## 2 Experimental Procedure
### Description of the CAPT Mobile Application  
To carry out all the experiments, we built a mobile app, Japanol, starting from a previous prototype app designed for self-directed training of English as an L2 [8]. Figure 2 shows the regular sequence of steps to complete a lesson in Japanol. After user authentication in (step 1), seven lessons are presented at the main menu of the application (step 2). Each lesson includes a pair of Spanish sound contrasts and users achieve a particular score, expressed as a percentage. Lessons are divided into five main training modes, Theory, Exposure, Discrimination, Pronunciation, and Mixed Modes (step 3), in which each one proposes several task types with a fixed number of mandatory task tokens. The final lesson score is the mean score of the last three modes. Users are guided by the system in order to complete all training modes of a lesson. When reaching a score below 60% in Discrimination, Pronunciation, or Mixed Modes, users are recommended to return to Exposure mode as a feedback resource and then return to the failed mode. Moreover, the next lesson is enabled when users reach a minimum score of 60%. The first training mode is **Theory** (step 4). A brief and simple video describing the target contrast of the lesson is presented to the user as the first contact with feedback. At the end of the video, the next mode becomes available, but users may choose to review the material as many times as they want. **Exposure** (step 5) is the second mode. Users strengthen the lesson contrast experience previously introduced in Theory mode, in order to support their assimilation. Three minimal pairs are displayed to the user. In each one of them, both words are synthetically produced by _Google_ TTS five times (highlighting the current word), alternately and slowly. After this, users must record themselves at least one time per word and listen to their own and the system's sound. Words are represented with their orthographic and phonemic forms. A replay button allows users to listen to the specified word again. Synthetic output is produced by _Google_'s offline Text-To-Speech tool for Android. After all previous required events per minimal pair (listen-record-compare), participants are allowed to remain in this mode for as long as they wish, listening, recording, and comparing at will, before returning to the Modes menu. Step 6 refers to **Discrimination** mode, in which ten minimal pairs are presented to the user consecutively. In each one of them, one of the words is synthetically produced, randomly. The challenge of this mode consists of identifying which word is produced. As feedback elements, words have their orthographic and phonetic transcription representations. Users can also request to listen to the target word again with a replay button. Speed varies alternately between slow and normal speed rates. Finally, the system changes the word color to green (success)  
Figure 2: Standard flow to complete a lesson in Japanol. or red (failure) with a chime sound. **Pronunciation** is the fourth mode (step 7), whose aim is to produce, as well as possible, both words, separately, of the five minimal pairs presented with their phonetic transcription. gASR determines automatically and in real time acceptable or non-acceptable inputs. In each production attempt, the tool displays a text message with the recognized speech, plays a right/wrong sound, and changes the word's color to green or red. The maximum number of attempts per word is five in order not to discourage users. However, after three consecutive failures, the system offers to the user the possibility of requesting a word synthesis as an explicit feedback as many times as they want with a replay button. **Mixed mode** is the last mode of each lesson (step 8). Nine production and perception tasks alternate at random in order to further consolidate the obtained skills and knowledge.