<NME> README.md
<BEF> ## WALT Latency Timer ##

**DISCLAIMER:** This is not an official Google product.

 * [Post about WALT on Android Developers Blog](http://android-developers.blogspot.ca/2016/04/a-new-method-to-measure-touch-and-audio.html)
 * [Instructional videos showing how to use WALT](https://www.youtube.com/playlist?list=PLd6Fi7WgXfcCEJg1FDqNCoQfpWo7W3J5a)
 * [Detailed usage instructions](docs/usage/WALT_usage.md)
 * Mailing list - [walt-discuss](https://groups.google.com/forum/#!forum/walt-discuss)
 * Low-traffic mailing list for major announcements [walt-announce](https://groups.google.com/forum/#!forum/walt-announce)

WALT is designed to measure the latency of physical sensors and outputs on phones and computers. It can currently perform the following measurements:

 * [Tap latency](docs/TapLatency.md) - time from the moment a finger-like probe touches down (or up) on the screen
   until the kernel timestamps an ACTION_DOWN (or ACTION_UP) event. This physical contact with
   the screen is timed using an accelerometer mounted on the probe.
 * [Drag latency](docs/DragLatency.md) (scroll).
 * [Screen draw latency](docs/ScreenLatency.md) - using a photodiode that detects whether the screen is black or white.
 * [Audio output and microphone latencies](docs/AudioLatency.md).
 * MIDI input and output latencies

The WALT app for Android can be
[installed from Google Play](https://play.google.com/store/apps/details?id=org.kamrik.latency.walt)
or downloaded in the [releases section](https://github.com/google/walt/releases); the iOS app must be built from source.
  differing clock frequencies. This means they will go out of sync after
  several minutes. The workaround is to use the app to re-sync the
  clocks. Some, but not all tests in the app will sync the clocks when starting a measurement.
* Python code used to communicate with WALT from Linux and ChromeOS can be found
  [here](https://chromium.googlesource.com/chromiumos/platform/touchbot/+/master/quickstep/)
* Clock synchronization details are described [here](android/WALT/app/src/main/jni/README.md).
* The Android/iOS device and Teensy clocks have a tendency to diverge due to
  differing clock frequencies. This means they will go out of sync after
  several minutes. The workaround is to use the app to re-sync the
  clocks. Some, but not all tests in the app will sync the clocks when starting a measurement.
* Python code used to communicate with WALT from Linux and ChromeOS can be found
  [here](https://chromium.googlesource.com/chromiumos/platform/touchbot/+/master/quickstep/).


<MSG> Update README.md
<DFF> @@ -24,5 +24,6 @@ WALT is designed to measure the latency of physical sensors and outputs on phone
   differing clock frequencies. This means they will go out of sync after
   several minutes. The workaround is to use the app to re-sync the
   clocks. Some, but not all tests in the app will sync the clocks when starting a measurement.
+* The Android app uses some native code. In order to build it run `ndk-build` in `<repo root>/android/WALT/app/src/main` before building in Android Studio.
 * Python code used to communicate with WALT from Linux and ChromeOS can be found
   [here](https://chromium.googlesource.com/chromiumos/platform/touchbot/+/master/quickstep/)

Update README.md