Budget DSP for Sound Systems: What to Check

A budget DSP for sound systems has three jobs: split the bands, align them, and stop the amps from destroying drivers. Cheap units vary least on the first two and most on the third. Check limiting, output level and noise before anything else.

Inputs, outputs and routing

Common formats are 2 in with 4, 6 or 8 out. A stereo three-way needs 6 outputs; a stereo four-way needs 8. A mono system frees outputs for delay stacks or a cardioid sub arrangement, which needs its own delayed and polarity-inverted feed. Check that any input can route to any output and that both inputs can be summed for mono subs.

Crossover filters

• Linkwitz-Riley 24 dB/oct on every high-pass and low-pass. Each side is −6 dB at the crossover point and the two are in phase, so they sum flat when the drivers are aligned.
• Butterworth and Bessel as alternatives; Butterworth sides are −3 dB at the crossover point.
• Steeper slopes (48 dB/oct) are useful for the sub high-pass.
• Fine frequency steps, not a handful of fixed points.

Every sub output needs a high-pass filter. Below port tuning a reflex box unloads the cone and excursion rises 12 dB per octave for constant SPL; the HPF is what keeps it inside Xmax. See crossover basics and Xmax and excursion.

EQ and delay

Several bands of fully parametric EQ per output (frequency, gain, Q), with shelving filters for the tops. Input EQ lets you make room changes at a dance without touching output EQ.

Delay per output aligns sources at different distances, such as a horn-loaded sub with a long internal path against a direct-radiating top. Sound covers about 34 cm per millisecond, so check the maximum delay and step size. Polarity invert per output is essential. See phase and polarity.

Limiters

This is where budget units differ most, and what saves drivers.

• One per output, not just a master.
• Peak limiter with fast attack, to catch transients and keep the amp out of clipping.
• RMS limiter with long time constants, to cap the average power that heats the voice coil.
• Adjustable attack and release, since sub and top time constants differ.
• Threshold in dBu or volts, so you can calculate it from amp gain and driver rating. For example, 1800 W into 8 Ω is √(1800 × 8) = 120 V RMS at the speaker terminal.

Set-up is covered in limiter settings for subs; the thermal reasons in voice coils and heat.

Output level and gain structure

Check maximum output in dBu against your amp's input sensitivity. If the DSP clips before the amp reaches full power, you lose headroom. If it can drive the amp far past full power, the limiter thresholds are all that stand between the amp and the drivers. See amp matching.

Lock-out, presets and software

• Password or lock-out mode, so nobody changes crossovers or limiters mid-dance.
• Settings retained through a power cut, and several stored presets.
• PC software that runs on your OS, is still supported, and saves setup files you can back up.

Analogue performance

Balanced XLR in and out, a published dynamic range or S/N figure, and converter sample rate and bit depth. Listen at idle with the amps up: hiss shows first through sensitive compression drivers. Check for turn-on and turn-off thumps, and power the processor on before the amps and off after them.

If you want hands-on analogue band-splitting with no software, the XR204 is a 4-way stereo active crossover with 2 × XLR in, 8 × XLR out, >106 dB dynamic range and <0.004% THD+N. Its spec sheet lists no limiters, so protection has to come from the amps or a separate limiter.

Checklist

Sources: XR204 figures from the Wavemark XR204 spec sheet. Filter summation, excursion and P = V² / R relationships are standard electrical and acoustic engineering. Values rounded.