Full Spectrum vs. Zero Cross
Why It May Matter to You
Although call parameters from the time-amplitude domain do increase species classification performance, the primary benefit of full-spectrum analysis comes from increasing the robustness, accuracy, and confidence of certain bat call frequency metrics familiar to users of zero-crossing methodology, yet these metrics are rendered more accurately in full-spectrum analyses due to the less-sensitive nature inherent in frequency-divided recordings. In summary: full-spectrum provides higher quality results.
The full-spectrum approach enables and supports automated processing and classification of bat echolocation sequences. Moreover, the enhanced information content of full-spectrum data compensates for and decreases the need for human attention and artistry in the interpretation of less information-rich data. Some examples of conditions that can confound accurate call analysis using a frequency-divided approach are illustrated below.
Full-spectrum recording viewed in SonoBat
Zero Cross/Frequency-divided rendering of the same call
Full-spectrum (with SonoBat call trending) and divide by 8 zero-crossing interpretations of the same Myotis californicus call signal in the presence of noise. The ability to view a single call pulse in a high resolution standard view in both full spectrum and zero-cross is a feature in SonoBat 3.1.3 and later.
For a more complete overview of the benefits of full-spectrum analysis of bat echolocation calls, and the differences in data interpretation between full-spectrum and zero-crossing, view this presentation.
Full-spectrum processing rendered a complete time-frequency trend and confident determination of call parameter data from this Indiana bat call despite insect noise. Because zero-crossing analysis can only detect the strongest frequency component at any time interval, zero-crossing analysis of the same signal could not render a usable time-frequency trend because of the stronger low frequency content of the insect signal.
The overpowering concurrent signal amplitude that prevents full zero-crossing recognition of bat time-frequency trends more seriously affects bats that vocalize more quietly such as this Corynorhinus spp., shown beside the same signal rendered by zero-crossing. The multiple frequency content available in full-spectrum data enables tracking the time-frequency trends of calls to completion even when the call amplitude falls below the maximum amplitude of other concurrent signals.
An example of a full-spectrum enabled time-frequency trend rendered through clutter echoes. In this case, the ending downward trend in frequency readily discriminates this Tadarida brasiliensis from a Lasiurus cinereus, whose calls tend to turn upward at the end. The zero-crossing analysis followed the stronger echo signals at the end of the call, suggesting an upward trend, and potentially confounding an accurate species determination.
Echoes from clutter will often obscure ending details of calls. In this example, the time-frequency trend processed from full-spectrum data revealed the downward ending frequency trend that assists in recognition of this as a Myotis spp. call.
The time-frequency trend as rendered from zero-crossing the same signal rises up at the end from the effects of noise and clutter echoes. The resulting trend has more features in common with an Eptesicus fuscus call than a Myotis spp. call.
Some bats like this Leptonycteris shift power among their harmonics, and zero-crossing trending follows the strongest power of these shifts generating interrupted time-frequency trends. Experienced zero-crossing users can recognize these shifts and make assumptions about call continuity, but they complicate automated analysis by zero-crossing. Full-spectrum processing readily generates an uninterrupted trend with such data.
In this Corynorhinus townsendii call, the power shifts to the second harmonic late in the call, and the trend points from zero-crossing processing jump up to the higher powered signal. With access to multiple frequency content, full-spectrum processing can generate uninterrupted time-frequency trends from which to determine call parameters with greater confidence and detail for accurate species identification.
With some species, the call fragments acquired from out of range bats, or noise-burdened signals processed by zero-crossing, can leave fragments that mimic the fully-formed calls of other species. In this example, only the higher powered core of this Myotis thysanodes call rendered trend points from zero-crossing analysis and these left a signal that mimics a Corynorhinus townsendii call. The higher quality time-frequency call trends supported by full-spectrum data minimize this source of error.
The amplitude and multiple frequency content of full-spectrum data enables assessment of signal quality. For example, one such measure, the signal to noise ratio (SNR), measures the relative strength of a signal of interest (the call) to the strength of the background signal level.
Calls with low SNRs may generate unreliable parameters and are best excluded from contributing to sequence-level species classifications. This measure would reject the call in the above example. Such metrics provide essential quality control for automated call and sequence classification.
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