[cpp]
/*****************************************************************************
* dvbpsi_PushPacket 數字電視中TS包解碼函數注解
*****************************************************************************
* Injection of a TS packet into a PSI decoder.
*****************************************************************************/
void dvbpsi_PushPacket(dvbpsi_handle h_dvbpsi, uint8_t* p_data)
{
uint8_t i_expected_counter; /* 存儲下一個數據包的連續計數器數字 */
dvbpsi_psi_section_t* p_section; /* 當前段指針 */
uint8_t* p_payload_pos; /* 當前位置指針 */
uint8_t* p_new_pos = NULL; /* Beginning of the new section,updated to NULL when the new section is handled */
int i_available; /* 包中可用字節數 */
/* TS start code */
if(p_data[0] != 0x47) //同步字節(sync byte:1B),應為0x47
{
DVBPSI_ERROR("PSI decoder", "not a TS packet");
return;
}
/* 連續檢查 */
i_expected_counter = (h_dvbpsi->i_continuity_counter + 1) & 0xf; /*連續計數器累加1*/
h_dvbpsi->i_continuity_counter = p_data[3] & 0xf; //TS頭部最後4比特為連續計數器
if(i_expected_counter != h_dvbpsi->i_continuity_counter) /*判斷是否是連續傳送的包*/
{
DVBPSI_ERROR_ARG("PSI decoder",
"TS discontinuity (received %d, expected %d)",
h_dvbpsi->i_continuity_counter, i_expected_counter);
h_dvbpsi->b_discontinuity = 1; //將不連續標志置1
if(h_dvbpsi->p_current_section) //刪除當前段
{
dvbpsi_DeletePSISections(h_dvbpsi->p_current_section); //釋放空間
h_dvbpsi->p_current_section = NULL;
}
}
/* 判斷是否有有效負載，沒有就返回*/
if(!(p_data[3] & 0x10)) //第28bit標識有無調整段
{
return;
}
/* 跳過自適應長度區 */
if(p_data[3] & 0x20) //第27bit標識有無自適應區(adaption field)
p_payload_pos = p_data + 5 + p_data[4]; //5為包頭4字節及1字節的自適應長度 p_data[4]自適應區長度
else
p_payload_pos = p_data + 4;
/* Unit start -> skip the pointer_field and a new section begins */
if(p_data[1] & 0x40) //即"payload unit start indicator"bit位 有效荷載單元起始批示符位,值為1表示存在確定的起始信息
{
p_new_pos = p_payload_pos + *p_payload_pos + 1; //指向包數據起始位置
p_payload_pos += 1; //跳過長度指示的1個字節
}
p_section = h_dvbpsi->p_current_section;
/* If the psi decoder needs a begginning of section and a new section
begins in the packet then initialize the dvbpsi_psi_section_t structure */
if(p_section == NULL)
{
if(p_new_pos)
{
/* 為結構分配內存空間 */
h_dvbpsi->p_current_section
= p_section
= dvbpsi_NewPSISection(h_dvbpsi->i_section_max_size);
/* 更新當前位置指針 */
p_payload_pos = p_new_pos;
/* 有指針指向新段 */
p_new_pos = NULL;
/* Just need the header to know how long is the section */
h_dvbpsi->i_need = 3; //在前3個字節中包含了表ID、section syntax indicator和長度批示，能確定是那種表，有多長
h_dvbpsi->b_complete_header = 0;
}
else
{
/* No new section => return */
return;
}
}
/* Remaining bytes in the payload */
i_available = 188 + p_data - p_payload_pos; //有效載荷長度
while(i_available > 0)
{
if(i_available >= h_dvbpsi->i_need)
{
/* 有足夠的字節存儲頭部或整個段的信息 */
memcpy(p_section->p_payload_end, p_payload_pos, h_dvbpsi->i_need);
p_payload_pos += h_dvbpsi->i_need;
p_section->p_payload_end += h_dvbpsi->i_need;
i_available -= h_dvbpsi->i_need;
if(!h_dvbpsi->b_complete_header)
{
/* Header is complete */
h_dvbpsi->b_complete_header = 1;
/* Compute p_section->i_length and update h_dvbpsi->i_need */
//不懂?????????
h_dvbpsi->i_need = p_section->i_length = ((uint16_t)(p_section->p_data[1] & 0xf)) << 8 | p_section->p_data[2]; //從第12個比特位到第24個比特位為長度批示段
/* 檢查段是否超過最大長度 */
if(h_dvbpsi->i_need > h_dvbpsi->i_section_max_size - 3) //包頭3個字節
{
DVBPSI_ERROR("PSI decoder", "PSI section too long");
dvbpsi_DeletePSISections(p_section);
h_dvbpsi->p_current_section = NULL;
/* 如果還有新段沒有分析，則繼續*/
if(p_new_pos)
{
h_dvbpsi->p_current_section
= p_section
= dvbpsi_NewPSISection(h_dvbpsi->i_section_max_size);
p_payload_pos = p_new_pos;
p_new_pos = NULL;
h_dvbpsi->i_need = 3;
h_dvbpsi->b_complete_header = 0;
i_available = 188 + p_data - p_payload_pos;
}
else
{
i_available = 0;
}
}
}
else
{
/* PSI section is complete */
p_section->b_syntax_indicator = p_section->p_data[1] & 0x80; //讀段語法標志 位
p_section->b_private_indicator = p_section->p_data[1] & 0x40; //固定的'0'，這是為了防止和ISO13818Video流格式中的控制字沖突而設置的
/* 如果存在CRC校驗數據，則更新p_payload_end指針 */
if(p_section->b_syntax_indicator)
p_section->p_payload_end -= 4;
if(dvbpsi_ValidPSISection(p_section)) /*調用函數對數據進行CRC校驗*/
{
/* 數據無誤 */
p_section->i_table_id = p_section->p_data[0]; //從第一個字節讀取表ID，0x00為PAT，0x02為PMT
if(p_section->b_syntax_indicator)
{
p_section->i_extension = (p_section->p_data[3] << 8)
| p_section->p_data[4]; //16bits的當前流ID，DVB內唯一.(事實上很多都是自定義的TS ID)
p_section->i_version = (p_section->p_data[5] & 0x3e) >> 1; //43到47的5bits為版本號碼，標注當前節目的版本．這是個非常有用的參數，當檢測到這個字段改變時，說明TS流中的節目已經變化了，程序必須重新搜索節目
p_section->b_current_next = p_section->p_data[5] & 0x1; //第48比特位，當前還是未來使用標志符,一般情況下為'0'
p_section->i_number = p_section->p_data[6]; //當前段號碼
p_section->i_last_number = p_section->p_data[7]; //最後段號碼(section_number和last_section_number的功能是當PAT內容>184字節時，PAT表會分成多個段(sections),解復用程序必須在全部接收完成後再進行PAT的分析)從for()開始，就是描述了當前流中的頻道數目(N),每一個頻道對應的PMT PID是什麼．解復用程序需要和上圖類似的循環來接收所有的頻道號碼和對應的PMT PID,並把這些信息在緩沖區中保存起來．在後部的處理中需要使用到PMT PID.
p_section->p_payload_start = p_section->p_data + 8; //前面已分析了8個字節，指向新的開始
}
else
{
p_section->p_payload_start = p_section->p_data + 3;
}
h_dvbpsi->pf_callback(h_dvbpsi, p_section);
h_dvbpsi->p_current_section = NULL;
}
else
{
/* PSI section isn't valid => trash it */
dvbpsi_DeletePSISections(p_section);
h_dvbpsi->p_current_section = NULL;
}
/* If there is a new section not being handled then go forward
in the packet */
if(p_new_pos)
{
h_dvbpsi->p_current_section
= p_section
= dvbpsi_NewPSISection(h_dvbpsi->i_section_max_size);
p_payload_pos = p_new_pos;
p_new_pos = NULL;
h_dvbpsi->i_need = 3;
h_dvbpsi->b_complete_header = 0;
i_available = 188 + p_data - p_payload_pos;
}
else
{
i_available = 0;
}
}
}
else
{
/* There aren't enough bytes in this packet to complete the
header/section */
memcpy(p_section->p_payload_end, p_payload_pos, i_available);
p_section->p_payload_end += i_available;
h_dvbpsi->i_need -= i_available;
i_available = 0;
}
}
