Implementing audio waveform view and audio timeline view in iOS?

Question

I am working on an app that will allow users to record from the mic, and I am using audio units for the purpose. I have the audio backend figured out and working, and I am starting to work on the views/controls etc. There are two things I am yet to implement:

1) I will be using OpenGL ES to draw waveform of the audio input, there seems to be no easier way to do it for real-time drawing. I will be drawing inside a GLKView. After something is recorded, the user should be able to scroll back and forth and see the waveform without glitches. I know it's doable, but having a hard time understanding how that can be implemented. Suppose, the user is scrolling, would I need to re-read the recorded audio every time and re-draw everything? I obviously don't want to store the whole recording in memory, and reading from disk is slow.

2) For the scrolling etc., the user should see a timeline, and if I have an idea of the 1 question, I don't know how to implement the timeline.

All the functionality I'm describing is do-able since it can be seen in the Voice Memos app. Any help is always appreciated.

Answer 1

I have done just this. The way I did it was to create a data structure that holds different "zoom levels" data for the audio. Unless you are displaying the audio at a resolution that will display 1 sample per 1 pixel, you don't need every sample to be read from disk, so what you do is downsample your samples to a much smaller array that can be stored in memory ahead of time. A naive example is if your waveform were to display audio at a ratio of 64 samples per pixel. Lets say you have an array of 65536 stereo samples, you would average each L/R pair of samples into a positive mono value, then average 64 of the positive mono values into one float. Then your array of 65536 audio samples can be visualized with an array of 512 "visual samples". My real world implementation became much more complicated than this as I have ways to display all zoom levels and live resampling and such, but this is the basic idea. It's essentially a Mip map for audio.