Expose HiFiBerry DAC+ADC Pro hardware controls in the UI?

[palavido-dev]

Right now the EQ in Vinyl Streamer is entirely software-based (5-band parametric biquad filters applied to PCM before playback). It works well, but the HiFiBerry DAC+ADC Pro actually exposes some interesting hardware-level controls through ALSA that we are not surfacing in the UI today.

Here is what the hardware offers that could be worth exposing:

DAC side (playback)

• DSP Program (digital reconstruction filter): Five options built into the DAC chip itself: FIR interpolation with de-emphasis, low latency IIR, high attenuation, fixed process flow, and ringing-less low latency FIR. These change how the DAC reconstructs the analog waveform from digital samples. Subtle but real differences, especially on transients and high frequencies.
• Digital Playback Volume: Hardware-level volume control (0 to 207).
• De-emphasis Switch: Applies a hardware de-emphasis curve. Relevant for older pressings that used pre-emphasis during mastering.

ADC side (recording)

• ADC Capture Volume: Controls the input gain at the hardware level (-12dB to +40dB in 0.5dB steps). Currently fixed at 0dB.
• PGA Gain: Pre-amplifier gain before the analog-to-digital converter.

The question is whether this adds real value or just creates complexity. The DSP filter selector feels like the most compelling one since it is doing something our software EQ cannot replicate (it runs on the DAC silicon after our code hands off the PCM). The ADC gain controls could also be useful for dialing in recording levels at the hardware stage rather than relying on software gain.

Most of the other controls (volume ramp rates, auto-mute timers, overclock limits, input routing) feel like "set once and forget" configuration that would just add clutter.

Would love to hear thoughts:

• Is the DSP filter selector something you would actually use?
• Would hardware-level recording gain controls be useful, or is the current setup good enough?
• Any other HiFiBerry features you wish were accessible?
• Or is this solving a problem that does not really need solving?