[Asterisk-video] if2 bit stuffing in app_h324m

[Francesco Emmi]

Hi Klaus,

An AMR file can be formatted in 2 different ways.

The first is according to the MIME file storage format used e.g. by the 
MMS service. The specification of this format can be found in RFC 3267 
"Real-Time Transport Protocol(RTP) Payload Format and File Storage 
Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate 
Wideband (AMR-WB) Audio Codecs". File starts with magic number #!AMR and 
a typical frame in mode 7 is like this:

3c 08 5a 5f 8c 44 50 44 4c cd 45 2f 25 75 43 13 80 00 01 19 e0 8a 22 10 
00 00 de 51 3e 87 3d 80

First byte is used to indicate the amr mode, and 244 core frame bits are 
fitted into 31 octets. Last 4 bits are zero padding bits. Total amount 
of bytes is 32

In a mp4 container, amr is saved in this way.

Second formatting way is IF2 (ETSI TS 26.101) and this is used by h324m 
terminals.
This is the same frame as before in IF2:

07 a1 a5 1f 23 a2 20 22 33 2b 4a 4f ea 2a 8c 1c 00 00 88 79 10 45 84 00 
  00 b0 a7 c8 17 ce 1b

In this frame only 4 bits are dedicated for indicating amr mode.
Bit stuffing is referred to the core frame bit size. So, in mode 7, core 
frame size is 244. When formatting in If2 you have 244 core bits + 4 
bits to indicate amr mode for a total of 248 bits which fit exactly in 
31 octets (bit stuffing then is 0).
On the contrary when formatting in MIME format you have 1 octet for amr 
mode and 244 bits to be fitted in 31 octets for a total of 32 octets and 
4 bits as zero padding



Klaus Darilion wrote:
> Hi Sergio!
> 
> I do not understand what for is the bit stuffing in 
> create_h324m_frame(). I thought if2 is used in h324m and also in RTP and 
> .mp4 files. Thus, the ast_frames are also if2 encoded - aren't they?
> 
IF2 is used by h324m terminals, e.g. a mobile phone and h324mlib, but 
not in RTP and mp4 files that use MIME format. So when app_h324m 
receives an AMR frame from an mp4 file (for example coming from 
mp4_read) it has to convert it from MIME format to IF2 format.

So, as the 2 formats may have different frame size (for a given mode), 
using array bitstuffing in create_h324m_frame() is simple a way to help 
correctly convert from a format to the other.

Francesco

ps. I hope to have answered to your doubts. If not, please let me know. 
I'll try to be clearer...



> regards
> klaus
> 
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