[Asterisk-video] if2 bit stuffing in app_h324m

[Klaus Darilion]

Thanks - got it.

But I'm still not sure about the Bitreverse during if2 stuffing.

There is Bitreverse() libh324m during H324MSessionRead and 
H324MSessionWrite. Thus, those should be transparent to h324m_gw.

But: There is Bitreverse() when creating the h324 frames - but there is 
no Bitreverse() when creating the ast_frame. Further, when creating the 
ast_frame, the mode is derived from the first byte, but then the AMR 
frame is just copied without if2 decoding (shifting everything for 4 bits).

thanks
klaus

Francesco Emmi wrote:
> 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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