[Asterisk-video] several questions to source code of app_h324m

[Sergio Garcia]

---------- Original Message ----------------------------------
From: Klaus Darilion <klaus.mailinglists at pernau.at>
Reply-To: Development discussion of video media support in Asterisk<asterisk-video at lists.digium.com>
Date:  Wed, 25 Jul 2007 16:10:32 +0200

>Hi Sergio!
>
>I'm reading the code to find out why some things don't work for me. Can 
>you please comment on my questions?
>
>
>static struct ast_frame* create_ast_frame(void *frame, struct video_tr *vtr)
>{
>   int mark = 0;
>   struct ast_frame* send;
>
>   /* Get data & size */
>   unsigned char * framedata = FrameGetData(frame);
>   unsigned int framelength = FrameGetLength(frame);
>
>   /* Depending on the type */
>   switch(FrameGetType(frame))
>   {
>     case MEDIA_AUDIO:
>       /*Check it's AMR */
>       if (FrameGetCodec(frame)!=CODEC_AMR)
>         /* exit */
>         return NULL;
>       /* Create frame */
>       send = (struct ast_frame *) malloc(sizeof(struct ast_frame) + 
>AST_FRIENDLY_OFFSET + framelength + 2);
>
>If I Understand Correctly (IIUC) you allocate the frame structure AND 
>the buffer for the data with one malloc. Thus, the frist part of the 
>allocated memory is the structure, followed by the data buffer?
>
>       /* Set data*/
>       send->data = (void*)send + AST_FRIENDLY_OFFSET;
>
>I think this is wrong - it overwrites the structure.
>send->data = (void*)send + sizeof(struct ast_frame) +
>              AST_FRIENDLY_OFFSET;

Ups you-'re rigth

>
>       send->datalen = framelength;
>       /* Set header cmr */
>       ((unsigned char*)(send->data))[0] = 0xF0;
>       /* Set mode */
>       ((unsigned char*)(send->data))[1] = (framedata[0] & 0x78) | 0x04;
>
>
>Here you set the Codec Mode Request 2 times? You set the byte 0 to 0xF0 
>(what is that - is that something standardized or just a proprietary 
>method for signaling something?).
>
>Then you reset the bits 7,2,1,0 and then you set the bit 2. What for? I 
>though the topmost 4 bits (7,6,5,4) are relevant for the codec mode?
>
>       /* Copy */
>       memcpy(send->data+2, framedata, framelength);
>
>Although the cmr is already copy to framedata[1], it will be copied 
>(with the original value) to framedata[2] too. Why?


What you receive in the framedata is the amr if2 frame, you have to add
a 2 byte header for rtp transmision. The problem is that I don't have any
softphone client with amr support to test it.. :(

See http://www.ietf.org/rfc/rfc3267.txt 
Chapter 4.4.1. The Payload Header

>
>       /* Set video type */
>       send->frametype = AST_FRAME_VOICE;
>       /* Set codec value */
>       send->subclass = AST_FORMAT_AMRNB;
>       /* Rest of values*/
>       send->src = "h324m";
>wouldn't it be nicer to define "h324m" as global variable?

Ok.

>
>       send->samples = 160;
>       send->delivery.tv_usec = 0;
>       send->delivery.tv_sec = 0;
>       send->mallocd = 0;
>IIUC setting mallocd=0 works as the data buffer is allocated together 
>with the frame structure. Thus it will be freed when to frame structure 
>is freed.


I'll have to check that, probably it's wrong also.

Best regards
Sergio