I’d like to see SoX considered. I don’t know about its availability, but I believe it’s open source. It’s available in Foobar on Windows and XLD on Mac. Also in Audacity, but not with all available settings.
I ran a test of SoX against Izotope RX4 and r8Brain Pro, converting a 96KHz 24 Bit 3:45 song to 44.1 32 bit float in each of the programs, using settings closest to “ideal” as shown on infinitewave. I was shocked to find that all 3 results nulled exactly to each other down to about -110db. I made difference files of the nulls, and turned them up 60db in Wavelab to hear what was there, listening in headphones at a loud volume, but there was no discernable audio in the differences at all. I would call the 3 programs identical at these settings. I could be wrong, but I don’t think anyone could tell these 3 apart in blind testing, using these settings, with nulls down to -110.
These are the settings I used:
Izotope RX4
filter steepness 2000
cutoff shift 1.0
time to convert 0:38s
SoX in Foobar
passband 99%
phase 50% (linear)
time to convert 0:13s
r8Brain Pro
linear phase
ultra steep mode
time to convert 0:26s
Crystal Resampler Ultra
time to convert 0:55s
The closest in XLD is VHQ Linear Phase, not quite as close as Foobar for some reason.
With these settings, as well as being equivalent in quality to Izotope imo, SoX is also almost 3 times faster than Izotope, and more than 4 times faster than Crystal Resampler, which makes a big difference when converting 96k and especially 192k albums.
The version of SoX in Foobar has sliders for continuous adjustment of passband and phase, and a control for aliasing.
Other more euphonic, analog-like settings could probably be compared, since they have become more common, but that should be possible with continuously variable parameters.
I’d be ok with Izotope (although I much prefer the speed of SoX), but if Izotope is a problem, and SoX is less of a problem, I think SoX could be a great alternative.