Simulink Streaming DSP

Build streaming (frame-based) signal processing models in Simulink using DSP System Toolbox. This skill prevents silent errors from wrong block selection, missed parameter settings, and outdated filter architectures.

The most damaging failures here are silent: the model compiles, simulates, and produces output — but the numbers are wrong, with no error or warning. Apply the Block Selection and Frame-Based Configuration rules on every build, even for tasks a strong model would usually get right — they are the cheap insurance against silent corruption and they hold across models and effort levels.

When to Use

• Building a Simulink model that processes signals in frames (not sample-at-a-time)
• Choosing source, sink, or visualization blocks for a DSP pipeline
• Adding filters to a streaming model (FIR, IIR, tunable)
• Buffering signals (scalar→frame, frame resizing, overlapping windows, unbuffering)
• Converting sample/frame rates (decimate, interpolate, fractional resample)
• Building multirate models (dual-path, different frame sizes, mixed rates)
• Working with variable-size signals (packet-based, adaptive frame sizes)
• Building a tunable/adjustable filter with runtime parameter control
• Visualizing signals or filter responses (Time Scope, Spectrum Analyzer, Filter Visualizer)
• Computing buffering latency or frame timing

When NOT to Use

• MATLAB-only scripting (no Simulink model) — use standard MATLAB functions
• Continuous-time Simulink modeling (no discrete/DSP blocks)

Model Construction — Use the MCP Model Tools Only (mandatory)

Build and edit every model through the MATLAB MCP model tools — model_edit (add/connect/configure/delete), model_read, model_check, model_test. Never construct or modify a model with raw MATLAB commands (add_block, add_line, set_param, delete_line, new_system, …) run through evaluate_matlab_code. The MCP tools handle autolayout, undo tracking, and error recovery; the raw-command fallback is slow (block-by-block round-trips), skips layout, and is prohibited. Use evaluate_matlab_code only to run/verify a model (sim, reading logged signals, rebuffer_delay, filter design math) — never to build one.

Get the schema right the first time — guessing field names wastes turns. The add-block field is type (a full library path), and connections use target:

add_block:  {"op":"add_block", "type":"dspsrcs4/Sine Wave", "ref":"src", "name":"SineWave", "params":{"SamplesPerFrame":"256"}}
connect:    {"op":"connect", "target":"#src.y1 -> #buf.u1"}          // #ref within the same call
configure:  {"op":"configure", "target":"blk_3", "params":{"InputProcessing":"Columns as channels (frame based)"}}

• type (NOT source, path, or block) names the block; ref is your handle for chaining, and the response returns ref → blk_id.
• connect uses target: "src.y1 -> dst.u1" (NOT from/to); y1 = output port 1, u1 = input port 1. Within one call reference just-added blocks as #ref; across calls use the returned blk_N id.
• Model-level settings (solver, display overlays) go through a configure op targeting config:<ModelName>, not set_param:

  {"op":"configure", "target":"config:MyModel", "params":{"ShowLineDimensions":"on","SampleTimeColors":"on"}}
  

Wrong field names fail with a misleading Unrecognized field name "type" — that error means the operation was missing type, not that the tool is broken.

Build Workflow

Build a streaming DSP model in this order. Each step has a verification.

1. Source — pick the DSP source block (see Block Selection). Set SamplesPerFrame.

2. Frame configuration — confirm the signal is [FrameSize × NumChannels] (column-oriented). See the Frame-Based Configuration Checklist.

3. Processing — add filters / rate conversion / buffering. For each filter, set/inherit frame-based InputProcessing.

4. Sink — pick a DSP display/output block (Time Scope, Spectrum Analyzer — never Scope).

5. Solver — Fixed-step, Discrete (no continuous states).

6. Verify — turn on the model's information overlays so dimensions and rates are visible on the diagram, then confirm signal dimensions read [N×1]/[N×C], not [1×N]. Confirm output against a reference where possible.

   • Non-scalar signals → enable signal dimensions via a configure op: {"op":"configure","target":"config:<model>","params":{"ShowLineDimensions":"on"}}. Wires then annotate their size — a [1×N] label flags a wrong orientation at a glance.
   • Multirate models (any two blocks running at different sample times) → enable sample time colors via a configure op: {"op":"configure","target":"config:<model>","params":{"SampleTimeColors":"on"}}. Each rate draws in a distinct color, so an unintended rate transition or a block stuck at the wrong rate is immediately obvious.

For the task at hand, load the matching reference (see Task References below) once the block-level details are needed.

Block Selection

Select blocks using this table. Never use the generic Simulink equivalent — it lacks frame-based processing support and silently corrupts framed signals.

Source Blocks

User intent	Correct block	Library	Key parameter
Read audio/video file	From Multimedia File	dspsrcs4	AudioFrameSize (dialog: "Samples per audio channel")
Load workspace signal (framed)	Signal From Workspace	dspsrcs4	SamplesPerFrame, SignalName
Generate sine/cosine test signal	Sine Wave (DSP)	dspsrcs4	SamplesPerFrame, Frequency
Generate chirp test signal	Chirp (DSP)	dspsrcs4	SamplesPerFrame
Generate noise	Random Source	dspsrcs4	SamplesPerFrame, SourceType

Never use From Workspace (Simulink built-in) — outputs [1×N] row vectors, not [N×1] column frames.

Sink Blocks

User intent	Correct block	Library
View time-domain waveform	Time Scope	dspsnks4
View frequency spectrum	Spectrum Analyzer	dspsnks4
View filter frequency response	Filter Visualizer	dspsnks4
View vector/array snapshot	Array Plot	dspsnks4
Write audio/video to file	To Multimedia File	dspsnks4

Discouraged use Scope (Simulink built-in) — no frame-based support, staircase display, no DSP measurements. This applies even when display is a secondary part of the task ("…and show the output").

Filter Blocks

Need	Use	Avoid
FIR filtering	dsparch4/Discrete FIR Filter (frame-based default)	simulink/Discrete/Discrete FIR Filter (sample-based default)
IIR via cascaded biquads	Second-Order Section Filter (R2023b+)	Biquad Filter (legacy)
Tunable filter	two-block Design→Implementation pattern	monolithic / MATLAB Function

The Discrete FIR Filter exists in two libraries with the same name — see Checklist Rule 1.

For exact library paths and parameter names of any block, see references/block-inventory.md — the master lookup; load it when you need a path for model_edit or need to confirm a parameter name.

Frame-Based Configuration Checklist

These settings produce silent wrong output if left at defaults. The agent typically knows these facts when asked directly but fails to apply them while building — run this checklist on every model.

Rule 1: Use the DSP Discrete FIR Filter, not Simulink's

Library	InputProcessing default	Use for streaming?
dsparch4/Discrete FIR Filter (DSP System Toolbox)	"Columns as channels (frame based)"	YES
simulink/Discrete/Discrete FIR Filter (Simulink built-in)	"Elements as channels (sample based)"	NO

If you must use any block whose InputProcessing defaults to sample-based, set:

InputProcessing = "Columns as channels (frame based)"

Affected blocks with sample-based defaults: simulink/Discrete/Discrete FIR Filter, Discrete Filter, Biquad Filter.

Why it matters: "Elements as channels (sample based)" treats each element in the frame as a separate channel. A 256-sample frame becomes 256 single-sample channels filtered independently — output is garbage, no error thrown.

Exceptions: Second-Order Section Filter, FIR Decimation, FIR Interpolation default to frame-based — no action needed. And when input is genuinely multichannel sample-based (e.g. [1×8] from 8 sensors at one instant), "Elements as channels" is the correct choice — do not reflexively force frame-based.

Rule 2: FrameBasedProcessingString on Time Scope

After connecting a frame-based signal to Time Scope, set:

FrameBasedProcessingString = "Columns as channels (frame based)"

Same mechanism as Rule 1 — the default shows a staircase display instead of a smooth waveform.

Rule 3: Signal Dimension Convention

DSP signals are column vectors: [FrameSize × NumChannels].

• [256×1] = 256-sample frame, 1 channel — CORRECT
• [1×256] = 1 sample, 256 channels — WRONG for streaming DSP
• [512×2] = 512-sample frame, 2 channels (stereo) — CORRECT

A [1×N] line means something upstream is wrong (a From Workspace block or a transposing MATLAB Function).

For any model with non-scalar signals, turn on ShowLineDimensions via a configure op ({"op":"configure","target":"config:<model>","params":{"ShowLineDimensions":"on"}}) so every wire is annotated with its size — this makes a stray [1×N] visible on the diagram instead of only discoverable by clicking each signal.

Rule 4: Solver Configuration

Apply via a configure op targeting config:<model> (not set_param): {"op":"configure","target":"config:<model>","params":{"SolverType":"Fixed-step","Solver":"FixedStepDiscrete","FixedStep":"auto"}}

• Solver type: Fixed-step
• Solver: Discrete (no continuous states) — unless mixing with continuous blocks
• Fixed-step size: auto (Simulink derives the base rate from block sample times)

Task References

When the build reaches one of these task domains, load the matching reference for block-level detail, parameters, and timing math. SKILL.md covers the cross-cutting rules above; these cover the specifics.

Task	Reference	Load when
Block / library lookup	references/block-inventory.md	Need the exact library path or parameter name for any DSP block referenced in the tables above
Buffering / multirate	references/buffering-multirate.md	Changing frame size, overlap windows, unbuffering, dual-path, multirate timing, buffering latency
Rate conversion	references/rate-conversion.md	Decimate / interpolate / resample; choosing FIR Decimation vs Downsample; setting RateOptions (single-rate vs multirate)
Tunable filtering	references/tunable-filtering.md	Building any runtime-tunable filter; the 8 Design blocks, two-block pattern, SOS vs Biquad, Parameter Smoother
Variable-size signals	references/variable-size-signals.md	Frame size changes during simulation; variable→fixed conversion; the Buffer/variable-rate traps
Visualization	references/visualization.md	Configuring Spectrum Analyzer (NumInputPorts, spectrogram), Filter Visualizer wiring, Array Plot, Time Scope frame config

Each reference is self-contained and loaded on demand — read only the one relevant to the current step.

Common Mistakes

What the agent does wrong	Why it fails	Correct approach
Uses From Workspace for audio/signal input	Outputs [1×N] row vector, not [N×1] column frame	Use From Multimedia File or Signal From Workspace
Uses Scope for signal display	No frame-based processing, staircase display	Use Time Scope (dspsnks4)
Uses simulink/Discrete/Discrete FIR Filter	Sample-based default — silent wrong output	Use dsparch4/Discrete FIR Filter
Leaves InputProcessing at default on a sample-based filter block	Silent wrong output — runs but garbage	Set "Columns as channels (frame based)" or use the DSP block
Adds a Buffer before Spectrum Analyzer for scalar input	SA buffers internally — extra Buffer is unnecessary	Connect scalar directly to SA
Uses Downsample/Upsample for a rate change	No anti-aliasing/anti-imaging — aliases or images	Use FIR Decimation / FIR Interpolation (or Sample-Rate Converter)
Reverses RateOptions single-rate vs multirate semantics	Wrong output frame size or frame rate, silently	Single-rate → frame size changes; multirate → frame rate changes (see rate-conversion.md)
Builds TFE+ArrayPlot or FFT workaround for filter response	Over-engineered, noisy, needs excitation	Use Filter Visualizer (1 block, exact response)
Uses Biquad Filter for IIR	Legacy block, not tunable, no Design-block integration	Use Second-Order Section Filter (R2023b+)
Calls the two-block pattern "old" and monolithic "new"	Inverted — two-block IS the modern R2023b+ pattern	Design block → Implementation block is correct
Treats the plain Buffer as a variable-size→fixed converter	Buffer requires a fixed compile-time input size	Use dsp.AsyncBuffer (in a MATLAB Function)
Models a variable frame size as a variable rate	Frame-size change does not alter the sample period	Variable-size needs only a discrete sample time
Uses Variable FIR Decimation to convert arbitrary variable-size→fixed frames	It accepts variable-size input, but its Output frame length mode only rebuffers when each input frame length is an integer multiple of Po — fails for chunk sets like {240,480,960}→256	Use dsp.AsyncBuffer (write variable chunks, read fixed frames) + a separate anti-alias filter
Computes buffer latency manually	Misses tasking-mode edge cases	Use rebuffer_delay()
Tries to reconcile rebuffer_delay against a hand-measured impulse latency	They answer different questions; overlap changes the output rate, so the numbers legitimately differ — chasing the gap is a time sink	rebuffer_delay returns input-rate samples and is an alignment delay, not an impulse-peak position; report it as the buffer latency and add downstream filter group delay separately (see buffering-multirate.md)

---

Copyright 2026 The MathWorks, Inc.

---