Returning to the original example of a broadband transmission consisting of just two short pure-tone pulses, imagine that the two short pulses progressively increase in length until the two just touch each other. This is illustrated in Figure 13. When this happens, the broadband transmission has become narrowband. A narrowband pulse can be treated as a special case of broadband with signal processing that is fundamentally the same. And narrowband is where ADCPs started, partly because it seemed conceptually simpler, but also because the microprocessors of that era had limited processing power. Broadband processing is more computationally intensive, but of course microprocessors are faster now.
Earlier we said that broadband ADCPs capture finer details because their shorter pulses produce more independent estimates. This is illustrated in Figure 12, where an ADCP was set up to profile currents using both narrowband and broadband, alternating between the two methods every 60 seconds. Both methods captured well the general structures in the flow, and both gave the same depth-averaged velocity.
However, as shown in the contour plots on the left-hand side of the figure, the broadband method (top left) provides a greater level of detail than the narrowband method (bottom left).
Another reason broadband ADCPs obtain better detail is that they have more flexibility to separate sequential pulses. Greater pulse separation reduces measurement uncertainty, provided that they still see the same scatterers. This is also illustrated in Figure 12, where on the right-hand side we show the current profiles for 30 individual broadband pulses (top right) and 30 individual narrowband pulses (bottom right). It’s clear to see how the variability between individual pulses is smaller for broadband than narrowband. So, even though both methods provide the same depth-averaged velocity, broadband provides a greater level of detail.
While greater pulse separation allows broadband to provide us more detailed data, waiting too long can cause signal processing problems too (e.g. it affects the range of velocities the instrument can measure), so broadband ADCPs demand more care than narrowband ADCPs when setting them up for measurements.