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iDevices are DC Coupled?
According to Sweetwater, "All Apple phones and tablets with headphone jacks are DC coupled. Check out apps like CV Mod, Brute LFO, Trigger Box, or just search for Control Voltage in the App store. "
Of course, all new iDevices are missing the headphone jack...
Comments
So far, many of us were stuck with iOS 13.7 for lack of a no-brainer iOS 14 release.
Now we're stuck with our older iDevices as well 😂
Hint: Most Windows tablets can run Ableton or Bitwig quite well and they usually have headphone jacks, expandable RAM and an SSD that can be swapped by the user. It's a totally different experience from iOS though.
I’m sure someone will correct me if incorrect, but I don’t think they are. The existence of these apps doesn’t prove that they are. For trigs and gates they’re probably ok in most cases. For modulation they may be enough (mod depth). For pitch I’m almost certain that they will fail. It’s all about the range they provide to be usable in a standardised environment (eurorack or others). You may be able to tweak it to cover an octave or two, but the point of DC coupled i/o is that you don’t have to and full range is available.
I've just done measurements on an iPhone SE and an iPad 6, neither of them seems to have DC coupled output. There's obviously a high pass filter on the output stage, even with "measurement mode" enabled.
Might work for triggers and modulations though...
I remember someone on this forum being very vocal about this, saying something like "everyone already has a DC-coupled interface in their pockets". If I remember correctly, they also said that it worked with the headphone dongles. I can't find the post right now though, but maybe you'll have better luck.
Edit:
It looks like I was attributing all of these posts to the same person, and thought that they had provided some proof, but it's not the case:
https://forum.audiob.us/discussion/comment/869825/#Comment_869825
https://forum.audiob.us/discussion/comment/938081/#Comment_938081
https://forum.audiob.us/discussion/comment/849784/#Comment_849784
Brute LFO was awesome
I’ve seen reports that the headphone output works, but needs amplification:
https://modwiggler.com/forum/viewtopic.php?t=122155
I've used triggers from mirack from an iPad mini (4?) and iPad Air 3 into the clock sync of the microfreak and the SQ-1. Both work after some tweaking.
I just tried looking, but cannot find it at the moment, but the headphone adapter also works.
Is the headphone adapter still capable of outputting CV when running on newer hardware/recent OS versions? I did an experiment where I dug out my old apple 3.5mm headphone adapter and I tried to send CV from the dongle over regular mono TS patch cables to my eurorack system (input patched directly into a gain/amp module to boost the signal), and I struggled to get static voltage offsets out of this setup. I was using Drambo to produce the signals. But the bigger issue is that I had a hard time physically holding the cables in place long enough to get a stable connection --- every time I changed my hand grip on my phone, it seemed to break the lighting connection and inject a burst of noise into the signal.
Audio is a voltage so in effect can be used but unless it’s DC coupled you won’t get the range and accuracy needed for controlling pitch accurately and so on.
@wim may want to change the category to be more visible to the whole forum, thanks.
I’m confused by your comment here. My understanding is that a DC coupled audio output would be capable of outputting unchanging, static voltages, whereas AC coupled outputs only transmit changes in voltage. With AC coupled outputs, for example, a static +1V signal would be no different from a 0V signal. On the other hand, a 100 Hz signal oscillating between +/-1V can be faithfully output from both AC and DC coupled outputs. As far as I understand it, it is precisely audible signals (as opposed to CV) that can be reproduced faithfully without DC coupled outputs. Audio, as opposed to sub-audible control voltage signals, will not present any problems for AC coupled outputs. And in fact, I am able to use the lightning to 3.5mm dongle to transmit audio rate signals from my iPad to my eurorack gear mostly without issue. The problems appear when I try to transmit CV.
Is it the original dongle from Apple?
With both the Apple headphone dongle and older devices with the built-in headphone jack, you have to use a 1/8" stereo-to-dual mono adapter FIRST, then plug your normal 1/8" mono cable to go to eurorack. The headphone jack will give you 2 CV outs. You don't need to boost it or anything, the range is just limited (about an octave or 2 in pitch if I remember correctly)
Did you manage to make this setup work ? What apps did work ? I tried a couple of apps that said they support CV out and tried Drambo as well. The output was always DC filtered. I checked with a scope and a multimeter and never got any DC out of the headphone jack. Tried on both an Air 2 running iOS 12 and a 9th generation iPad with the latest iOS 17.x on it. I wonder if there is some “damage limiter” built into newer iOS versions.
@catherder I haven't used a multimeter, but anecdotally this result seems more consistent with what I've been experiencing. I only tried Drambo, so I didn't know if it was the app or the OS filtering the output. But in any case, I haven't been able to get good CV signals.
Yeah it works fine...I've used both the built-in headphone jack on various devices over the years and the Apple headphone dongle that came with my last iPhone. I've used various CV apps (CV Mod, Brute LFO, etc.) and modules in MiRack with a little Moog Werkstatt and some Littlebits modules. Sounds like you're either using a 3d-party dongle (not Apple branded) or the wrong 1/8" stereo-to-dual mono splitter.
That sounds encouraging. I am not using any dongles, so the problem must be the splitter. Does the splitter you are using have a TRS or a TRRS plug to connect to the headphone jack ?
I've used both succesfully. For instance using a TRRS headphone/Mic splitter then a TRS stereo-to-mono splitter gave one mono input and two CV-compatible mono outputs, so I could use my ipad to add FX and a additional LFO to a hardware monosynth.
Today I used an oscilloscope to check the output from my 9th Gen iPad's 3.5mm aux port while running iPadOS 18.3.1. I plugged in a 3.5mm TRS cable, and used alligator clips on the tip and sleeve to probe the signal. In the image attached, you can see that a square wave LFO generated in Drambo is not faithfully reproduced in the output signal. The output appears DC filtered.
To me that looks like the output (or the oscilloscope input) may have a high pass filter of some kind enabled!?
(If you use a 1-pole high pass filter at 50hz on a square wave it looks almost identical to the picture).
Yes, it looks like a high pass because there probably is a high pass in the signal chain somewhere. But the high pass must be happening before it reaches the oscilloscope. You can see in the photo that in the bottom right corner of the oscilloscope's screen, it says "DC" to indicate that it's set to DC-coupled mode, as opposed to AC-coupled mode. This means that the scope is not performing a high pass (or AC coupling, which is the same thing AFAIK). And I can confirm that it's working properly because when I probe a source that is verified DC coupled (ie. outputs on my eurorack system), the oscilloscope correctly detects and displays the DC signal.
Since the high pass isn't happening on the oscilloscope side of things, I would reason that the iPad itself has a high pass filter before the output. If that were the case, then the iPad's 3.5mm output would be AC coupled, not DC coupled.
Gotcha, one thing to try here would be to enable measurement mode / disable iOS processing by for example using AUM and enable its high quality mode in the settings and then loading Drambo as an AUv3 and see if it makes any difference, this as iOS by default does some pretty heavy processing on the signal when using 3.5mm dongles, this processing is NOT done when using regular audio interfaces.
This is a good suggestion but it unfortunately did not work. I just ran some tests on different iPads, and all showed the DC blocker / high pass filter behavior. My oscilloscope was working well with DC because I could see the DC provided for the mic.
The main test was using Drambo with a simple knob connected to the output. I can see the VU meter in AUM showing the DC offset but nothing DC wise get's out - even in measurement mode. On the pre-AUM device I tested with CV Mod, but with no positive result either.
I tested the following iPads (all with headphone jack):
In the test setup the oscilloscope was connected to the headphone jack via different cable configurations:
So my conclusion is that there is either some "magic" configuration I am not aware of, the DC output works only on very specific iPads / iPhones / iOS versions, or this is just an "urban myth". The iPad / iPhone headphone output could very well be DC coupled as this saves components (capacitors). But looking at the datasheets of some audio DAC chips that are designed for DC coupled operation, many of them have built in DC compensation.