G711 raw file law-a law-µ

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newuserfr
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Joined: Tue Nov 17, 2009 11:41 am
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G711 raw file law-a law-µ

Post by newuserfr » Fri Nov 27, 2009 10:38 am

Hello,

I would check the sound level of a VOIP communication. For this I catch a VOIP communication using wireshark. And I extract voice packets (RTP Sinus 1000 Hz). I get a. Raw. The communication uses the G711 codec A-law.
Then I import this file with audacity.
Encoding: A-Law
Sampling frequency: 8000 Hz
Byte order: little endian mode / big endian (no difference)
Channels 1 Channel Mono
Start Offset 0
...

Well then I get the desired signal (1000 Hz).

Following this test I have some questions:
1) What is the amplitude displayed on the vertical bar on the left?
2) What is the relationship between the amplitude displayed on audacity and the level indicated in the standard G711 (ex max 3.14dbm0 for the law-a).
3) Is this when we import a. Raw effects are applied to it (compression / ...)?
4) In which case the byte order is used (endian)?

Thank you in advance

My configuration Windows XP version 1.3.9

steve
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Posts: 80677
Joined: Sat Dec 01, 2007 11:43 am
Operating System: Linux *buntu

Re: G711 raw file law-a law-µ

Post by steve » Sat Nov 28, 2009 10:45 am

I don't know much about voip, so just your first question...

Audio samples have values in the range of +/- 1
A wave that touches both extremes (+1 to -1) is "full scale" 0dB
A wave that goes +0.5 to -0.5 is -6dB

If you click on the track name there is a drop down menu where you can select a dB scale - note that in this view the waveform will look weird because the dB values are equally spaced. You can also grab the lower edge of a track and drag it down to increase the vertical size of the track.
9/10 questions are answered in the FREQUENTLY ASKED QUESTIONS (FAQ)

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