Signal Flow, Routing, Summing, And Dynamics

Advanced DSP Video Companion

Routing, Phase, Crossovers, Dynamics, And Verification uses the Wondom ADAU1701 reference project to expose matrix arithmetic, mute/slew, timing, acoustic filters, sidechains, limiters, FIR/IIR resources, export, reload, and regression.

What You Will Be Able To Do

• Draw and explain the complete audio, control, detector, and meter paths.
• Build and verify an input-to-output routing matrix.
• Calculate stereo-to-mono coefficients and coherent worst-case headroom.
• Switch routes and presets without uncontrolled clicks or pops.
• Read compressor and limiter curves without confusing threshold with output ceiling.
• Choose peak, RMS, combined, filtered, external, and linked detector behavior.
• Bench-verify the DSP's digital nodes and physical analog outputs.

Four Different Jobs

The Reference Bench Program

The first practical version uses one enclosed two-input, four-output Wondom ADAU1701 unit and a full bench oscilloscope. The same logic applies to other DSPs, but block names, number formats, meters, time constants, and analog limits vary.

ADC input L/R
→ input trim and input meters
→ routing / matrix stage
→ four parallel output paths
→ output meters
→ DAC outputs 1–4

1Draw The Complete Signal Flow

Include physical input, analog input buffer, ADC, digital input node, input trim, matrix, processing blocks, dynamics detector, controlled gain stage, output trim, DAC, line driver, physical connector, amplifier, and speaker.

Mark each meter as pre- or post-processing and record its detector type and reference. A meter is evidence only for its own node.

2Build A Unity Reference Path

1. Route input L to output 1 at unity.
2. Route input R to output 2 at unity.
3. Bypass EQ, crossover and dynamics unless required for safe testing.
4. Use a conservative tone and record input and output meter values.
5. Measure physical RCA output with the scope and DMM if appropriate.
6. Confirm that output 1 is physically output 1 and output 2 is physically output 2.

3Read The Matrix In Native Units

Do not assume a display value means percent. Translate the actual unit:

linear gain = 10^(dB / 20)
dB = 20 log10(linear gain)

Useful anchors:

• 1.000 = 100% = 0 dB
• 0.707 = 70.7% ≈ -3.01 dB
• 0.500 = 50% ≈ -6.02 dB

4Prove Coherent Summing

1. Feed identical in-polarity tones to L and R.
2. Set output 3 to 1.0 L + 1.0 R and observe up to +6.02 dB.
3. Set 0.707 L + 0.707 R and observe about +3.01 dB.
4. Set 0.5 L + 0.5 R and observe unity amplitude.
5. Invert one input and observe cancellation.
6. Repeat with uncorrelated noise and representative stereo music.

The worst coherent condition sets the deterministic headroom requirement. Music correlation varies with time and frequency.

5Build The Practical Mono Path

Use 0.5 L + 0.5 R as a conservative starting point for fully correlated center material, then verify anti-phase loss, source-specific EQ or delay, the post-sum meter, physical output voltage, and downstream gain requirements.

A -3 dB-per-input convention can be useful for preserving the average power of uncorrelated inputs, but it produces +3 dB for identical coherent inputs. The target must be stated.

6Test Parallel And Duplicate Paths

Create a deliberate duplicate route to one output, measure the level increase, then remove it. This makes a common configuration error visible: a signal can be routed directly and through another bus at the same time.

7Add Clickless Mute And Switching

1. Capture a no-slew mute or source switch at low level.
2. Repeat with a ramp or slewed mute.
3. Compare linear, constant-dB, RC-style, or available platform options.
4. Test preset load, reset, communication loss, power-up and power-down.
5. Confirm linked channels transition together.

A user fade and a protection mute are different functions. One is chosen for feel; the other is chosen for fault response.

8Build The Dynamics Block As Separate Parts

audio path ─────────────→ controlled gain ─→ output
       └→ sidechain → detector → curve → timing ─┘

Place meters on the audio input, detector envelope, gain reduction, and output. The detector can use the same audio, a filtered copy, another channel, or a linked channel combination.

9Map The Compressor Curve

Use stepped input levels to record the static curve. For a simple 4:1 hard-knee compressor, 8 dB above threshold produces approximately 2 dB of output rise above threshold. Then compare hard and soft knee.

Do not call the threshold a ceiling. Timing, detector and makeup gain are not shown by the static curve.

10Compare Detectors

• Use a short transient and sustained tone at the same peak.
• Compare peak-oriented, RMS-oriented and combined detection.
• High-pass the sidechain and confirm the audible bass path remains unchanged.
• Record the exact detector tap and time controls.

11Adjust Attack, Hold And Release

Use bursts and observe overshoot, gain-reduction rise, hold, recovery and waveform modulation. The displayed time value may be defined differently by each algorithm, so measure the result rather than transferring numbers blindly between platforms.

12Link And Unlink Channels

Drive one side harder than the other. Independent compression can move the stereo image. Linked maximum, average, or other detector rules preserve different aspects of the relationship. Record the link rule instead of only writing “stereo linked.”

13Measure A Limiter's Real Ceiling

1. Set a limiting curve at a conservative digital level.
2. Use a burst to measure overshoot.
3. Add gain after the limiter and show that the intended ceiling is defeated.
4. Raise the digital level only within the DSP's safe path and observe the physical output.
5. Verify that the DAC or analog line stage can still clip after a clean digital limiter.
6. When an amplifier is added, calibrate again at the real gain, voltage, load and bridge mode.

14Document And Revalidate

Record the block diagram, matrix, native coefficient units, meters, summing assumptions, mute sequencing, detector tap, detector type, threshold or curve, ratio, knee, attack, hold, release, channel link, makeup gain, limiter ceiling, analog output, amplifier state, test signals and measured failures.

Revalidate after any preset, routing, EQ, bass-restoration, source, firmware, controller or amplifier-gain change.

Troubleshooting Matrix

Continue

• Print the DSP Routing And Dynamics Workflow
• DSP Signal Flow
• Summing And Mixing
• Compressor
• Limiter