Chapter 4. The Converter and the Reader
- Table of Contents
- The Converter
- Interfaces
- The Reader
The role of the couple (Reader, Converter) is to get raw data from several medium (dvd, file, video encoder, satellite card), and to convert it into valid TS packets. I choose to gather them in the same section since they belong to the same thread. Refer to General architecture to see the position of converter between other parts of vls.
The Converter
The Converter - or MpegConverter - is a module that is responsible for providing TS packets. Today, the modules implemented are ps2ts and ts2ts, which functions talk by themselves.
As any modules in the VideoLAN server, the main class C_MpegConverter is implemented in the main program : src/mpeg/converter.cpp . It is in general better for a first overview to have a look at the .h defs (src/mpeg/converter.h) :
class C_MpegConverter : public C_Thread
{
public:
(...)
void Resume();
void Suspend();
void ShortPause();
protected:
virtual void InitWork();
virtual void StopWork();
virtual void CleanWork();
(...)
};
The C_MpegConverter inherits from the C_Thread, since it is the motor of the source thread (Reader & Converter).
This class implements the basic functions of the converter, which will be common to all the Converter modules (ps2ts, ts2ts). Resume(), Suspend(), ShortPause() do all the common stuff to Init/Close the Reader, and deal with the m_cCondition stuff for Suspending/Resuming the streaming (see further).
But what is interesting are the virtual functions. In the present case, they are declared as 'virtual' since there are re-implementation of the basic InitWork(), StopWork() and CleanWork() of the C_Thread class. But there are also virtuals as every Converter module will also re-implement them : that will contitute the essence of a module. All the differences between the modules will take place in thoses functions, and that's why MpegConverter modules have been created.
As the C_MpegConverter inherits from C_Thread, the thread has to be created. This is made in the Input modules (for example in the Localinput.cpp, OnStartStreaming) :
// Create the converter
C_MpegConverterModule* pConverterModule = (C_MpegConverterModule*)
C_Application::GetModuleManager()
->GetModule("mpegconverter",
strConverterType);
(...)
pConverter = pConverterModule->NewMpegConverter(cConfig);
// Create the Thread
pConverter->Create();That's why we can say that the Input spawns the Converter (as well as other parts of the TS chain : Reader, Streamer).
To suspend a stream, we use the thread structure : we block the condition m_cCondition.
Interfaces
The two interfaces are Reader and the SyncFifo (to the TsStreamer).
Reader-Converter
The member m_pReader is a reference to the MpegReader module used in the streaming chain. This value is initialized during the creation of the Converter, in the appropriate Input Module (have a look in C_LocalInput::OnStartStreaming for example).
So, to get data, the converter makes direct calls to the Reader :
m_pReader->Read(); m_pReader->Seek();
Converter SyncFifo
The SyncFifo is called in the converter m_pHandler. m_pHandler is declared in converter.h as an interface : I_TsPacketHandler, which is the mother class of C_SyncFifo.
We call it SyncFifo because it is a fifo which is thread-safe, and allows synchro between source thread and consumer thread. Moreover, the I_TsPacketHandler used is declared as C_SyncFifo in the Input modules :
C_SyncFifo* pBuffer; (...) cConfig.m_pHandler = pBuffer;
So, the packet is send to the SyncFifo using the command :
m_pHandler->HandlePacket(pPacket);
If for some reason the packet has to be trashed (pre-parsing, end of stream, read error....), then the call to m_pHandler->HandlePacket() is not made. However, one should not forget to set it back available in the TsProvider though the command :
m_pTsProvider->ReleasePacket(pPacket);
The Reader
There is not much to say more than reading the code... All the interest locates in the implementation of Read() and Seek().