I haven’t purchased a Little Labs I.B.P hardware box yet… But recently I was lucky to find out that an audio engineer, friend of mine, had one (thanks a lot Marco Posocco!).
I immediately borrowed it and put it on the test bench!!!
What is the best way to describe the I.B.P.? I guess it can be thought of as an analog mono delay, capable of continuously shifting in time the signal from 0 up to almost 4 ms . Therefore it can re-align frequencies out of phase but, as you might already foresee if you’ve read previous post, it can only provide you with perfect phase for ONE FREQUENCY at a time.
I tested the unit really in depth and I was able to understand (and see on the oscilloscope) that the unit is not consistent in performing its task. At first I imagined the delay was one and the same for all frequencies. No, it doesn’t work like that.
I set the knob at “full shift”, CW position, no buttons pushed and then I sent different frequencies to a pair of channels of my digital recorder, one with the IBP inserted. Here’s what I found: at 40 Hz, the delay is 190 samples (@ 48 kHz). Or 3,958 ms, if you like. At 50 Hz the delay is 182 samples. At 63 Hz is 173 samples. At 80 Hz is 162. As the frequency rises, the delay decreases.
Now, the quarter wavelength for 40 Hz is 300 samples (@ 48 kHz). 50 Hz is 240, 63 Hz is 190, 80 is 150 samples…
| Frequency | 1/4 wl | IBP Max. Shift |
| (Hz) | (samples/48k) | (samples/48k) |
| 40 | 300 | 190 |
| 50 | 240 | 182 |
| 63 | 190 | 173 |
| 70 | 171 | 169 |
| 72 | 167 | 167 |
| 75 | 160 | 165 |
| 80 | 150 | 162 |
| 100 | 120 | 147 |
| 125 | ||
| 160 | ||
| 200 | 60 | 102 |
| 250 | ||
| 320 | ||
| 400 | 30 | 91 |
What this means? It means that I.B.P. won’t be able to re-align completely frequencies below 72 Hz. For those frequencies I.B.P. doesn’t cover the full span of 1/4 wl: if the misalignment occurs within the first 1/4 wl and its dimension is beyond capability of the I.B.P., there is no way to re-align.
Let’s better understand this with an example: you want to tune (i.e.align) @ 50 Hz a “Bass D.I.” track and a “Bass Cab Mic” track (same take of course!). On the D.A.W. timeline you measure a time shift of 215 samples between the two, which is less than the first 1/4 wl of 50 Hz. The “D.I”. preceds the “Cab Mic”. Here’s the point: the I.B.P. inserted on the “D.I.” won’t be able to delay the track more than 182 samples and therefore it’ll remain short of 33 samples. Probably not a great deal for 50 Hz but still… phase is not perfect.
Let’s see another example: you want to tune @ 80 Hz a “Kick In” mic and a “Kick Out” mic: adopting the procedure explained in previous post, you measure a difference of 124 samples between the two (still within the first 1/4 wl). In this case, the I.B.P. applied to the preceding “Kick In” track will provide perfect phase because it can delay 80 Hz up to 162 samples and the difference is only 124.
Let’s consider now a different scenario: you have a third, “FrontOfKit” mic which you want to tune @ 80 Hz as well. By the way: is it a condenser? A ribbon? Consider that! Whatever the typology, time shift between the two tracks is 173 samples, which falls within the second 1/4 wl of 80 Hz and beyond capabilities of I.B.P.
BUT… if you insert I.B.P. on the “FrontOfKit” track and reverse polarity by pressing the “phase invert” button, then youll’be able to tune that mic as well.
Going back to the first example, bass alignment @ 50 Hz, you could do the same i.e. realign the “Bass Cab Mic” track if the difference fell within the 182 samples preceding the 1/2 wl, 480 samples (between 298 samples and 480).
How does all this theory sound in real life out of a pair of speakers?
Well… It sounds good.
Real life test:
Sent a 50 Hz, -20 dBu tone to my BSS AR-133 active D.I. box linked to a bass head+8×10” cab bass system. Placed an Electro-Voice RE20 and a SOLOMON MiCS LoFreQ in front of the cab (on different speakers). Recorded the three tracks.
Compared to RE20, D.I was misaligned, almost 180° out of phase. LoFreQ as well.
Flipped the phase on both.
Result: RE20 preceds D.I. by 17 samples (FS: 48 Hhz); also preceds LoFreQ by 60 samples.
What I did? 1): on the timeline I nudged forward the D.I. track by 17 samples to align it with RE20. 2): I duplicated the RE20 track. 3): one copy of RE20 I nudged back 60 samples together with DI to align both with LoFreQ. 4): the other copy I left it where it was and I inserted IBP on that channel (remember I work on an analog console, therefore I don’t deal with latency).
I listened to both versions.
Version 1:
- D.I. track nudged back by 43 samples (17-60)
- RE20 track nudged back by 60 samples (I.B.P. on bypass!!!)
- LoFreQ track
Version 2:
- D.I. track nudged back by 43 samples (17-60)
- RE20 track NOT nudged and with I.B.P. active
- LoFreQ track
Version 2 sounded better, without any doubt. More open and just… better!
This test was really important for me to understand if IBP is worth having: my answer is yes.
On version 1 ALL frequencies of RE20 were nudged back by 60 samples.
On version 2, by virtue of I.B.P. non linearity, NO frequency was delayed by the same amount and this action was definitely sounding better. Period.
Since I had only one I.B.P., I quit for a moment RE20 and inserted I.B.P. on a copy of D.I. track non-nudged and mixed it with LoFreQ. Again with oscilloscope I found perfect phase.
Version 1:
- D.I. track nudged back by 43 samples (-17+60)
- LoFreQ track
Version 2:
- D.I. track NOT nudged and with inserted I.B.P. active
- LoFreQ track
Again version 2 sounded better.
Addendum
First thing I noticed is that when the circuit is inserted, a “phase issue” occurs by default. What this means? I’ll explain what I did.
At the beginning of my test I ran a 440 Hz tone from the console’s oscillator into channel #21. I took the tone out of the insert send of that channel and reinjected it straight into the insert return of channel #22. Why 440 Hz? No particular reason: it was the last setting used.
I took the #21 & #22 direct outs and brought ‘em to the oscilloscope: the two sine waves were in perfect phase.
Then I patched the Little Labs I.B.P. between insert send #21 and insert return #22… looked at the oscilloscope: the sine wave of channel #22 (the one with the I.B.P.) was slightly delayed. Ooops… by default NO PERFECT PHASE there!
OK: the I.B.P. circuit introduces a phase shift by default when inserted and with the knob at minimum, fully ccw.
Could this be a problem? Maybe… I wondered: “which frequency is put 180° out of phase because of this delay?” An higher frequency, for sure! I increased the frequency on the oscillator until I reached a 180° out of phase situation on the oscilloscope: it happened at 5228 Hz. But the knob was at minimum, i.e. no delay, which is not the working condition of I.B.P.