineralisation/salt);

3) The fact that up to 10% of some earlier design British £1 coins were fakes!

4) The fact that each search mode transmits a different spectrum of energy into the ground and so the energy spectrum of the return signal will be different to match.

Having read this thread I've come to realize that there's more to this.  Frequency shift/scan has an effect on discrimination/unmasking as well.  I put this down to the fact that every specific target location will have an absolutely unique frequency response pattern/signature (signal returns from ground/minerals/salts/ferrous and non-ferrous undesired items and desired targets all combined into an overall unique 'signature').  The transmitted (and therefore) returned signals are quantized (along the frequency axis) and so there's a sort comb-filtering effect going on. A 'frequency shift/retune simply moves the 'comb' sideways a little and in so doing, can mask parts of the overall response signature (sometimes enough to affect discrim/unmask/VDI functions).

I don't think that there's anything that can be done about this - very interesting - effect, but it's just one more reason why a permission can never be said to be completely 'done'.