Focusrite Scarlett Input and Output Polarity are Correct

Tests over the past two nights showed the Focusrite Scarlett 2i4 has correct polarity on inputs and outputs.  The world of Focusrite Scarlett using musicians can rest easier now.

To perform these tests, I used a modified version of the SpeakerPop signal created for the SpeakerPop app.  My modified version has lower amplitude (safer) and no DC offset (safer).  I call this signal SpeakerPopSofter.

The SpeakerPopSofter signal looks like this in Audacity:

It looks like an inverse (downward sloping) sawtooth wave, but you need to examine the time scale.   The upward spikes only occur about once per second.  Most devices do not have flat response below 1 Hz, so instead of uniformly ramping down, as the digital signal does in Audacity, they will drop to zero at a rate determined by the low frequency response of the device.

Recorded by the ARTA Triggered Signal Recording feature, the results looked exactly like they should have.  I first recorded using a balanced cable from my Tact preamp into the Focusrite, and then with an unbalanced cable.

I played the SpeakerPopSofter signal through a Sonos Connect digital output, into my Tact 2.0 RCS Preamp and through its analog outputs (which I rarely use except for testing purposes...I use the AES digital outputs otherwise).  The Tact has both balanced and unbalanced analog outputs.  I tested those over 14 years ago to verify they are correct.  (The Tact also has a polarity control in the Menu.)

The balanced connection was direct from the Tact to the Focusrite through an XLR terminated cable made by Blue Jeans Cable.  For the unbalanced cable, I used an RCA to mono 1/4 plug adapter to plug into the combi inputs of the Focusrite.  I zoomed the entire 5 second recording to full scale, and in both cases the Speaker Pop signal appeared in positive polarity, with downward fall suggesting approximately 10 Hz multipole low frequency cutoff.

I strongly expected results would not work out this way.  Based on previous tests, and what seemed like reasonable assumptions, I expected to see the input polarity inverted.

But when possible one should always do the tests instead of relying on expectations.  Just before this polarity tests, I noticed I was having a problem with my bedroom video monitor playing through HDBaseT setup from my computer in the kitchen.  The video colors on my desktop were periodically shifting.  I'd seen this a few weeks ago but then the problem went away, I figured it might be caused by sunlight hitting the back of the monitor triggering some "autodim" feature, or it could be the 2 month old HDBaseT unit, which caused hum on the first system I used it so I swapped it to the bedroom system where the TV only connects to the audio via fiber optic, but perhaps the new HDBasteT unit, which I paid $350 for, had a fault beyond the lack of total ground isolation that was causing the problem.  I think it would still have warranty but especially now, it would be a hassle to ship it back for replacement, if they'd even agree it was faulty.  And then, the ultimate fear, my 4+ year old Samsung TV might have an issue!  I'd want to sort that out quickly as I might still have a few months on the extended warranty.  BUt mostly, I suspected the new HDBaseT unit, under the idea that what has changed most recently is probably the culprit.  Well, I swapped the new and possibly faulty HDBaseT unit with the old one that I had used in the bedroom for years, and, guess what, the problem went away.  Typical audiophiles would call it a day after such a test, and never use the now "proven" faulty unit again.  But one should never assume that in the process of swapping things other things might have changed too.  Maybe it was just the power cycling!  Or maybe it was the time of day, or the temperature of the units involved that shifted just enough during the tests, or the quality of the AC power.  Anyway, there are an infinite number of possible relevant tests one might do, depending on your lack of confiedence in them.   But the easiest would be just to swap back to "A" after trying "B" after "A."  Making the minimum reliable test sequence A/B/A, never just A/B.  So I did swap back, and guess what, the problem went away!  Now, sadly, it may come back, and if it does come back I might be inclined to call it done at that point.  But in this case, there's one more fact I knew.  As I was starting to replace the units, I started pulling out the HDMI cable, but then decided I didn't need to change that, I'd just use each unit with the HDMI cables they were currently with.  But at that point, I started plugging the HDMI connector back in, and it might have been a little crooked.  So THAT might have been the problem right there, and perhaps even the first time.  I've ordered a higher spec HDMI cable whose connector might be more secure.  So never jump to conclusions, always do A/B/A, and try to remember if not write down every last detail.

Because there is still a possibility my speakers are out of polarity, as the SpeakerPopSofter signal was playing, I ran the SpeakerPop app on my iPhone.  It indicated positive polarity everywhere, strangely even in back of the Acoustats.  So I turned off the subwoofers (which are behind the Acoustats).  Then I measured positive polarity in front of the Acoustats, and negative polarity behind the Acoustats, just as it should be.  I hadn't actually tested the polarity in about a year or so, and many changes have been done since then.  (However, I always test or use cable marking to ensure speaker cables are connected in correct polarity.  Most of my equipment has been checked for polarity at one time or another, even my Oppo UDP 205.  The only thing I've ever found having incorrect polarity is an iPod.  Not trusting my hearing in these regards, I perform only objective tests, using an oscilloscope if possible, but speaker outputs are remarkably hard to interpret with a microphone and oscilloscope because of phase shift and reflections, so on speakers I generally use the SpeakerPop app.)

As this proved to my satisfaction that the input polarity of the Focusrite is correct, I then believed that it must then be that the output polarity is incorrect.  However, that was disproven on the next day.

This time I connected an oscilloscope to the unbalanced outputs of the Focusrite.  I played the SpeakerPop signal using the Groove Music app (the default music player when you click on music files) on my Windows 10 laptop.  The Focusrite had already been selected as the audio output, as I verified by clicking on the volume button.

However, I immediately discovered that outputs 3 and 4 had nothing but noise on them.  Only outputs 1 and 2 had the speaker pop signal.  I used Input 1 on a Rigol oscilloscope to record the signal, and I verified that Input 1 was not "Inverted" in the input dialog.

At this point (haha) I believe it is the ARTA program, for some strange reason, that puts out signals in inverted polarity.  I had long discounted that possibility, assuming ARTA was the most technically sophisticated and therefore likely correct item in this story.  But apparently not.

However, I do not recall which outputs I previously used when testing the Acoustats!  Possibly I used the 3 and 4 outputs, which did not put out a signal using Groove Music.  I will need to check that using ARTA.

Update: I must have used output 1 or 2 (first left/right pair on the 2i4).  The second set of outputs does not put out any signal using ARTA.  I have also directly verified that outputs 1 and 2 put out the ARTA "Transient" signal in negative polarity, using my oscilloscope.

So the polarity inversion when using Triggered Signal Recording is a "feature" of ARTA having nothing to do with the Focusrite Scarlett 2i4.