The LY Corporation tech blog revealed how LINE Premium Call was developed. Premium Call is a feature released through LINE Labs that lets you record audio or video of a chosen segment during a 1:1 voice or video call and save it to Keep Memo after the call. The other party is notified that recording is in progress, and the post mentions availability in Japan, Taiwan, and Thailand.
The first challenge was keeping the recording continuous when a voice call switches to a video call. The team prepared audio and video tracks in a single file from the start, and during voice calls inserted an image generated from profile information as the video frame. When the call switches to video, only this screen is replaced with the actual video, so the same file is maintained.
Another problem was that your video and the other party's video arrive separately, and their resolution and aspect ratio can change. The team reflects whichever side's video is ready first and keeps the profile image for the other, and fixes the final recording frame size and draws everything to fit inside it. It uses the video the call engine hands over instead of screen capture, so elements like notifications don't get mixed in.
In voice calls, speech-to-text (STT) is available for Japanese only, and processing is handled by an on-device model rather than a server. To reduce the load on the device, it receives a high-quality stream for call playback and a separate optimized stream for recording and STT, and recognizes only the segments where speech occurs. Silence between utterances is left in the recording file as is.
After a call ends, it first stops receiving new data, finishes the remaining processing, and then closes the file. STT is finalized separately from the recording file with an upper limit on the waiting time, and the screen shows the two states separately.
Source: LY Corporation Tech Blog
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