Transcoding from SLIN to Ulaw and back, why?

FreePBX
1

I don’t understand why my server is transcoding from ulaw to slin.

I am making a call from PJSIP endpoint 5314 using ulaw out on a trunk that is also using ulaw.
There shouldn’t be any transcoding, but why is it?

Channel 1:

   Name: PJSIP/5314-00000051
   Type: PJSIP
   UniqueID: 1557531408.9768
   LinkedID: 1557531408.9768
  Caller ID: 5243888444
 Caller ID Name: (N/A)
Connected Line ID: 2124354243
Connected Line ID Name: (N/A)
Eff. Connected Line ID: 2124354243
Eff. Connected Line ID Name: (N/A)
DNID Digits: (N/A)
   Language: en
  State: Up (6)
  NativeFormats: (ulaw)
WriteFormat: slin
 ReadFormat: slin
 WriteTranscode: Yes (slin@8000)->(ulaw@8000)
  ReadTranscode: Yes (ulaw@8000)->(slin@8000)

Channel:2

 Name: SIP/Outbound_SBC-00000309
           Type: SIP
       UniqueID: 1557531408.9770
       LinkedID: 1557531408.9768
      Caller ID: 94342000
 Caller ID Name: (N/A)
Connected Line ID: 2124354243
Connected Line ID Name: (N/A)
Eff. Connected Line ID: 2124354243
Eff. Connected Line ID Name: (N/A)
    DNID Digits: (N/A)
       Language: en
          State: Up (6)
  NativeFormats: (ulaw)
    WriteFormat: slin
     ReadFormat: slin
 WriteTranscode: Yes (slin@8000)->(ulaw@8000)
  ReadTranscode: Yes (ulaw@8000)->(slin@8000)

Asterisk 13.18.3

2

Take a look at parameter transcode_via_sln on asterisk.conf

3

Those are the contents of asterisk.conf

[directories]
astetcdir => /etc/asterisk
astmoddir => /usr/lib64/asterisk/modules
astvarlibdir => /var/lib/asterisk
astagidir => /var/lib/asterisk/agi-bin
astspooldir => /var/spool/asterisk
astrundir => /var/run/asterisk
astlogdir => /var/log/asterisk

[options]
transmit_silence_during_record = yes
languageprefix=yes
execincludes=yes
4

By default, Asterisk uses SLIN as internal codec so everything is transcoded to SLIN. Supposedly, you could disable that behaviour with parameter transcode_via_sln=no

5

If both channels use the same codec, what’s the benefit of transcoding to slin vs passthrough?

6

The cost of transcoding between slin and any other uncompressed codec is minimal, don’t sweat it.

2 Likes
7

I guess Asterisk prefers to work internally with SLIN, but that is just my thought.

8

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