Waveform to single ai (again)

["Dr. Simon Friederich via Tech-talk" <tech-talk at aps.anl.gov>]

Dear EPICS-Community,

I have a 128 channel power supply for magnets.
Every 10 seconds a waveform record reads the measured voltages and 
currents for all channels (transmitted as one string from the device):
Pattern send by device = U0, I0, U1, I1, U2, I2,..., U127, I127

    record(waveform, "$(WEDLKI)imem_get")
    {
      field(DESC, "Read current of all channels 0-127")
      field(DTYP, "stream")
      field(INP,  "@wedlkiste.proto getmemarea $(PORT)")
      field(SCAN, "10 second")
      field(NELM, "256")
      field(FTVL, "DOUBLE")
      field(PREC, "7")
    }


Now I want to put/assign these values to single ai-records for the 
different magnets, i.e. in my substition file I have the channel number 
as macro (+Prefix,+Subprefix,etc.):
channel = $(CH) = 0, 1, 2, ..., 127

I now need to extract
    Index_voltage = 2*$(CH)
    Index_current = 2*$(CH) + 1


I am aware of array filters, e.g. $(WEDLKI)imem_get.VAL[INDEX], which 
would fit my needs, but I don't know how to calculate the INDEX for this.
    record (ai, "$(P)$(R)i_get_ai")
    {
        field(DESC, "Extr curr for each channel")
        field(INP,  "$(WEDLKI)imem_get.[$(2*CH+1)] CP")  # 
unfortunately that does not work
        field(EGU,  "A")
        field(PREC, "7")
    }


I could calculate this by hand and enter the INDEX via Macro, but I'd 
like to keep my substition file simple:
file ./db/debug_wedlerchannels.db {
  pattern
  {CH, P, R, INDEX} # not desired to calculate INDEX by hand
  { 0, wed:, c0:, 1}
  { 1, wed:, c1:, 3}
  ...
  { 6, wed:, c6:, 13}
  { 7, wed:, c7:, 15}
  ...
  { 13, wed:, c13:, 27}
  { 14, wed:, c14:, 29}
  ....
}

Do you have a suggestion for me how to solve this?

I've also tried a single element subarray and calculated INDX via a calc 
record. But unfortunately the single values do only refer to the first 
value (index = 0) and are not updated although I've added the "CP" flag.
    record(calcout, "$(P)$(R)__calci_INDX")
    {
        field(DESC, "Calcuate the index for the subArray")
        field(INPA, "$(P)$(R)i_get CP")
        field(CALC, "(2*$(CH))+1")
        field(OUT,  "$(P)$(R)i_get.INDX NPP") # works, or at least 
caget shows that i_get.INDX is correct
        field(DISP, 1)
    }

    record (subArray, "$(P)$(R)i_get")
    {
        field(DESC, "Extr curr for each channel")
        field(SCAN, "Passive")
        field(INP,  "$(WEDLKI)imem_get CP") # Process linking record 
every time the target record processes
        field(FTVL, "DOUBLE")
        field(MALM, "1")
        field(NELM, "1")
        field(EGU,  "A")
        field(PREC, "7")
    }


Some background:
The imem_get-PV is on the same IOC. Currently we use  EPICS R7.0.7.

Thank you very much in advance.
Best regards
Simon