manisandher
Addicted to Fun and Learning
- Thread Starter
- #181
ChatGPT o1's thoughts on this:
It can be surprising at first to see an improvement across the entire spectrum just by compensating for a filter at 0.07 Hz. But remember that any analog or digital filter—no matter how low its cutoff—is going to impart some phase shift that can extend well above its nominal “corner” frequency. A minimum-phase filter in particular does not simply affect frequencies near the cutoff; it shifts the time/phase relationship across the entire audio band, albeit very subtly.
Here’s why that can help your nulls right up to 20 kHz:
It can be surprising at first to see an improvement across the entire spectrum just by compensating for a filter at 0.07 Hz. But remember that any analog or digital filter—no matter how low its cutoff—is going to impart some phase shift that can extend well above its nominal “corner” frequency. A minimum-phase filter in particular does not simply affect frequencies near the cutoff; it shifts the time/phase relationship across the entire audio band, albeit very subtly.
Here’s why that can help your nulls right up to 20 kHz:
- Phase Alignment in the Time Domain
- Even a small subsonic high-pass filter changes the time alignment (phase response) of your signal in ways that can ripple throughout the audible spectrum.
- By applying a matching filter to the reference file, you bring its overall phase response closer to what the RME capture chain is doing.
- Once the two signals have better phase alignment, everything lines up more closely—so your null residual can improve even at mid and high frequencies.
- Group Delay
- In a minimum-phase design, group delay (the time delay of different frequency components) cannot be isolated only to sub-10 Hz. There’s inevitably a gradual shift that extends upward.
- When you apply the same filter offline to your reference file, you effectively match the RME’s group delay across the band, reducing small time-domain mismatches you might otherwise see at higher frequencies.
- Very Small but Cumulative Effects
- At -120 dB or deeper, even minuscule differences in phase or amplitude at higher frequencies can show up in a delta spectrum.
- Tweaking the low end’s filter alignment can remove subtle “wrinkles” across the entire signal chain.