How to decode a packetized MPEG4A-LATM

 

http://www.rfc-editor.org/rfc/rfc3016.txt

If you need compatibility with 3g , then use it . LATM .

This is explained LATEM follow to RFC.

1.2 MPEG-4 Audio RTP payload format

   MPEG-4 Audio is a new kind of audio standard that integrates many

   different types of audio coding tools.  Low-overhead MPEG-4 Audio

   Transport Multiplex (LATM) manages the sequences of audio data with

   relatively small overhead.  In audio-only applications, then, it is

   desirable for LATM-based MPEG-4 Audio bit streams to be directly

   mapped onto the RTP packets without using MPEG-4 Systems.

   While LATM has several multiplexing features as follows;

 

   -  Carrying configuration information with audio data,

   -  Concatenation of multiple audio frames in one audio stream,

 

RTP sample codes

http://webcvs.freedesktop.org/gstreamer/gst-plugins-good/gst/rtp/gstrtph264depay.c?view=co&content-type=text%2Fplain

 

RTP_LATM sample code

http://open.phonyx.eu/browser/mpeg4ip-1.5.0.1/player/plugin/rtp/rfc3016/latm.cpp?rev=21

 

SO, couldn’t find it, don’t know where describe this logic of LATM.

Just said with this code.

Facing 0xff On header of MPEG4A-LATM when phrasing data.

It means be data more after this 0xff. So, this Audio data size is more than 255.

You can find about it at 129 } while (value == 0xff);

 

97	static bool start_next_frame (rtp_plugin_data_t *pifptr,
98	uint8_t **buffer,
99	uint32_t *buflen,
100	frame_timestamp_t *ts,
101	void **userdata)
102	{
103	latm_rtp_data_t *iptr = (latm_rtp_data_t *)pifptr;
104	uint64_t timetick;
105	rtp_packet *rpak;
106	uint32_t rtp_ts;
107	uint8_t *dptr;
108	uint32_t dlen;
109	uint32_t calc_len;
110	uint64_t ntp_ts;
111
112	if (iptr->m_rpak != NULL) {
113	(iptr->m_vft->free_pak)(iptr->m_rpak);
114	iptr->m_rpak = NULL;
115	}
116	rpak = (iptr->m_vft->get_head_and_check)(iptr->m_ifptr, false, 0);
117	if (rpak == NULL) return false;
118
119	dlen = rpak->rtp_data_len;
120	dptr = rpak->rtp_data;
121	rtp_ts = rpak->rtp_pak_ts;
122	ntp_ts = rpak->pd.rtp_pd_timestamp;
123	calc_len = 0;
124	uint8_t value;
125	do {
126	value = *dptr++;
127	calc_len += value;
128	dlen--;
129	} while (value == 0xff);
130	#if 0
131	latm_message(LOG_DEBUG, latmrtp, "len %u header %u",
132	calc_len, rpak->rtp_data_len - dlen);
133	#endif
134	if (rpak->rtp_pak_m != 0) {
135	// just one packet - we should have frame length
136	if (calc_len != dlen) {
137	latm_message(LOG_ERR, latmrtp, "header length not correct %u %u",
138	calc_len, dlen);
139	return false;
140	}
141	iptr->m_rpak = rpak;
142
143	*buffer = dptr;
144	*buflen = dlen;
145	} else {
146	if (calc_len > iptr->m_frag_buffer_len) {
147	iptr->m_frag_buffer = (uint8_t *)realloc(iptr->m_frag_buffer, calc_len);
148	iptr->m_frag_buffer_len = calc_len;
149	}
150	memcpy(iptr->m_frag_buffer, dptr, dlen);
151	calc_len -= dlen;
152	uint32_t offset = dlen;
153	(iptr->m_vft->free_pak)(rpak);
154	do {
155	rpak = iptr->m_vft->get_head_and_check(iptr->m_ifptr,
156	true,
157	rtp_ts);
158	if (rpak == NULL) return false;
159	if (calc_len < rpak->rtp_data_len) {
160	latm_message(LOG_ERR, latmrtp, "Illegal frag len - remaining %u pak len %u", calc_len, rpak->rtp_data_len);
161	return false;
162	}
163	memcpy(iptr->m_frag_buffer + offset,
164	rpak->rtp_data,
165	rpak->rtp_data_len);
166	calc_len -= rpak->rtp_data_len;
167	offset += rpak->rtp_data_len;
168	} while (calc_len > 0);
169
170	}
171
172	timetick = iptr->m_vft->rtp_ts_to_msec(iptr->m_ifptr,
173	rtp_ts,
174	ntp_ts,
175	0);
176	ts->audio_freq_timestamp = rtp_ts;
177	ts->msec_timestamp = timetick;
178	ts->timestamp_is_pts = true;
179	return (true);
180	}