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Function Table Control: Read/Write Operations

tableiw, tablew, tablewkt

  kr      tableiw    kndx, ifn[, ixmode[, ixoff[, iwrap]]]
          tablew     isig, indx, ifn[, ixmode[, ixoff[, iwgmode]]]
          tablew     ksig, kndx, ifn[, ixmode[, ixoff[, iwgmode]]]
          tablew     asig, andx, ifn[, ixmode[, ixoff[, iwgmode]]]
          tablewkt   ksig, kndx, kfn[, ixmode[, ixoff[, iwgmode]]]
          tablewkt   asig, andx, kfn[, ixmode[, ixoff[, iwgmode]]]

Description

These opcodes operate on existing function tables, changing their contents. tableiw is used when all inputs are init time variables or constants and you only want to run it at the initialization of the instrument. tablew is for writing at k- or at a-rates, with the table number being specified at init time. tablewkt is the same, but uses a k-rate variable for selecting the table number. The valid combinations of variable types are shown by the first letter of the variable names.

Initialization

isig, ksig, asig – The value to be written into the table.

indx, kndx, andx – Index into table, either a positive number range matching the table length (ixmode = 0) or a 0 to 1 range (ixmode != 0)

ifn, kfn – Table number. Must be = 1. Floats are rounded down to an integer. If a table number does not point to a valid table, or the table has not yet been loaded (GEN01) then an error will result and the instrument will be de-activated.

ixmode – index more Default is 0.

ixoff – index offset. Default is 0.

iwgmode – table writing mode. Default is 0.

Performance

Limit mode (0)

Limit the total index (ndx + ixoff) to between 0 and the guard point. For a table of length 5, this means that locations 0 to 3 and location 4 (the guard point) can be written. A negative total index writes to location 0. Total indexes 4 write to location 4.

Wrap mode (1)

Wrap total index value into locations 0 to E, where E is one less than either the table length or the factor of 2 number which is one less than the table length. For example, wrap into a 0 to 3 range - so that total index 6 writes to location 2.

Guardpoint mode (2)

The guardpoint is written at the same time as location 0 is written - with the same value.

This facilitates writing to tables which are intended to be read with interpolation for producing smooth cyclic waveforms. In addition, before it is used, the total index is incremented by half the range between one location and the next, before being rounded down to the integer address of a table location.

Normally (igwmode = 0 or 1) for a table of length 5 - which has locations 0 to 3 as the main table and location 4 as the guard point, a total index in the range of 0 to 0.999 will write to location 0. ("0.999" means just less than 1.0.) 1.0 to 1.999 will write to location 1 etc. A similar pattern holds for all total indexes 0 to 4.999 (igwmode = 0) or to 3.999 (igwmode = 1). igwmode = 0 enables locations 0 to 4 to be written - with the guardpoint (4) being written with a potentially different value from location 0.

With a table of length 5 and the iwgmode = 2, then when the total index is in the range 0 to 0.499, it will write to locations 0 and 4. Range 0.5 to 1.499 will write to location 1 etc. 3.5 to 4.0 will also write to locations 0 and 4.

This way, the writing operation most closely approximates the results of interpolated reading. Guard point mode should only be used with tables that have a guardpoint.

Guardpoint mode is accomplished by adding 0.5 to the total index, rounding to the next lowest integer, wrapping it modulo the factor of two which is one less than the table length, writing the table (locations 0 to 3 in our example) and then writing to the guard point if index = = 0.

tablew has no output value. The last three parameters are optional and have default values of 0.

Caution with k-rate table numbers :

The following notes also apply to the tablekt and tableikt opcodes which can now have their table number changed at k-rate.

At k-rate or a-rate, if a table number of < 1 is given, or the table number points to a non-existent table, or to one which has a length of 0 (it is to be loaded from a file later) then an error will result and the instrument will be deactivated. kfn and afn must be initialized at the appropriate rate using init. Attempting to load an i-rate value into kfn or afn will result in an error.

Changed Name

As of Csound version 3.52, the opcode name itablew is changed to tableiw.

Author

Robin Whittle
Australia
May 1997


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Function Table Control: Read/Write Operations