Two Transducers?

Garrison

Garrison
May 21, 2008
571
610
Temecula
Name
Garrison
Boat Name
Mikelson 43’
Our current system is a mix of Furuno Tztouch2’s and one 3. The existing transducer is a 1Kw B260 50/200 KHz. Just purchased a B275LHW wanting the High Wide CHIRP functionality for offshore. I plan on switching between the two as the situation requires. We were able to schedule a haul out and install next month. Planning on discussing this question with the installer next week, but thought I would ask here.

It has always been my understanding that running two transducers at the same time is a no no due to interference. I was watching some YouTube videos, doing a little homework on transducers and the Airmar rep seemed to refute this assumption. Can the High Wide CHIRP from the B275 and the Low frequency from the B260 be used together for better coverage/results as he describes in the video? Or is the B275’s Low CHIRP superior to the B260 single frequency lower in the water column? He seems to make it sound, it would take a little one time tweaking in the settings to isolate frequencies.

Any thoughts or experience with this? The short video clip is attached below.

Thanks,
Garrison

 
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ShadowX

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Oct 10, 2010
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Our current system is a mix of Furuno Tztouch2’s and one 3. The existing transducer is a 1Kw B260 50/200 KHz. Just purchased a B275LHW wanting the High Wide CHIRP functionality for offshore. I plan on switching between the two as the situation requires. We were able to schedule a haul out and install next month. Planning on discussing this question with the installer next week, but thought I would ask here.

It has always been my understanding that running two transducers at the same time is a no no due to interference. I was watching some YouTube videos, doing a little homework on transducers and the Airmar rep seemed to refute this assumption. Can the High Wide CHIRP from the B275 and the Low frequency from the B260 be used together for better coverage/results as he describes in the video? Or is the B275’s Low CHIRP superior to the B260 single frequency lower in the water column? He seems to make it sound, it would take a little one time tweaking in the settings to isolate frequencies.

Any thoughts or experience with this? The short video clip is attached below.

Thanks,
Garrison


If its the same frequency range and overlaps, then it could be a no-no. If you are using high chirp on one and low chirp on another, it should be no problem because the two frequencies never overlap each other. In fact, most people with a 3-in-1 transducer is already using multiple transducers within one unit. The 50/200KHz signal and the 400/800KHz signal for the downscan/sidescan can be operating at the same time without interference.

Sounds don't interfere in the conventional sense. Like waves, if two signals are close in frequency, it can have a low point (null) or a high point if they are high (resonance). However, computers can separate those signals into separate tones as long as they are far enough apart to be distinct peaks. Computers can use fast Fourier transforms to convert a complex sound into discrete frequency tones to decompose the signal into separate signals at various amplitudes.
1280px-FFT_of_Cosine_Summation_Function.svg.png


If you can imagine if the sound was coming from a speaker, the tweeters will send sound out and the bass speaker would do it at the same time. Our brains can mix the two signals, but electronics can differentiate the two as separate outputs.

The interference was a bigger issue using conventional sonar. In conventional sonar, its a single monotone ping that is sent. If another transducer sends out a ping in the same frequency, it may confuse one response for another since they look identical to the fish finder.

With chirp sonar, it has changed. Not only is there a single tone, it sweeps at a specific frequency curve and it sends another signal right when the first sweep it completed. However, the second sweep uses a different frequency curve that is distinct from the first sweep. When the signals are detected, the computer can differentiate the response of one signal from another even though there may be overlaps. It uses the distinct sweep curve to determine the time period between when the sonar signal was sent to when it was detected. The computer then has to compile all these return signals in order to display a meaningful image on the screen.

Different Chirp signal profiles:
1631501800514-png.1317958


Because of that, I believe the chirp system has a higher immunity against interference. Unless the other transducer is sending at the same frequency and the same sweep signature, it can separate the two signals as if they are two different distinct signals. The chirp system looks at the entire signal sweep received as opposed to a specific frequency. The CPU can even discard the interference signals when it compiles the image. Every vendor has different algorithms for compiling the image, so it depends on the vendor's code.

On the other hand if you have one chirp system and a conventional sonar on another. The conventional sonar may have more interference from the chirp sonar if the frequencies are in the same range. The conventional sonar would not be able to differentiate the sonar signal it sent from the one sent from the chirp sonar at the same frequency.

In the example below, the conventional sonar would not be able to tell the 200 KHz signal it sent compared to the Chirp signal when the Chirp sweep reaches the 200 KHz frequency. The chirp sonar can tell the difference because the conventional is a single tone while the chirp is a distinct sweep signal over a frequency range.
CHIRP-10.jpg
 
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Garrison

Garrison
May 21, 2008
571
610
Temecula
Name
Garrison
Boat Name
Mikelson 43’
If its the same frequency range and overlaps, then it could be a no-no. If you are using high chirp on one and low chirp on another, it should be no problem because the two frequencies never overlap each other. In fact, most people with a 3-in-1 transducer is already using multiple transducers within one unit. The 50/200KHz signal and the 400/800KHz signal for the downscan/sidescan can be operating at the same time without interference.

Sounds don't interfere in the conventional sense. Like waves, if two signals are close in frequency, it can have a low point (null) or a high point if they are high (resonance). However, computers can separate those signals into separate tones as long as they are far enough apart to be distinct peaks. Computers can use fast Fourier transforms to convert a complex sound into discrete frequency tones to decompose the signal into separate signals at various amplitudes.
1280px-FFT_of_Cosine_Summation_Function.svg.png


If you can imagine if the sound was coming from a speaker, the tweeters will send sound out and the bass speaker would do it at the same time. Our brains can mix the two signals, but electronics can differentiate the two as separate outputs.

The interference was a bigger issue using conventional sonar. In conventional sonar, its a single monotone ping that is sent. If another transducer sends out a ping in the same frequency, it may confuse one response for another since they look identical to the fish finder.

With chirp sonar, it has changed. Not only is there a single tone, it sweeps at a specific frequency curve and it sends another signal right when the first sweep it completed. However, the second sweep uses a different frequency curve that is distinct from the first sweep. When the signals are detected, the computer can differentiate the response of one signal from another even though there may be overlaps. It uses the distinct sweep curve to determine the time period between when the sonar signal was sent to when it was detected. The computer then has to compile all these return signals in order to display a meaningful image on the screen.

Different Chirp signal profiles:
1631501800514-png.1317958


Because of that, I believe the chirp system has a higher immunity against interference. Unless the other transducer is sending at the same frequency and the same sweep signature, it can separate the two signals as if they are two different distinct signals. The chirp system looks at the entire signal sweep received as opposed to a specific frequency. The CPU can even discard the interference signals when it compiles the image. Every vendor has different algorithms for compiling the image, so it depends on the vendor's code.

On the other hand if you have one chirp system and a conventional sonar on another. The conventional sonar may have more interference from the chirp sonar if the frequencies are in the same range. The conventional sonar would not be able to differentiate the sonar signal it sent from the one sent from the chirp sonar at the same frequency.

In the example below, the conventional sonar would not be able to tell the 200 KHz signal it sent compared to the Chirp signal when the Chirp sweep reaches the 200 KHz frequency. The chirp sonar can tell the difference because the conventional is a single tone while the chirp is a distinct sweep signal over a frequency range.
CHIRP-10.jpg
Thanks for taking the time to give such a thorough explanation. So, in theory it should not be an issue. Similar to using the two tilted flush mount transducers that are designed with separate frequencies and designed to work together, or as you described the multi elements within one transducer.

I am still interested to find out if there are any practical benefits to this arrangement, and if my system can support running both at the same time in isolated frequencies and at full power to get a better picture of what is deeper in the water column at the same time the high wide is covering the upper water column as described.

I guess I may need to go to the boat and dive into the manual a bit more. Or patiently wait until I can play with it and report back.

Thanks,
Garrison
 
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ShadowX

I Post A Lot But I Can't Edit This
Oct 10, 2010
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4,681
Los Angeles
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Anonymous
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None
Thanks for taking the time to give such a thorough explanation. So, in theory it should not be an issue. Similar to using the two tilted flush mount transducers that are designed with separate frequencies and designed to work together, or as you described the multi elements within one transducer.

I am still interested to find out if there are any practical benefits to this arrangement, and if my system can support running both at the same time in isolated frequencies and at full power to get a better picture of what is deeper in the water column at the same time the high wide is covering the upper water column as described.

I guess I may need to go to the boat and dive into the manual a bit more. Or patiently wait until I can play with it and report back.

Thanks,
Garrison

I know the Simrad units can run both transducers at the same time in separate split screens. You can also use two separate units if you have more than one. I'm not sure if units from other vendors can do the same.


As for practical use, I'm sure the different cone angles if they are different would result in some useful data. For instance, if the high-wide transducer shows fish marks but the standard high transducer doesn't, it means the fish are around, but not directly under narrower sonar cone of the high chirp transducer. You can deduct roughly how far away the fish are away from the boat. Most people see fish marks and assume its under the boat, but it could be a hundred feet away if it is very deep.

In the example below, you the fish would be in the gray area (high-wide) but not directly under the yellow cone (high chirp). The difference becomes larger as the fish is deeper.

1653296853396.png
 
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