How to listen with the screen locked in Sandbook

1. Open the book at the chapter you want. Tap the ⋯ menu in the reader and choose Prepare chapter for lock screen. In a PDF the item is Prepare page for lock screen, and it prepares one page.
2. Keep Sandbook open. A card titled Preparing audio for lock screen shows how far it has got, a progress bar, the time left and “Audio so far”. The screen stays on by itself while it works, and Cancel stops it.
3. Lock the phone. When the menu reads Chapter ready for lock screen, tap it. The whole chapter is prepared, and playback picks up from where you are in it.

A prepared chapter plays like a podcast. On the lock screen and in Control Center you get play and pause, 15-second skips back and forward, and a scrubber, with the chapter, the book, the author and the cover in Now Playing. Headphone buttons work too.

What you do not get is the read-along: words are not highlighted during prepared playback. When you unlock and want to follow the text again, press play in the reader and it carries on in real time.

Sandbook has no speech server. The voice model, Kokoro-82M, runs on your iPhone's GPU and holds GPU memory while it speaks, and iOS does not let an app keep that running behind a locked screen. That is the price of text to speech that works offline; preparing a chapter ahead is the way around it.

• One chapter, or one PDF page, at a time, and up to an hour of audio each.
• One preparation runs at a time, and the next chapter is not prepared automatically.
• Changing the voice or the speed means preparing the chapter again, because the audio was made with the old ones.
• An app update can require preparing chapters again.
• Prepared audio has no word highlighting, and the next chapter does not start by itself.

If you need whole books to play on in your pocket for hours, a reader built around cloud voices or the system voices may suit you better; the comparison of reading apps sets out which ones state that they do.

Prepared chapters are kept between launches and replay instantly. In Settings, the Audio cache limit can be set to 1, 2, 3 or 4 GB (2 GB is the default), with the space in use shown next to it and a Clear Audio Cache button. When the cache is full, the chapters you played least recently go first, never the one that is playing. Opening another book does not delete anything.

Preparing uses the GPU with the screen on, so it draws power like any heavy app; prepare a long chapter while the phone is charging. Playing it back afterwards is ordinary audio playback.