One thing that hasn’t been mentioned much is the fact that tranducers have improved by a factor of probably 10 in the last 20 years. The basic concepts are all the same, but advances in coil materials, dielectrics, and suspensions make todays transducers much more sensitive and much less susceptible to noise (and self-jamming) than the old days.
I have a schematic I drew up of an ancient flasher if anyone wants to see it. Pure analog, don’t make them like the used to. It uses a pulse generator, resonator, and three gain stages to trigger the light as the dial spins. Pretty ancient compared to today’s flasher units, which likely use a single gain stage (low noise amp) and an A/D converter, allowing them to do all the thresholding, noise rejection, and sensing in software.
The ‘hot steady’ power that your unit draws does not translate to trasmitter power on the x-ducer, as most of today’s units are probably running rather high-end real-time processors (something akin to an ARM4 family) to execute the discriminator and generate the display. Think of that desktop PC you threw out the window 5 years ago getting run through a trash compactor and squeezed into a flasher.
The reason graphs show so much more detail is their ability to generate essentially an infinite greyscale. Think about it as a pen riding on the paper surface with pressure applied in proportion to the return signal (okay, not that simple, but the analogy is good I think). It’s easy to see the difference by comparing the 10-level greyscale Eagle I bought last year to my old ‘on/off’ pixels on the Humminbird 400 I ran for three years prior.
The speed of sound through water is still a crawl compared to what the unit is seeing. To generate an example, probably 90% or more of the units out there use a 200kHz transducer. This means when they transmit, they generate a ping 1/200,000th of a second. It doesn’t have to dissipate that much power through the transducer in that amount of time to rapidly get to 1000 watts (a few milliamps will do it)! A more advanced unit might be transmit a series of pulses, in order to eliminate noise from nearby transmitters; it really doesn’t (shouldn’t) make a difference, because it’s attempting to correlate the same sequence on return, thus the integrated power of the sequence is all brought to bear and though the power dissipation is spread over a longer time interval, integrated power is equal. It sounds like nonsense, but that’s the power of processing in the digital domain.
Back to 200kHz and what that means: Sound travels approx 3x faster in water, or about 900 meters/sec. At 1/200,000th of a second, that gives you about a 2 inch separation from successive return pulses. Today, A/D coverters run up in the 100+MHz range (yep, that’s mega), meaning that if a vendor was willing to stand the power, they could probably run the transducer frequency up to something that would allow you to see the difference in depth between a waxworm’s tail and his head if they wanted to. Since not many people are willing to shovel out $30K for a sonar to fish with, I would expect that further advances in that area will probably be limited to researchers and university types.
