Buffering and Frame-Based Processing

    Note:   Starting in R2010b, many DSP System Toolbox™ blocks received a new parameter to control whether they perform sample- or frame-based processing. The following content has not been updated to reflect this change. For more information, see the Frame-Based Processing section of the Release Notes.

Frame Status

The frame status of a signal refers to whether the signal is sample based or frame based. In a Simulink® model, the frame status is symbolized by a single line ,→, for a sample-based signal and a double line, ⇒ for a frame-based signal. One way to convert a sample-based signal to a frame-based signal is by using the Buffer block. You can convert a frame-based signal to a sample-based signal using the Unbuffer block. To change the frame status of a signal without performing a buffering operation, use the Frame Conversion block in the Signal Attributes library.

Buffer Sample-Based Signals into Frame-Based Signals

Multichannel sample-based and frame-based signals can be buffered into multichannel frame-based signals using the Buffer block.

The following figure is a graphical representation of a sample-based signal being converted into a frame-based signal by the Buffer block.

In the following example, a two-channel sample-based signal is buffered into a two-channel frame-based signal using a Buffer block:

  1. At the MATLAB® command prompt, type ex_buffer_tutex_buffer_tut.

    The Buffer Example model opens.

  2. Double-click the Signal From Workspace block. The Source Block Parameters: Signal From Workspace dialog box opens.

  3. Set the parameters as follows:

    • Signal = [1:10;-1:-1:-10]'

    • Sample time = 1

    • Samples per frame = 1

    • Form output after final data value = Setting to zero

    Based on these parameters, the Signal from Workspace block outputs a sample-based signal with a sample period of 1 second. Because you set the Samples per frame parameter setting to 1, the Signal From Workspace block outputs one two-channel sample at each sample time.

  4. Save these parameters and close the dialog box by clicking OK.

  5. Double-click the Buffer block. The Function Block Parameters: Buffer dialog box opens.

  6. Set the parameters as follows:

    • Output buffer size (per channel) = 4

    • Buffer overlap = 0

    • Initial conditions = 0

    Because you set the Output buffer size parameter to 4, the Buffer block outputs a frame-based signal with frame size 4.

  7. Run the model.

    Note that the input to the Buffer block is sample based (represented as a single line) while the output is frame-based (represented by a double line).

    The figure below is a graphical interpretation of the model behavior during simulation.

    Note:   Alternatively, you can set the Samples per frame parameter of the Signal From Workspace block to 4 and create the same frame-based signal shown above without using a Buffer block. The Signal From Workspace block performs the buffering internally, in order to output a two-channel frame-based signal.

Buffer Sample-Based Signals into Frame-Based Signals with Overlap

In some cases it is useful to work with data that represents overlapping sections of an original sample-based or frame-based signal. For example, in estimating the power spectrum of a signal, it is often desirable to compute the FFT of overlapping sections of data. Overlapping buffers are also needed in computing statistics on a sliding window, or for adaptive filtering.

The Buffer overlap parameter of the Buffer block specifies the number of overlap points, L. In the overlap case (L > 0), the frame period for the output is (Mo-L)*Tsi, where Tsi is the input sample period and Mo is the Buffer size.

    Note:   Set the Buffer overlap parameter to a negative value to achieve output frame rates slower than in the nonoverlapping case. The output frame period is still Tsi*(Mo-L), but now with L < 0. Only the Mo newest inputs are included in the output buffers. The previous L inputs are discarded.

In the following example, a four-channel sample-based signal with sample period 1 is buffered to a frame-based signal with frame size 3 and frame period 2. Because of the buffer overlap, the input sample period is not conserved, and the output sample period is 2/3:

  1. At the MATLAB command prompt, type ex_buffer_tut3ex_buffer_tut3.

    The Buffer Example T3 model opens.

    Also, the variable sp_examples_src is loaded into the MATLAB workspace. This variable is defined as follows:

    sp_examples_src=[1 1 5 -1; 2 1 5 -2; 3 0 5 -3; 4 0 5 -4; 5 1 5 -5; 6 1 5 -6];
  2. Double-click the Signal From Workspace block. The Source Block Parameters: Signal From Workspace dialog box opens.

  3. Set the block parameters as follows:

    • Signal = sp_examples_src

    • Sample time = 1

    • Samples per frame = 1

    • Form output after final data value by = Setting to zero

    Based on these parameters, the Signal from Workspace block outputs a sample-based signal with a sample period of 1 second. Because you set the Samples per frame parameter setting to 1, the Signal From Workspace block outputs one four-channel sample at each sample time.

  4. Save these parameters and close the dialog box by clicking OK.

  5. Double-click the Buffer block. The Function Block Parameters: Buffer dialog box opens.

  6. Set the block parameters as follows, and then click OK:

    • Output buffer size (per channel) = 3

    • Buffer overlap = 1

    • Initial conditions = 0

    Because you set the Output buffer size parameter to 3, the Buffer block outputs a frame-based signal with frame size 3. Also, because you set the Buffer overlap parameter to 1, the last sample from the previous output frame is the first sample in the next output frame.

  7. Run the model.

    Note that the input to the Buffer block is sample based (represented as a single line) while the output is frame based (represented by a double line).

    The following figure is a graphical interpretation of the model's behavior during simulation.

  8. At the MATLAB command prompt, type sp_examples_yout.

    The following is displayed in the MATLAB Command Window.

    sp_examples_yout =
    
         0     0     0     0
         0     0     0     0
         0     0     0     0
         0     0     0     0
         1     1     5    -1
         2     1     5    -2
         2     1     5    -2
         3     0     5    -3
         4     0     5    -4
         4     0     5    -4
         5     1     5    -5
         6     1     5    -6
         6     1     5    -6
         0     0     0     0
         0     0     0     0
         0     0     0     0
         0     0     0     0
         0     0     0     0
    

    Notice that the inputs do not begin appearing at the output until the fifth row, the second row of the second frame. This is due to the block's latency.

See Excess Algorithmic Delay (Tasking Latency) for general information about algorithmic delay. For instructions on how to calculate buffering delay, see Buffer Delay and Initial Conditions.

Buffer Frame-Based Signals into Other Frame-Based Signals

In the following example, a two-channel frame-based signal with frame size 4 is rebuffered to a frame-based signal with frame size 3 and frame period 2. Because of the overlap, the input sample period is not conserved, and the output sample period is 2/3:

  1. At the MATLAB command prompt, type ex_buffer_tut4ex_buffer_tut4.

    The Buffer Example T4 model opens.

    Also, the variable sp_examples_src is loaded into the MATLAB workspace. This variable is defined as

    sp_examples_src = [1 1; 2 1; 3 0; 4 0; 5 1; 6 1; 7 0; 8 0]
  2. Double-click the Signal From Workspace block. The Source Block Parameters: Signal From Workspace dialog box opens.

  3. Set the block parameters as follows:

    • Signal = sp_examples_src

    • Sample time = 1

    • Samples per frame = 4

    Based on these parameters, the Signal From Workspace block outputs a two-channel, frame-based signal with a sample period of 1 second and a frame size of 4.

  4. Save these parameters and close the dialog box by clicking OK.

  5. Double-click the Buffer block. The Function Block Parameters: Buffer dialog box opens.

  6. Set the block parameters as follows, and then click OK:

    • Output buffer size (per channel) = 3

    • Buffer overlap = 1

    • Initial conditions = 0

    Based on these parameters, the Buffer block outputs a two-channel, frame-based signal with a frame size of 3.

  7. Run the model.

    The following figure is a graphical representation of the model's behavior during simulation.

    Note that the inputs do not begin appearing at the output until the last row of the third output matrix. This is due to the block's latency.

See Excess Algorithmic Delay (Tasking Latency) for general information about algorithmic delay. For instructions on how to calculate buffering delay, and see Buffer Delay and Initial Conditions.

Buffer Delay and Initial Conditions

In the examples Buffer Sample-Based Signals into Frame-Based Signals with Overlap and Buffer Frame-Based Signals into Other Frame-Based Signals, the input signal is delayed by a certain number of samples. The initial output samples correspond to the value specified for the Initial condition parameter. The initial condition is zero in both examples mentioned above.

Under most conditions, the Buffer and Unbuffer blocks have some amount of delay or latency. This latency depends on both the block parameter settings and the Simulink tasking mode. You can use the rebuffer_delay function to determine the length of the block's latency for any combination of frame size and overlap.

The syntax rebuffer_delay(f,n,v) returns the delay, in samples, introduced by the buffering and unbuffering blocks during multitasking operations, where f is the input frame size, n is the Output buffer size parameter setting, and v is the Buffer overlap parameter setting.

For example, you can calculate the delay for the model discussed in the Buffer Frame-Based Signals into Other Frame-Based Signals using the following command at the MATLAB command line:

d = rebuffer_delay(4,3,1)
d = 8

This result agrees with the block's output in that example. Notice that this model was simulated in Simulink multitasking mode.

For more information about delay, see Excess Algorithmic Delay (Tasking Latency). For delay information about a specific block, see the "Latency" section of the block reference page. For more information about the rebuffer_delay function, see rebuffer_delay.

Unbuffer Frame-Based Signals into Sample-Based Signals

You can unbuffer multichannel frame-based signals into multichannel sample-based signals using the Unbuffer block. The Unbuffer block performs the inverse operation of the Buffer block's "sample-based to frame-based" buffering process, and generates an N-channel sample-based output from an N-channel frame-based input. The first row in each input matrix is always the first sample-based output.

The following figure is a graphical representation of this process.

The sample period of the sample-based output, Tso, is related to the input frame period, Tfi, by the input frame size, Mi.

Tso=Tfi/Mi

The Unbuffer block always preserves the signal's sample period (Tso =  Tsi). See Convert Sample and Frame Rates in Simulink for more information about rate conversions.

In the following example, a two-channel frame-based signal is unbuffered into a two-channel sample-based signal:

  1. At the MATLAB command prompt, type ex_unbuffer_tutex_unbuffer_tut.

    The Unbuffer Example model opens.

  2. Double-click the Signal From Workspace block. The Source Block Parameters: Signal From Workspace dialog box opens.

  3. Set the block parameters as follows:

    • Signal = [1:10;-1:-1:-10]'

    • Sample time = 1

    • Samples per frame = 4

    • Form output after final data value by = Setting to zero

    Based on these parameters, the Signal From Workspace block outputs a two-channel, frame based-signal with frame size 4.

  4. Save these parameters and close the dialog box by clicking OK.

  5. Double-click the Unbuffer block. The Function Block Parameters: Unbuffer dialog box opens.

  6. Set the Initial conditions parameter to 0, and then click OK.

    The Unbuffer block unbuffers the frame-based signal into a two-channel sample-based signal.

  7. Run the model.

    The following figures is a graphical representation of what happens during the model simulation.

      Note:   The Unbuffer block generates initial conditions not shown in the figure below with the value specified by the Initial conditions parameter. See the Unbuffer reference page for information about the number of initial conditions that appear in the output.

  8. At the MATLAB command prompt, type sp_examples_yout.

    The following is a portion of the output.

    sp_examples_yout(:,:,1) =
    
         0     0
    
    
    sp_examples_yout(:,:,2) =
    
         0     0
    
    
    sp_examples_yout(:,:,3) =
    
         0     0
    
    
    sp_examples_yout(:,:,4) =
    
         0     0
    
    
    sp_examples_yout(:,:,5) =
    
         1    -1
    
    
    sp_examples_yout(:,:,6) =
    
         2    -2
    
    
    sp_examples_yout(:,:,7) =
    
         3    -3
    

    The Unbuffer block unbuffers the frame-based signal into a two-channel, sample-based signal. Each page of the output matrix represents a different sample time.

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