In this document
This article describes some tips and tricks for debugging Android audio.
The "tee sink" is an AudioFlinger debugging feature, available in custom builds only, for retaining a short fragment of recent audio for later analysis. This permits comparison between what was actually played or recorded vs. what was expected.
For privacy the tee sink is disabled by default, at both compile-time and run-time. To use the tee sink, you will need to enable it by re-compiling, and also by setting a property. Be sure to disable this feature after you are done debugging; the tee sink should not be left enabled in production builds.
The instructions in the remainder of this section are for Android 4.4, and may require changes for other versions.
- push or sync the new
libaudioflinger.soto the device's
adb shell getprop | grep ro.debuggable
Confirm that the output is:
ls -ld /data/misc/media
Confirm that the output is:
drwx------ media media ... media
If the directory does not exist, create it as follows:
mkdir /data/misc/media chown media:media /data/misc/media
echo af.tee=# > /data/local.prop
af.teevalue is a number described below
chmod 644 /data/local.prop
The value of
af.tee is a number between 0 and 7, expressing
the sum of several bits, one per feature.
See the code at
for an explanation of each bit, but briefly:
- 1 = input
- 2 = FastMixer output
- 4 = per-track AudioRecord and AudioTrack
There is no bit for deep buffer or normal mixer yet, but you can get similar results using "4."
Test and acquire data
- Run your audio test
adb shell dumpsys media.audio_flinger
- Look for a line in dumpsys output such as this:
tee copied to /data/misc/media/20131010101147_2.wav
This is a PCM .wav file
/data/misc/media/*.wavfiles of interest; note that track-specific dump filenames do not appear in the dumpsys output, but are still saved to
/data/misc/mediaupon track closure
- Review the dump files for privacy concerns before sharing with others
Try these ideas for more useful results:
- Disable touch sounds and key clicks
- Maximize all volumes
- Disable apps that make sound or record from microphone, if they are not of interest to your test
- Track-specific dumps are only saved when the track is closed; you may need to force close an app in order to dump its track-specific data
- Do the
dumpsysimmediately after test; there is a limited amount of recording space available
- To make sure you don't lose your dump files, upload them to your host periodically. Only a limited number of dump files are preserved; older dumps are removed after that limit is reached.
As noted above, the tee sink feature should not be left enabled. Restore your build and device as follows:
- Revert the source code changes to
- Push or sync the restored
libaudioflinger.soto the device's
The standard Java language logging API in Android SDK is android.util.Log.
The corresponding C language API in Android NDK is
Within the native portion of Android framework, we
prefer macros named
ALOGV, etc. They are declared in
<utils/Log.h>, and for the purposes of this article
we'll collectively refer to them as
All of these APIs are easy-to-use and well-understood, so they are pervasive
throughout the Android platform. In particular the
process, which includes the AudioFlinger sound server, uses
Nevertheless, there are some limitations to
ALOGx and friends:
They are suspectible to "log spam": the log buffer is a shared resource
so it can easily overflow due to unrelated log entries, resulting in
missed information. The
ALOGVvariant is disabled at compile-time by default. But of course even it can result in log spam if it is enabled.
The underlying kernel system calls could block, possibly resulting in
priority inversion and consequently measurement disturbances and
inaccuracies. This is of
special concern to time-critical threads such as
- If a particular log is disabled to reduce log spam, then any information that would have been captured by that log is lost. It is not possible to enable a specific log retroactively, after it becomes clear that the log would have been interesting.
NBLOG, media.log, and MediaLogService
NBLOG APIs and associated
service together form a newer logging system for media, and are specifically
designed to address the issues above. We will loosely use the term
"media.log" to refer to all three, but strictly speaking
NBLOG is the
C++ logging API,
media.log is a Linux process name, and
is an Android binder service for examining the logs.
media.log "timeline" is a series
of log entries whose relative ordering is preserved.
By convention, each thread should use it's own timeline.
The benefits of the
media.log system include:
- doesn't spam the main log unless and until it is needed
- can be examined even when
mediaservercrashes or hangs
- is non-blocking per timeline
- less disturbance to performance (of course no form of logging is completely non-intrusive)
The diagram below shows the relationship of the
init process, before
media.log is introduced:
initforks and execs
initdetects the death of
mediaserver, and re-forks as necessary
ALOGxlogging is not shown
The diagram below shows the new relationship of the components,
media.log is added to the architecture:
NBLOGAPI to construct log entries and append them to a circular buffer in shared memory.
MediaLogServicecan dump the contents of the circular buffer at any time.
The circular buffer is designed in such a way that any corruption of the
shared memory will not crash
MediaLogService, and it will still be able to dump as much of the buffer that is not affected by the corruption.
- The circular buffer is non-blocking and lock-free for both writing new entries and reading existing entries.
- No kernel system calls are required to write to or read from the circular buffer (other than optional timestamps).
Where to use
As of Android 4.4, there are only a few log points in AudioFlinger
that use the
media.log system. Though the new APIs are not as
easy to use as
ALOGx, they are not extremely difficult either.
We encourage you to learn the new logging system for those
occasions when it is indispensable.
In particular, it is recommended for AudioFlinger threads that must
run frequently, periodically, and without blocking such as the
How to use
First, you need to add logs to your code.
FastMixer thread, use code such as this:
logWriter->log("string"); logWriter->logf("format", parameters); logWriter->logTimestamp();
NBLog timeline is used only by the
there is no need for mutual exclusion.
In other AudioFlinger threads, use
mNBLogWriter->log("string"); mNBLogWriter->logf("format", parameters); mNBLogWriter->logTimestamp();
For threads other than
NBLog timeline can be used by both the thread itself, and
by binder operations.
NBLog::Writer does not provide any
implicit mutual exclusion per timeline, so be sure that all logs occur
within a context where the thread's mutex
mLock is held.
After you have added the logs, re-build AudioFlinger.Caution:
NBLog::Writer timeline is required per thread,
to ensure thread safety, since timelines omit mutexes by design. If you
want more than one thread to use the same timeline, you can protect with an
existing mutex (as described above for
mLock). Or you can
NBLog::LockedWriter wrapper instead of
However, this negates a prime benefit of this API: its non-blocking
NBLog API is at
media.log is disabled by default. It is active only when property
1. You can enable it by:
adb root adb shell echo ro.test_harness=1 > /data/local.prop chmod 644 /data/local.prop reboot
The connection is lost during reboot, so:
adb shellThe command
ps mediawill now show two processes:
Note the process ID of
mediaserver for later.
Displaying the timelines
You can manually request a log dump at any time. This command shows logs from all the active and recent timelines, and then clears them:
Note that by design timelines are independent, and there is no facility for merging timelines.
Recovering logs after mediaserver death
Now try killing
kill -9 #, where # is
the process ID you noted earlier. You should see a dump from
in the main
logcat, showing all the logs leading up to the crash.