## 3 Vehicle-Embedded Speech Recognition Front-End for Multi-UAV Control
### Voice Trigger for Multiple UAVs Control
#### 3.3.2 Multi-Channel Voice Trigger for Multi-UAV Control  
The conventional voice trigger method is very simple and suitable for operation on user devices; however, it has several limitations in multi-UAV environments. First, the conventional method can only handle a target device using a single trigger word, thereby providing insufficient conditions for multiple UAVs. Next, this approach is highly dependent on a predefined threshold to determine the trigger word. The dependency on this threshold may induce significant determination errors because it may be vulnerable to the speaking variations of multiple users and interference by background noise. In the distributed speech recognition scheme (Section 2.2), a single user device controls multiple UAVs. Thus, this device should process multiple trigger words to distinguish UAVs. In addition, although general smart devices, e.g., smartphones, tend to be tolerant of trigger recognition errors, such errors may lead to significant damages to UAVs, particularly in military facilities. Therefore, trigger word determination should be processed carefully in UAV environments. The proposed voice trigger approach for multi-UAV control can be characterized as a multi-channel voice trigger in which each UAV has a distinctive name used as a trigger word, and the user device establishes a connection between the user and the target vehicle among multiple UAVs. The trigger word detection module embedded in the user device has two important purposes, i.e., connecting to a target UAV corresponding to the detected trigger word and waking up the speech recognition engine on the UAV system. Figure A4 illustrates the communication flow of voice trigger based multi-UAV control. Once a trigger word is detected, the device attempts to connect to the corresponding UAV. After receiving an acknowledgment message from the target UAV, the user delivers a speech command. Then, the UAV system recognizes the speech and begins to execute the command. After delivering speech commands, the user can switch to a new target UAV by calling its trigger word. There are several trigger word detection methods according to the recognition unit. Some systems recognize trigger words based on continuous speech recognition that handles a sub-word unit, e.g., a phoneme or a tri-phone. Such systems require a set of time-consuming tasks, including speech segmentation, sub-word model computation, beam search pruning, and word matching. In this study, we employ word model-based trigger recognition in which trigger word models are  
Figure 6: Conventional voice trigger method. constructed as a recognition unit and only a task of model computation is required. This approach incurs lower computational costs compared to the sub-word model approach, thereby providing sufficient conditions to be processed by the user device. Figure 7 illustrates the procedures of the proposed multi-channel voice trigger-based front-end and speech recognition for multi-UAV control. The main property of the proposed approach is improving trigger word detection correctness by employing three consecutive processes for filtering non-trigger speech data. Once signals enter the microphone on the user device, the device first detects speech regions using the VAD module. Then, the duration of the detected speech region is compared to predetermined ranges of trigger words using the duration filtering module. If the duration is out of range, the module determines that the given region is pertinent to a non-trigger voice and discards the region. This first filtering process is a soft decision based on a sufficiently large range. Once a speech region passes the duration filtering process, acoustic features are extracted from the speech signals and submitted to the following process for trigger word recognition. This second filtering process involves the main feature of the proposed trigger.