+/* $Id$ */
+/** @file AliFMDDigitizer.cxx
+ @author Christian Holm Christensen <cholm@nbi.dk>
+ @date Mon Mar 27 12:38:26 2006
+ @brief FMD Digitizers implementation
+ @ingroup FMD_sim
+*/
+//////////////////////////////////////////////////////////////////////////////
+//
+// This class contains the procedures simulation ADC signal for the
+// Forward Multiplicity detector : SDigits->Digits
+//
+// Digits consists of
+// - Detector #
+// - Ring ID
+// - Sector #
+// - Strip #
+// - ADC count in this channel
+//
+// Digits consists of
+// - Detector #
+// - Ring ID
+// - Sector #
+// - Strip #
+// - Total energy deposited in the strip
+// - ADC count in this channel
+//
+// As the Digits and SDigits have so much in common, the classes
+// AliFMDDigitizer and AliFMDSDigitizer are implemented via a base
+// class AliFMDBaseDigitizer.
+//
+// +---------------------+
+// | AliFMDBaseDigitizer |
+// +---------------------+
+// ^
+// |
+// +----------+---------+
+// | |
+// +-----------------+ +------------------+
+// | AliFMDDigitizer | | AliFMDSDigitizer |
+// +-----------------+ +------------------+
+// |
+// +-------------------+
+// | AliFMDSSDigitizer |
+// +-------------------+
+//
+// These classes has several paramters:
+//
+// fPedestal
+// fPedestalWidth
+// (Only AliFMDDigitizer)
+// Mean and width of the pedestal. The pedestal is simulated
+// by a Guassian, but derived classes my override MakePedestal
+// to simulate it differently (or pick it up from a database).
+//
+// fVA1MipRange
+// The dymamic MIP range of the VA1_ALICE pre-amplifier chip
+//
+// fAltroChannelSize
+// The largest number plus one that can be stored in one
+// channel in one time step in the ALTRO ADC chip.
+//
+// fSampleRate
+// How many times the ALTRO ADC chip samples the VA1_ALICE
+// pre-amplifier signal. The VA1_ALICE chip is read-out at
+// 10MHz, while it's possible to drive the ALTRO chip at
+// 25MHz. That means, that the ALTRO chip can have time to
+// sample each VA1_ALICE signal up to 2 times. Although it's
+// not certain this feature will be used in the production,
+// we'd like have the option, and so it should be reflected in
+// the code.
+//
+//
+// The shaping function of the VA1_ALICE is generally given by
+//
+// f(x) = A(1 - exp(-Bx))
+//
+// where A is the total charge collected in the pre-amp., and B is a
+// paramter that depends on the shaping time of the VA1_ALICE circut.
+//
+// When simulating the shaping function of the VA1_ALICe
+// pre-amp. chip, we have to take into account, that the shaping
+// function depends on the previous value of read from the pre-amp.
+//
+// That results in the following algorithm:
+//
+// last = 0;
+// FOR charge IN pre-amp. charge train DO
+// IF last < charge THEN
+// f(t) = (charge - last) * (1 - exp(-B * t)) + last
+// ELSE
+// f(t) = (last - charge) * exp(-B * t) + charge)
+// ENDIF
+// FOR i IN # samples DO
+// adc_i = f(i / (# samples))
+// DONE
+// last = charge
+// DONE
+//
+// Here,
+//
+// pre-amp. charge train
+// is a series of 128 charges read from the VA1_ALICE chip
+//
+// # samples
+// is the number of times the ALTRO ADC samples each of the 128
+// charges from the pre-amp.
+//
+// Where Q is the total charge collected by the VA1_ALICE
+// pre-amplifier. Q is then given by
+//
+// E S
+// Q = - -
+// e R
+//
+// where E is the total energy deposited in a silicon strip, R is the
+// dynamic range of the VA1_ALICE pre-amp (fVA1MipRange), e is the
+// energy deposited by a single MIP, and S ALTRO channel size in each
+// time step (fAltroChannelSize).
+//
+// The energy deposited per MIP is given by
+//
+// e = M * rho * w
+//
+// where M is the universal number 1.664, rho is the density of
+// silicon, and w is the depth of the silicon sensor.
+//
+// The final ADC count is given by
+//
+// C' = C + P
+//
+// where P is the (randomized) pedestal (see MakePedestal)
+//
+// This class uses the class template AliFMDMap<Type> to make an
+// internal cache of the energy deposted of the hits. The class
+// template is instantasized as
+//
+// typedef AliFMDMap<std::pair<Float_t, UShort_t> > AliFMDEdepMap;
+//
+// The first member of the values is the summed energy deposition in a
+// given strip, while the second member of the values is the number of
+// hits in a given strip. Using the second member, it's possible to
+// do some checks on just how many times a strip got hit, and what
+// kind of error we get in our reconstructed hits. Note, that this
+// information is currently not written to the digits tree. I think a
+// QA (Quality Assurance) digit tree is better suited for that task.
+// However, the information is there to be used in the future.
+//
+//
+// Latest changes by Christian Holm Christensen
+//
+//////////////////////////////////////////////////////////////////////////////