Static/white noise heard during music playback


I’m not a developer but more of a sound guy. I’m currently having problems during playback of music during a call. During playback, some parts of the music suddenly generates white noise/static. Kinda lost at how to eliminate the noise heard.

I use Adobe Audition to convert .mp3 to .alaw
Source file is usually at 44,100Hz, 24bit, normalized to -1.0dB
Format is libsndfile
Resample to 8,000Hz mono, 16bit
Format settings is Waveform Audio, a-Law 8-bit PCM
Then i just change ext name from .wav to .alaw

I cant seem to figure out why there is static/whitenoise since not all music i used has this problem.

Is there any other settings/optimal settings -e.g. compression, hard limiter, etc- i should be applying to the source file before converting?


A-law is 8 bit not 16 bit. A-law files are pure media samples, not RIFF (.wav) format files. Normally one would use 16 bit signed linear RIFF with .wav extensions, as the cost of converting them to A-law, on the fly, is low, and they are easy for Windows to handle.

Hi David,

Audition does the conversion to 8bit. I just prep the source file to be at 16bit so that it’ll be converting from a higher bit sample which is supposedly better from what i read in other audio forums. So the final properties of the converted file is 8kHz, 8bit, 64kbps. I use waveform as the format since it’s one of the few format that supports a-law/u-law encoding. It saves it as .wav but i just rename it to .alaw when i need to load it to our system.

There are other formats avaialble as shown in the screenshot attached. I tried Raw but the output is still the same.

My concern is basically, after conversion, if i playback the file in the PC, it sounds good and ok but once i upload it to asterisk and play it via making a call from a mobile phone, it still plays ok but some parts of the song is heard with some whitenoise/static noise. It’s not the case with all the music I use. only with some that the noise is present. I couldn’t figure out why that noise is being generated.


I would expect raw, but I’m not familiar with the terminology used by that program. Renaming a .wav to .alaw will produce some noise when Asterisk encounters the meta data in the file.

To avoid raw versus RIFF being an issue, I would convert to 16 bit, mono, 8kHz signed linear .wav, then either use the Asterisk convert tool, or let it convert on the fly.

As you specifically mention mobile phones, mobile phone codecs are designed only for speech, they use a voice tract model. Music will not play accurately, although I’m not sure I would expect bursts of white noise, although it is possible that coder is detecting a sibilant, which needs to be encoded outside outside the basic RELP system, as a white noise burst.

You’ve verified that the noise is always heard during the same parts of the file playback? So it’s not network or system conditions that might be causing it?

Yes. The noise is found at the same part/s of the song but it may seem that network might have something to do with it? david551’s last post got me thinking again and made some other tests with one of our developers. I don’t think the converted alaw file is the problem. I understand that there is another encoding that takes place from telco system to cellular site in which it converts to a much lower bit rate. Since i’m using cellular network to playback the song, is it possible that the problem is not from the .alaw file but from how the cellular network encodes the .alaw to gsm?

again, thanks for replying in this thread.