Inverse discrete cosine transform (IDCT) of input
Transforms
dspxfrm3
The IDCT block computes the inverse discrete cosine transform
(IDCT) of each channel in the MbyN input
matrix, u
.
y = idct(u) % Equivalent MATLAB code
When the input is a samplebased row vector, the IDCT block computes the inverse discrete cosine transform across the vector dimension of the input. For all other ND input arrays, the block computes the IDCT across the first dimension. The size of the first dimension (frame size), must be a power of two. To work with other frame sizes, use the Pad block to pad or truncate the frame size to a poweroftwo length.
When the input is an MbyN matrix, the block treats each input column as an independent channel containing M consecutive samples. The block outputs an MbyN matrix whose lth column contains the lengthM IDCT of the corresponding input column.
$$y\left(m,l\right)={\displaystyle \sum _{k=1}^{M}w(k)u(k,l)\mathrm{cos}\frac{\pi (2m1)(k1)}{2M}},\begin{array}{ccc}& & m=1,\mathrm{...},M\end{array}$$
where
$$w(k)=\{\begin{array}{l}\frac{1}{\sqrt{M}}\\ \sqrt{\frac{2}{M}}\end{array}\begin{array}{c}\begin{array}{l}\\ ,\\ \\ ,\end{array}\\ \end{array}\begin{array}{ll}\hfill & \hfill \\ \hfill & k=1\hfill \\ \hfill & \hfill \\ \hfill & 2\le k\le M\hfill \\ \hfill & \hfill \end{array}$$
The Output sampling mode parameter allows
you to select the sampling mode of the output. If the input is an
ND array with 3 or more dimensions, the Output sampling
mode must be Sample based
. The
output sample rate and data type (real/complex) are the same as those
of the input.
The Sine and cosine computation parameter determines how the block computes the necessary sine and cosine values. This parameter has two settings, each with its advantages and disadvantages, as described in the following table.
Sine and Cosine Computation Parameter Setting  Sine and Cosine Computation Method  Effect on Block Performance 

 The block computes and stores the trigonometric values before the simulation starts, and retrieves them during the simulation. When you generate code from the block, the processor running the generated code stores the trigonometric values computed by the block in a speedoptimized table, and retrieves the values during code execution.  The block usually runs much more quickly, but requires extra memory for storing the precomputed trigonometric values. 
 The block computes sine and cosine values during the simulation. When you generate code from the block, the processor running the generated code computes the sine and cosine values while the code runs.  The block usually runs more slowly, but does not need extra data memory. For code generation, the block requires a support library to emulate the trigonometric functions, increasing the size of the generated code. 
The following diagrams show the data types used within the IDCT block for fixedpoint signals. You can set the sine table, accumulator, product output, and output data types displayed in the diagrams in the IDCT block dialog as discussed in Dialog Box.
Inputs to the IDCT block are first cast to the output data type and stored in the output buffer. Each butterfly stage processes signals in the accumulator data type, with the final output of the butterfly being cast back into the output data type.
The output of the multiplier is in the product output data type when at least one of the inputs to the multiplier is real. When both of the inputs to the multiplier are complex, the result of the multiplication is in the accumulator data type. For details on the complex multiplication performed, see Multiplication Data Types.
Note: When the block input is fixed point, all internal data types are signed fixed point. 
The Main pane of the IDCT block dialog appears as follows.
Sets the block to compute sines and cosines by either looking
up sine and cosine values in a speedoptimized table (Table
lookup
), or by making sine and cosine function calls
(Trigonometric fcn
). See the table in the Description
section.
Select Sample based
or Frame
based
output. If the input to the IDCT block has 3 or
more dimensions, you must select Sample based
output.
The Data Types pane of the IDCT block dialog appears as follows.
Select the rounding mode for fixedpoint operations.
The sine table values do not obey this parameter; they always round
to Nearest
.
Select the overflow mode for fixedpoint operations. The sine table values do not obey this parameter; instead, they are always saturated.
Choose how you specify the word length of the values of the sine table. The fraction length of the sine table values always equals the word length minus one. You can set this parameter to:
A rule that inherits a data type, for example, Inherit:
Same word length as input
An expression that evaluates to a valid data type,
for example, fixdt(1,16)
The sine table values do not obey the Rounding mode and Overflow
mode parameters; instead, they are always saturated and
rounded to Nearest
.
Specify the product output data type. See FixedPoint Data Types and Multiplication Data Types for illustrations depicting the use of the product output data type in this block. You can set this parameter to:
A rule that inherits a data type, for example, Inherit:
Inherit via internal rule
An expression that evaluates to a valid data type,
for example, fixdt(1,16,0)
Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Product output data type parameter.
See Specify Data Types Using Data Type Assistant for more information.
Specify the accumulator data type. See FixedPoint Data Types for illustrations depicting the use of the accumulator data type in this block. You can set this parameter to:
A rule that inherits a data type, for example, Inherit:
Inherit via internal rule
An expression that evaluates to a valid data type,
for example, fixdt(1,16,0)
Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Accumulator data type parameter.
See Specify Data Types Using Data Type Assistant for more information.
Specify the output data type. See FixedPoint Data Types for illustrations depicting the use of the output data type in this block. You can set this parameter to:
A rule that inherits a data type, for example, Inherit:
Inherit via internal rule
.
When you select Inherit: Inherit via internal rule
,
the block calculates the output word length and fraction length automatically.
The internal rule first calculates an ideal output word length and
fraction length using the following equations:
$$W{L}_{idealoutput}=W{L}_{input}+floor({\mathrm{log}}_{2}(DCTlength1))+1$$
$$F{L}_{idealoutput}=F{L}_{input}$$
Using these ideal results, the internal rule then selects word lengths and fraction lengths that are appropriate for your hardware. For more information, see Inherit via Internal Rule.
An expression that evaluates to a valid data type,
for example, fixdt(1,16,0)
Click the Show data type assistant button to display the Data Type Assistant, which helps you set the Output data type parameter.
See Specify Block Output Data Types for more information.
Select this parameter to prevent the fixedpoint tools from overriding the data types you specify on the block mask.
Port  Supported Data Types 

Input 

Output 
