I'd like to design my usb-c audio preamp, but that would be possible only if the analog audio comes directly from the phone, so that my piece of hardware can take the power supply from the usb-c and spit out an amplified version of that signal

I looked around the internet and found a lot of contrasting information. Do the dongles receive a digital stream and convert it to analog audio, or are the dongles just a mechanical conversion between connectors and the dac is in the system (typically a phone)?

What is the industry standard practice? What does the USB-IF say about audio via usb-c? How an host can recognize when an analog output is required?

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    \$\begingroup\$ Hit the edit link and add a photo and link to an example of what you are asking about. \$\endgroup\$ – Transistor Nov 15 '18 at 17:53
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    \$\begingroup\$ I'm guessing you're talking about Apple USB-C to Audio dongles? The USB-C standard does not include any pins for audio connections. That dongle almost certainly contains the DAC (citation needed). \$\endgroup\$ – Chris Fernandez Nov 15 '18 at 18:14
  • \$\begingroup\$ @Transistor it is a pretty common system in all android and apple smartphone these days \$\endgroup\$ – valerio_new Nov 15 '18 at 18:30
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    \$\begingroup\$ @ChrisFernandez That is incorrect. USB-C includes both a digital audio profile, as well as Audio Accessory Mode where traditional analog audio signals are routed over the D+ and D- pins (for left and right channels). So yes, it does include pins for audio, it's in the official USB C spec. Remember, USB-C isn't just a connector with pinouts, it permits dynamic rerouting of signals (which began out of necessity to support reversibility without symmetry, which would cut the effective number of pins in half) and many of the pins have multiple alternate functions. \$\endgroup\$ – metacollin Nov 19 '18 at 8:02
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    \$\begingroup\$ How is this a use question? OP SPECIFICALLY SAYS THEY ARE TRYING TO DESIGN ONE, and is asking for industry standards. \$\endgroup\$ – Passerby Nov 20 '18 at 23:49

It depends. There are a few different ways to send audio over USB-C.

The first way is the same way USB to Audio dongles work right now: The dongle is an actual USB device itself, one that supports one of the USB Audio profiles. This is a DAC that appears on a USB bus as an audio device, and will work just as readily on a desktop or any other device with USB-C and an OS that supports it.

So, in this case, the DAC is very much in the dongle itself. It's more like a USB 'sound card', and will often be a little bulkier and more expensive.

However, for most phones with a USB-C to 3.5mm dongle, the DAC is in the host device, meaning that the signals going out of the USB-C port and into the dongle are, in fact, analog ones. They're the exact same signals that get sent through any other 3.5mm headphone jack, and generated in the exact same way, by some kind of DAC inside the device with the USB C port. So in most cases, yes, the dongle is merely an electrical/mechanical connection that is adapting a USB-C port to a 3.5mm audio jack directly.

It would not make much sense to use a dongle that had an entire DAC and USB audio profile chipset inside it, as this would incur a good deal of cost, consume more power, and be physically bulkier. If the device is a phone, then it already must have a DAC of some kind or it would be unable to drive the speaker that lets you hear the person on the other end of a phone call. These usually come in the form of single chip solutions and will include multiple input and output channels, so this can be done effectively for free.

I do not know why, but there seems to be a ton of misinformation regarding this online which it sounds like you've likewise encountered. What I see most often is something along the lines of 'USB-C is digital audio only' or 'USB-C doesn't have any audio pins'.

...and neither statement is true, at least according to the USB-IF, the only body who has any authority on the matter.

enter image description here

Or just download the official PDF (warning, 20MB .zip) and scroll down to page 213, there is a whole section on it.

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  • \$\begingroup\$ Note that USB C analog audio is likely only supported by mobile devices, though. On computer hosts, they most likely won't work, and you'll have to use a digital adapter which includes the DAC (and that may be the origin of the "USB-C doesn't have audio pins" misconception). \$\endgroup\$ – dim Nov 19 '18 at 11:04

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