Checking the quality of the sound device

In some cases, some of the audio problems may come from the sound device itself, causing problems such as:

• Audio drop-outs or “stutters”,

• Audio is breaking up

It may not be the sound device itself that is causing the problem, but could be the operating system driver for the device. For example, on Linux, the ALSA driver tends to have a very good quality while using OSS driver for the same device would give a less satisfactory result.

It is also observed from the mailing list discussions that many embedded Linux device with on-board sound adapter give a bad audio quality with OSS driver, although normally it would play a WAV file fine. We conclude that these sound adapter (or the driver) is not really designed for streaming, bidirectional communication like audio call, but rather for trivial tasks like playing a file to the speaker.

Sound Device Problems

Some problems with sound device are as follows.

Jitter

Common problem with most sound device is the jitter. Where for example PJMEDIA expects audio frames to be delivered at exactly 20ms interval, the sound device (or driver) may deliver it at 10ms, 10ms, 30ms, 30ms, etc. Normally the total number of frames delivered will match the clock rate (i.e. there’s no lost frames), but it’s just that these frames are not delivered in timely manner.

Audio jitter in the capture direction will cause outgoing RTP packet to be delivered in uneven time. This shouldn’t cause too much problem because remote should be able to accomodate the jitter.

Audio jitter in the playback direction should be okay too. Much worse problem is audio burst (see below).

Burst

A worsening problem with the jitter is bursting, where the sound device (or driver) delivers the audio frames in burst and then followed by silent period, and burst again. If the sound device is open in full-duplex mode, this would normally cause the recorder callback to be called in burst of several calls, then followed by burst call to the playback callback, and back to burst call to the recorder callback, and so on.

PJMEDIA should be capable of handling audio burst to some level, as explained in Sound device timing problem in Understanding Audio Media Flow.

Underflows Overflows

Another problem with audio application is underflows and overflows, where application is not processing the audio frames quickly enough. When underflow or overflow occurs in the playback direction, you would hear a click sound in the speaker.

The PortAudio audio abstraction in PJMEDIA prints the number of underflow/overflow when the sound device is closed. With pjsua, you need to set the log level to 5 (--app-log-level 5), and when the application exits the underflow/overflow statistic will be printed to console/log.

Clock drifting

A not so common problem with some sound device is clock drifting, where the sound device is not delivering audio samples at the exact clock rate. For example, when the sound device is opened at 8KHz, the sound device may deliver a little less or more than 8000 samples per second.

Testing the Sound Device

Use pjsystest to test the performance of the sound device. See Testing and optimizing audio device with pjsystest for more information.

Specifically, run pjsystest Device Test (menu 01) to test the audio device’s burst and clock drifts problem. Below is a sample output of Device Test:

Audio Device Test
Here are the audio statistics:
Rec : interval (min/max/avg/dev)=
        0/31/20/11 (ms)
    max burst=2
Play: interval (min/max/avg/dev)=
        10/26/20/1 (ms)
    burst=2
There could be 1 problem(s) with the sound device:
1: Clock drifts detected. Capture is 16 samples/sec faster than the playback device

From the results above, the burst is good, and there is a little clock drifts, both should be able to be handled by PJMEDIA.