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I have two Eagle 320 one on the trolling motor and one for driving rigged thru the hull. I've been using them both for about a month and did'nt notice any feedback. How can I tell for sure.
 

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Captain McSh*tty
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The feedback I have seen was on Miguel (Mulitiple Migs) boat...the front graph was reading, but looks like there is a bunch of clutter, or a huge ball of shad from hell, on the graph. You can still see the bottom though.
 

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As Jason mentioned, I have that problem w/ mine. I run an X-85 in the bow and LMS-160 at the console. The interference shows up like what appears to be grass lines from the bottom. On occasion, the digital readout in the upper left corner will 'loose bottom'. I don't recall shad balls from hell, though. :roll: Funny thing is, I never had this problem on my shorter aluminum boat, where I ran Lowrance units at the same frequency. Hmmmm....
 

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Champ: It depends. I have 2 t'ducers installed for the graph in the front. One on the t'motor and the other is shooting through the hull. If I use the one that's shooting through the hull, I can't see my spoon. It doesn't reach the 'cone'. I am getting in the practice of shutting down the unit at the console. If the backseater wants to see what's going on, he's welcomed on the front deck.
 
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Each graph is going to work differently (makes/models/transmitted freq). Basically, interference between graphs may occur in two forms. One will be interference or clutter on the display while the other will be in masking echo's that you may want to see. Either case, you’re receiving returns that you don't want. However, you might not be aware of the interference when the visual clutter is not present.

If you're going to use two graphs, the best way to work around this would be to use two different freq graphs. Since they're sending different freq's and receiving different freq's, you won't have the interference problems. Lowrance makes some good deep water graphs that operate at lower fewq's ("DF" model's). You could use the 50kHz on one graph and the 200kHz on the other (you're pref) Or you could just get a graph that operated strictly at 50kHz. I think most of the DF/50kHz models transmit more power because the 50kHz is more suited for deeper, ocean uses. Because of this, they typically transmit more power than you might need for fishing our lakes. Using too much power with manual sensitivity and ASP modes can result in second time returns. Have you ever seen two identical displays displayed on your graph (one above the other)? This is from second time returns, when signals bounce off of the bottom, come back up and then bounce off of the bottom of your boat or the surface clutter and then head back down to produce the second display. This is from being way too sensitive or using too much power (but we can't choose power settings on our graphs so we need to decrease the sensitivity)

If you're using two graphs on the same freq and don't notice any interference on your displays, you might want to check for receive processing interference (jamming) issues. You can do this by taking a spoon or anything you can mark in deeper waters and use ONLY ONE graph (the second one should be OFF). Mark your spoon at a depth and then adjust the sensitivity until you the spoon is no longer marked. Then bring back some sensitivity until you just start to mark your spoon (use just enough sensitivity to mark your spoon at a depth of say 40'). Now you can turn on your second graph and see if you can still mark your spoon on the first graph. The sensitivity of the second graph doesn't matter because we're looking to see if it causes interference that hides your spoon on the other graph. You can try this for the other graph as well. This important in cases like d a shared with us the other week. That "thicker" mark on the bottom of the lake might not show up if a second graph is used that causes masking of returns.

I’d suggest using two different freq graphs unless you’re fishing in conditions that peak sensitivity wasn’t an issue.

Just my 2.5 cents…
Ray
 

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Keep in mind nothing is absolute when talking about cross-talk interference between two sonar units. For example, DigDug is seeing his X-70 192khz signal on his 200khz Fishmark 320. Shouldn’t happen right? Two different frequencies? Without getting into edge characteristics of bandpass filters suffice it to say that you can often tune your car radio to 630AM and still hear a station broadcasting on 620AM if you turn your volume up enough.

There are many variables that affect whether sonar Unit A will hear echoes resulting from Unit B’s sonar ping. Things like the distance between Units A and B, the power of Unit B, and the frequency of Unit B to some extent. But remember nothing is absolute. Also if Unit B is positioned over 10 ft of water and Unit A is past the dropoff and is positioned over 40 feet of water, Unit A might not see echoes from Unit B. However, when Unit B moves over the dropoff and its signal starts bouncing off that wall underwater, then Unit A might get overwhelmed by echoes from Unit B.

There are also variables that affect whether you will see the echoes from Unit B on the display on Unit A. Unit A’s sensitivity setting is one but I think the biggest variable is the timing between the two units. Say Unit A is on the TM and Unit B is on the transom. They are two independent units and generate pings at different times. And Unit A’s screen is set to display from 0 to 40 feet. Keep in mind Unit A listens for signals all the way down to say 800 feet, but it displays only what it hears from 0 to 40 feet. If the timing between A & B is close and they both ping at about the same time then Unit A hears and displays Unit B in the 0 to 40 ft range. That’s interference you will see. But if the timing between the two is not very close and Unit B pings when Unit A is listening for echoes at 200 feet then Unit A will not display the echoes from B in the 0 to 40 ft range. (Think of this as watching the football game and your wife starts talking to you, you hear her but it doesn’t register.)

The bottom line is I’ve got to trust what I see on the front graph to be comfortable. I can’t trust the front graph display if the back graph is turned on. Whether I see interference or not. I turn the back graph off.

Ray, I’ve been thinking about the masking issue a lot. I know it happens, I just can’t explain why. I know it happens because arches don’t look as smooth when the back graph is on. Dotted lines versus solid lines. That masking thing is one reason I don’t trust the front display when the back graph is on.
 

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dENNY,

...wHY IS MY CAPS LOC ON??? There! That's better... :lol:

If you're seeing masking from a different freq graph, then, like you had mentioned, there's a LOT of things that can cause this.

When those transducers ping away, there's a very wide range of noises being produced, transmitted and received as well as harmonic freq's that can cause issues. I wonder about the scattered returns from other graphs and how much effect they can have between two boats in close proximity or even the noise floor being picked up in different graphs. That timing issue is something as well. I've often wondered why they don't have a "sync" input you can connect between graphs so one isn't transmitting while the other is listening. They use blanking pulses for interference issues in all kinds of electronic transmission applications so the technology for that is out there. They could easily figure out a way of syncronizing two graphs to work without causing interference.

The only way to get around all of these these issues would be to induce coded info (or a unique signature) on each transmitted pulse and process it like digital signal processing where you're not concerned with the 200kHz (or any other 200kHz) but the exact code you sent out. I know you know this but there is a big difference between digital signal processing and processing a signal digitally... That would open some new doors to the world of the "every day graph".

Now, figure out how to do that and market it in a way that the average Joe Fishermen can afford it, and I'll gladly share your millions... :wink:

There is a LOT to this when you want to get down to it. It couldn't be covered over just a couple of beers and lunch, that's for sure... It's all interesting stuff. ...well to me it is.
 

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Capt. Ramius in The Hunt for Red October: Give me a ping, Vasily. One ping only please!

Obviously I find this interesting too.

I see masking on graphs using the same frequency. But no matter, your logic would apply there as well. The must blank the reciever while the transducer is pulsing. I think the masking must be caused by the software when it correlates the echo to the depth and then writes it to the display. The reason I think its the software is because the transducer must produce electricity when it receives a pulse.

High dollar sonar units, commercial stuff not our stuff, have controls to eliminate interference. Could be by shifting the timing or frequency or some other way, I don't know how.

Yes, Digital Signal Processing is good stuff. People make use of it all the time. Cell phones, digital cable TV, cable modems, voice recognition systems, etc. many of todays hi-tech devices rely on DSPs.
 
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