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Diffstat (limited to 'doc/codec-64bit.txt')
| -rw-r--r-- | doc/codec-64bit.txt | 47 |
1 files changed, 0 insertions, 47 deletions
diff --git a/doc/codec-64bit.txt b/doc/codec-64bit.txt deleted file mode 100644 index 8f2ceec59..000000000 --- a/doc/codec-64bit.txt +++ /dev/null @@ -1,47 +0,0 @@ -CODEC BIT EXPANSION -------------------- - -The code base up to and including Asterisk 1.6.2 has a basic limit of 32 codecs -recognizable, due to the use of a 32-bit integer for the codec bitmask. We -have expanded the number of available codecs from 32 to 64, through the use of -an immutable type, called format_t. This should make future expansion to even -more bits more easily done. - -The design of this expansion has made some changes to the architecture of codecs -in order to accomplish this task. I will attempt to enumerate them here. - -The initial set of 32-bits were allocated as the first 16 to audio codecs, the -next 8 to video codecs, and the remaining to text codecs (which are used for -fax capabilities). Initially, there is an assumption in the code that all -audio codecs are contiguous, followed by a contiguous set of video codecs. -After the conversion, this assumption will no longer be true. The codec bits -for the existing codecs will continue to be allocated as-is, and the additional -codec bits should be allocated on an as-needed basis, with audio codecs -occupying slots 32-47 and video codecs occupying slots 48-62 (with a 0-based -offset). Slot 63 is reserved and should not be allocated; it is used in code -as an end condition for iterating through the entire set of codecs. - -The frame structure has been altered. Initially, the subclass held an integer -whose meaning was specified by the frametype. If the frametype was -AST_FRAME_VOICE, the subclass specified the audio codec. If the frametype was -AST_FRAME_VIDEO, the subclass specified the video codec, with the 0-bit set to -specify a key frame. This was done with a union on the subclass, where the -"integer" union member specifies the traditional 32-bit subclass and the "codec" -union member specifies the new 64-bit codec bitmask. This additionally -guarantees that code compiled under the old scheme will need to be altered to -compile under the new scheme, which helps avoid incorrect assumptions about the -state of code which might otherwise compile without errors. - -The IAX2 code initially used a 32-bit integer IE to specify both the codec as -well as the preferred format. An additional IE has been added, which specifies -a single byte version number as the initial part of the data. This version -number is initially specified as 00 and requires 8 bytes to follow, specifying -the 64-bit codec bitmask, in network-byte order. This schema should allow -further codec expansion in the future without allocation of any additional IEs. - -Little changes are required to support further codec expansion in the future, -though the majority of the work has already been accomplished. Specifically, -the bitwise operations that are immutable operations in the gcc compiler will -need to be altered to handle larger bitmasks. Additionally, the constants that -define specific codecs will need to be changed from integers to structures. - |