Wai Wu wrote:
1) Is each channel use their own thread if a codec is used?
Each *inbound* channel uses it's own thread. The inbound channel then executes
dialplan applications, which may then create an outbound channel. In that case,
you then have these two channels in the same thread. This has nothing to do
with what codecs are in use.
2) Which part of the asterisk code actually makes the call to the
necessary codec? I notice in the applications. They save and set the
frame format, then read from the channel. So I trace the read function,
but all it does is reading from a file descriptor. So I need some help here.
Codec translation happens can happen in various places, really. It is anywhere
ast_translate() is called. In the most general case, the translation occurs in
ast_read() and ast_write(). Channels have a read-side translation path and a
write-side translation path.
For example, you may have two channels with one using ulaw and one g729. This
is how the frames would be translated when the channels are bridged together.
You can see that there is a translation path that must be followed when reading
a frame from the channel and passing it to the bridge (this happens in
ast_read), as well as when receiving a frame from the bridge and writing it back
to the channel (ast_write).
Channel 1 Channel 2
g729 ----> slinear ||--------|| slinear ----> ulaw
|| Bridge ||
g729 <---- slinear ||--------|| slinear <---- ulaw
When the channel is executing an application, the situation is similar. Perhaps
in your application, you explicitly set the read and write formats to slinear.
In that case, the translation looks the same. It still happens in ast_read and
ast_write.
Channel 1
g729 ----> slinear ||----------------||
|| My Application ||
g729 <---- slinear ||----------------||
--
Russell Bryant
Software Engineer
Digium, Inc.
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