| Geant4 User's Guide For Application Developers Detector Definition and Response

4.5 Digitization

## 4.5.1 Digi

A hit is created by a sensitive detector when a step goes through it. Thus, the sensitive detector is associated to the corresponding G4LogicalVolume object(s). On the other hand, a digit is created using information of hits and/or other digits by a digitizer module. The digitizer module is not associated with any volume, and you have to implicitly invoke the `Digitize()` method of your concrete G4VDigitizerModule class.

Typical usages of digitizer module include:

- simulate ADC and/or TDC
- simulate readout scheme
- generate raw data
- simulate trigger logics
- simulate pile up

G4VDigi

G4VDigi is an abstract base class which represents a digit. You have to inherit this base class and derive your own concrete digit class(es). The member data of your concrete digit class should be defined by yourself. G4VDigi has two virtual methods, `Draw()` and `Print()`.

G4TDigiCollection

G4TDigiCollection is a template class for digits collections, which is derived from the abstract base class G4VDigiCollection. G4Event has a G4DCofThisEvent object, which is a container class of collections of digits. The usages of G4VDigi and G4TDigiCollection are almost the same as G4VHit and G4THitsCollection, respectively, explained in the previous section.

## 4.5.2 Digitizer module

G4VDigitizerModule

G4VDigitizerModule is an abstract base class which represents a digitizer module. It has a pure virtual method, `Digitize()`. A concrete digitizer module must have an implementation of this virtual method. The Geant4 kernel classes do not have a ``built-in'' invocation to the `Digitize()` method. You have to implement your code to invoke this method of your digitizer module.

In the `Digitize()` method, you construct your G4VDigi concrete class objects and store them to your G4TDigiCollection concrete class object(s). Your collection(s) should be associated with the G4DCofThisEvent object.

G4DigiManager

G4DigiManager is the singleton manager class of the digitizer modules. All of your concrete digitizer modules should be registered to G4DigiManager with their unique names.

         G4DigiManager * fDM = G4DigiManager::GetDMpointer();
         MyDigitizer * myDM = new MyDigitizer( "/myDet/myCal/myEMdigiMod" );
         fDM->AddNewModule(myDM);

Your concrete digitizer module can be accessed from your code using the unique module name.

         G4DigiManager * fDM = G4DigiManager::GetDMpointer();
         MyDigitizer * myDM = fDM->FindDigitizerModule( "/myDet/myCal/myEMdigiMod" );
         myDM->Digitize();

Also, G4DigiManager has a Digitize() method which takes the unique module name.

         G4DigiManager * fDM = G4DigiManager::GetDMpointer();
         MyDigitizer * myDM = fDM->Digitize( "/myDet/myCal/myEMdigiMod" );

How to get hitsCollection and/or digiCollection

G4DigiManager has the following methods to access to the hits or digi collections of the currently processing event or of previous events.

First, you have to get the collection ID number of the hits or digits collection.

         G4DigiManager * fDM = G4DigiManager::GetDMpointer();
         G4int myHitsCollID = fDM->GetHitsCollectionID( "hits_collection_name" );
         G4int myDigiCollID = fDM->GetDigiCollectionID( "digi_collection_name" );

Then, you can get the pointer to your concrete G4THitsCollection object or G4TDigiCollection object of the currently processing event.

         MyHitsCollection * HC = fDM->GetHitsCollection( myHitsCollID );
         MyDigiCollection * DC = fDM->GetDigiCollection( myDigiCollID );

In case you want to access to the hits or digits collection of previous events, add the second argument.

         MyHitsCollection * HC = fDM->GetHitsCollection( myHitsCollID, n );
         MyDigiCollection * DC = fDM->GetDigiCollection( myDigiCollID, n );

where, `n` indicates the hits or digits collection of the `n`th previous event.

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