Video Book Resolution
From time to time we are asked why our books aren't 4k resolution. To that we say: as far as you can tell they are!
A Primer on Resolution
4k commonly refers to a video which is 3840 pixels wide. 4k looks very different depending on the size of the screen. On a cell phone that is only 5" wide, 4k would represent almost a thousand pixels in a single inch of space. On a 75" TV, that same 4k would represent only about 60 pixels per inch. If you got as close to the TV as you do to your phone, it would look like it was much lower resolution. Fortunately we tend to sit further away from bigger screens.
Our video books have relatively small screens, at only 5". This 5" size was carefully chosen after experimenting with many book sizes, looking for the ideal trade-off between cost and experience. For these small screens we also wanted a high-quality HD-looking video experience, but it didn't require us to use the same resolutions necessary for HD on a bigger screen. In fact, we realized that 854 pixels of width translated to about 200 pixels per inch on our small screens.
That 200 pixels per inch number, is about the same as what you get when you play 4k video on a 21.5" computer monitor. Given how spectacular 4k looks on such a small screen, it was very clear that the 854x480 resolution was more than sufficient for our needs. And if you, enjoy the pixel density of a 4k monitor, you will also love how your videos look on our books.
By choosing a suitable resolution, we also are able to optimize the limited storage we have on our books. 4k video consumes almost eight times more space than the video we use, but in our tests looks identical on our small screens.
Choosing File Sizes
We recommend that users store and keep the highest possible resolution of video, and when displaying it on a large screen they should absolutely use the highest resolution they can. But keep it there! There's no advantage to trying to fit those files onto a 5" video screen.
We recommend video be encoded as follows:
|Bitrate||VBR with CRF between 24 and 34|
|Audio Channels||Mono (Stereo is Acceptable)|
|Audio Bitrate||32 to 160 kbps|
|Audio Gain||+4 dB|