12.2 Input Configuration and Source Integration

🔰 BEGINNER LEVEL: Connecting Your Source

Analog Line-Level Input

The simplest connection: head unit RCA preamp outputs → DSP analog inputs.

Signal level: Typically 1–6V RMS depending on head unit. The DSP input stage accommodates this range with adjustable sensitivity.

RCA cable quality: For DSP input connections (relatively short runs, inside the cabin), quality matters less than for long amplifier runs. Standard quality shielded RCA is adequate. Focus budget on the longer power wiring runs.

Mono sum for subwoofer:

If your DSP has separate L and R inputs but you want a mono subwoofer signal, configure the DSP's routing matrix to sum both channels to the subwoofer output:

Sub_out = (Input_L + Input_R) / 2

The division by 2 (−6 dB) prevents overload. Most DSP software handles this automatically in the routing matrix.

High-Level Input (Speaker-Level)

For factory head units without RCA outputs, the DSP connects directly to speaker wires.

The high-level input converts speaker voltage to line-level:

Sensitivity range: typically 1–20V RMS
Input impedance: 30–100kΩ (high, to not load the speaker output)

Where to tap:

Tap at the head unit's speaker terminals, not at the speaker. This ensures you get the full-range signal before any factory amplifier processing.

If vehicle has factory amplifier: Tap after the factory amplifier at the speaker terminals. The signal will have factory EQ applied — plan to correct this in the DSP's EQ section.

🔧 INSTALLER LEVEL: Factory Integration Strategies

AccuBASS and Bass Recovery

Factory head units apply heavy bass cuts at higher volumes (loudness compensation in reverse — bass reduction as volume increases) to protect small factory speakers. When you tap the factory signal and amplify it, these bass cuts carry through to your subwoofer.

The symptom: Sub sounds good at low volume, disappears at higher volumes.

Detection: Record the factory signal output while increasing volume. If bass decreases relative to midrange as volume increases: factory bass management confirmed.

Solutions:

Option 1 — AccuBASS (Helix/Audison): A proprietary algorithm that detects and restores factory bass reduction in real time. Available in Helix and Audison DSPs.

Option 2 — Manual EQ: Measure the factory output at different volume levels. Build an EQ curve that applies progressive bass boost as the DSP detects rising input level (dynamic EQ). Complex but effective.

Option 3 — Find a constant-level factory tap: Some vehicles have a constant-level audio output (navigation system audio output, auxiliary input signal path). Research your specific vehicle.

Illustration note: Signal flow from factory head unit through integration DSP to aftermarket amplifiers showing EQ correction

Input Mixing Matrix

The routing matrix defines how each input connects to each output, with adjustable gain for each path:

Output_n = Σ (Input_m × Gain_mn)

Example configuration for 2-input → 6-output system:

Output 6 sums both channels at −3 dB each — creating a mono subwoofer signal at the correct level.

⚙️ ENGINEER LEVEL: Sample Rate Conversion and Dithering

Sample Rate Conversion

When the DSP's internal sample rate differs from the input, sample rate conversion (SRC) is required.

Synchronous SRC: Input and output clocked from the same reference. No jitter introduced. Used when input is from a digital source (S/PDIF, USB audio).

Asynchronous SRC: Input and output have independent clocks. Rate conversion involves resampling with interpolation/decimation. Introduces subtle artifacts.

Polyphase FIR SRC filter:

Upsampling from 44.1 kHz to 96 kHz requires: 1. Upsample by 320 (insert 319 zeros between samples) 2. Apply anti-imaging FIR lowpass at 22.05 kHz (the original Nyquist) 3. Downsample by 147

Net ratio: 320/147 ≈ 2.177 (matches 96000/44100)

The FIR filter must have sufficient transition band performance to prevent aliasing. Typical automotive DSPs use 128–256 tap polyphase FIR for SRC — transparent quality.

Dithering:

When reducing bit depth (24-bit to 16-bit, or for output DAC precision), dithering adds shaped noise to minimize quantization distortion.

TPDF dithering:

output_n = round(input_n + uniform_noise₁ - uniform_noise₂)

Where the two independent uniform noise values create a triangular PDF (TPDF). This linearizes quantization error — converting harmonic distortion into random noise, which is less audible.