Comparisons

Well, when I compared free IR files with a commercial library that I have from Ernest Cholakis, I was struck by how disappointing many free IRs are. They often have a very specific "color". They can have have high noise levels and tend to be simply truncated at the end, instead of being nicely faded out just before the background noise takes over.

So when I started to think about creating an impulse library myself, I wanted to go for maximum resolution, lowest noise levels and for a pristine and transparent sound.

For the hardware links I used two separate DAW's with RME audio cards. They were connected via ADAT to an RME ADI-8 DD digital format converter, which in turn was linked with AES/EBU cables to the digital reverb. All devices were synced with RME Word Clock. For playback and recording I used SoundForge 6. Audio processing was done in 32 bit floating point format, using Waves plugins in SoundForge. I decided not to use my Logic or Cubase SX multi-track applications to keep the digital signal path as simple and clean as possible. 44.1 and 48 Khz versions were recorded and processed separately. No automatic batch processing was used.

With these dry and wet noise files, it is now also easy to compare my final IRs with other, free IRs from the same digital reverb. The results confirmed my earlier disappointments with free IRs: they can be very colored indeed, they can have rather high noise levels (around -70 to -80 dB) and they do not have gentle fade-outs.

For these comparisons, I used the Church program and ran the original, dry white noise through my T600 IR version and through a free IR found on NoiseVault. I have captured a few screenshots from the frequency plots (see below). It should be clear that the curve of the T600 Church program is very much like the curve of the original program. The free IR version however has a serious bump in the lower frequencies, corresponding to the boxy sound. It also has more high frequencies than the original Church program, making it sound too bright for a real church.

I also zoomed in on the start of the IR files in SoundForge. From the wave form is obvious that there already is high frequency content (noise) present around the very first reflections in the free IR, whereas the T600 version displays very distinct and clean spikes, representing the first reflections (echoes) of the original signal. This noise in the free IR is actually starting before the first reflection peak, supporting my suggestion that it is noise what we see here. Furthermore, the first reflections in the IR are not at their correct position: the leading "silence" has been removed. This missing leading silence is actually the "pre-delay" as configured in the preset. So, it really should be present in the IR as well. I discovered that this actually holds for most of the free IR versions of this digital reverb. In the T600 library the embedded predelays, or silences to the first reflections, are accurate to a single sample with respect to the original program.