DSP Signal Flow
Canonical Definition
Signal flow is the ordered path and branching structure through which audio, control, and detector signals move from source to output, including every gain, routing, processing, metering, conversion, and protection stage.
A control is meaningful only when you know where it sits, what feeds it, and what comes after it.
Four Paths That Must Not Be Confused
| Path | What it carries | Common examples |
|---|---|---|
| Audio path | The samples or analog signal being heard | ADC, mixer, EQ, crossover, delay, compressor gain stage, DAC |
| Control path | Values that change processing | potentiometer, mute state, preset selector, threshold, matrix coefficient |
| Detector or sidechain | A signal used to calculate gain or trigger an action | peak envelope, RMS level, linked-channel maximum, external sidechain |
| Meter path | A tapped view of one node | input meter, post-EQ meter, gain-reduction meter, output meter |
A Complete Car-Audio DSP Chain
source and source processing
→ analog or digital input interface
→ input buffer / ADC / digital receiver
→ input trim and input meter
→ routing and summing matrix
→ crossover, EQ, delay and other processing
→ dynamics detector and gain stage
→ output trim and output meter
→ DAC / analog line driver
→ amplifier input and output
→ loudspeaker, enclosure and vehicleEvery arrow is a possible level, noise, latency, routing, or clipping boundary. A clean meter after the ADC does not prove the analog input buffer was clean. A clean pre-DAC meter does not prove the line driver or amplifier input remained clean.
Processing Order Changes The Result
- EQ before a detector: the detector reacts to the equalized spectrum.
- EQ after a limiter: a positive boost can defeat the limiter's intended ceiling.
- Summing before a meter: correlation-dependent level growth appears at that meter.
- Meter before summing: the meter cannot show the combined node's overload.
- Mute before an unstable block: the unstable block may still feed another route.
- Mute after the final DSP stage: the DAC or analog bypass may still require separate control.
Parallel Paths And Hidden Gain
A signal can reach the same output through more than one route. Two identical unity-gain paths add coherently and can raise voltage by 6.02 dB. Parallel processing is useful only when the blend, latency, polarity, phase, and total gain are deliberate.
Trace every output backwards. If two branches rejoin, identify the coefficients and expected worst-case correlation.
Fixed Point, Floating Point, And Physical Boundaries
Word length and accumulator design affect internal range and precision, but the implementation must be documented rather than guessed from a marketing label. For example, the ADAU1701 accepts normal audio data into a 28-bit core representation with additional processing headroom and uses a 56-bit accumulator for multiplication and accumulation. That does not remove the ordinary ADC, DAC, serial-output, and analog-stage ceilings.
Floating-point processing also does not make the complete system unclippable. Output conversion, deliberately saturated blocks, coefficient ranges, analog stages, and downstream hardware still have limits.
Verification Workflow
- Label every physical input and output.
- Build a unity-reference path with unnecessary processing bypassed.
- Verify one route and one physical output at low level.
- Add summing and monitor the actual summing node.
- Add filters, delay, and EQ while checking level before and after each level-changing stage.
- Add dynamics and identify both detector and controlled audio paths.
- Verify DAC or line output externally.
- Save a diagram, preset, meter map, and physical connector map.