To be able to convert the echo recieved to video, you need to determine the depth levels from which echoes are produced. Let's assume an experimental setup: Let's assume we have an ultrasonic sensor and three targets. The targets are aligned (they have to be, because we are using a single transducer) and installed with a distance of 1 cm between each other. Here's a schematic representation:
(Transducer) -- 1 cm-- (Target 1)--1 cm-- (Target 2)--1 cm--(Target 3)
As we are using more than 1 object, we have to be able to transmit ultrasonic pulses instead of continous ultrasonic waves, otherwise we can't determine the distence between objects, or the number of the objects. Let's assume we send a very short pulse and then the sensor starts to listen for echoes. The pulse will hit target 1 and a quantity of the pulse will reflect back from target 1, but if the target is not a specular reflector (meaning it won't reflect all of the pulse back), a quantity will continue to go in the same direction and hit target 2. The same reaction will happen there and a portion will reflect back, while the remaining will hit target three. We will assume the speed of sound to be constant, so by measuring the time it takes for the returning echoes, we can accurately calculate the distence of these reflectors (x=V*t/2) and we can use a graphic display to plot these as dots. The upper border of the display would be the transducer, the lower part could be the range of the sensor (it would depend on its frequency), so we can draw dots on suitable locations. The size of the dot could be determined by the amplitude of the returning signal (assuming we could measure this amplitude).