[Theory] An introduction to the OTG standard for turning your mobile device into an audiophile source

The ever-increasing diffusion and computing power of different mobile devices, such as smartphones and tablets, would lead one to think of being able to exploit them as convenient sources for HiFi listening. The headphone output is typically of low quality and unsuitable for driving high impedance or low sensitivity headphones decently. Furthermore, its low quality would not suggest using it as a line output to be connected to an external amplifier, via minijack/RCA or Lightning/RCA cable, depending on the device in use, whether Android or iOS.

It would be interesting to be able to access the digital signal - not internally converted by the device - and to transform it into analog with higher quality external devices (DAC). This can be done thanks to the different DACs with USB input that crowd the market. There are both standard and portable models. Adapters have recently appeared on the market, for example from USB-C to mini-jack, which contain a DAC and perhaps an operational amplifier capable of driving headphones. Typically these accessories rely on the USB OTG (USB On-The-Go) connection protocol. This so-called specification allows any device capable of functioning as a host to communicate with USB peripherals such as external HDDs, pendrives, keyboards, mouse or joypads through a simple interface cable. There are also simple OTG cables, without an internal DAC, but these naturally require the use of an external DAC for audio purposes.

Initially introduced for the USB 2.0 standard in 2000, today OTG also manages USB 3.0, with a specification called High-Speed USB On-The-Go, that allows a very fast data transfer of 4.8 Gbit/s, ideal for complex data communications as audio and video signals without delay.

One of the differences compared to standard USB connection is that the connected devices are driver-free, i.e. they do not need additional drivers. Communication between OTG devices can occur bidirectionally: an OTG device can be either master or slave, so for example a phone can read data from an external hard disk or itself function as mass storage. It should be remembered that a USB hub prevents the possibility of using the OTG specification if multiple devices are connected. Battery consumption, as you can imagine, is considerable, and to date it is not possible to use the battery charger simultaneously to overcome this inconvenience. It would obviously be advisable to use self-powered external hosts or clients.

Another difference of the OTG standard lies in the structure of the cable and connectors: OTG sockets and plugs contain a fifth pin, added to the four standard USB pins (VBUS, GND, D+ and D-). This is a fifth pin in the connector, not a fifth wire in the cable. The mini-A plug has the fifth pin connected to ground while the mini-B plug leaves the fifth pin disconnected. A dual-role device therefore requires the ability to read the state of this fifth pin (whether connected or not) with the help, for example, of a pull-up resistor, in such a way as to be able to determine which end of the cable is inserted. In other words, the OTG cable is a directional one. Just reverse it and the master device becomes slave or vice versa (or host and client). For other technical details you can refer to this very complete paper by Infineon.

There is also a SuperSpeed OTG (SS-OTG) version, following the release of the USB 3.0 standard, but support for all operational capabilities is not yet widespread. Not all mobile devices support OTG without restrictions, so you need to check the settings of your phone or tablet for which option to select.

On iOS (iPhone etc.) it may be necessary to explicitly allow the use of the external port for purposes other than charging, better consult the instructions or Apple online support. As for Android, the latest versions of the operating system have pre-installed support for OTG; in case of doubt, if you do not find the option in the settings menu, you can install a special free app (e.g. USB OTG Checker on PlayStore) which will check your device and tell you if it is compatible with the OTG specifications. On my Android 13 the option appears in the “Additional Settings” submenu, at the bottom of the list (see screenshot below). My Android 13.0 is however proprietary (therefore modified) by OnePlus (OxygenOS 13.0), it may be that other devices have the option in other menu locations. As of today, February 2024, Android 13 is not yet installed on all phones, the diffusion process seems to be slower than expected. Once you have verified that OTG is present on your operating system you can go wild and connect your phone to many external USB devices.

As for audio applications of the protocol, the OTG connection allows you to add a headphone output - or a higher quality one - in smartphones or tablets that do not have it, for example. Furthermore, by not requiring any additional drivers, the result is software independent. To completely bypass the operating system's internal restrictions on the audio resolution of output files, you can use a specific app, like this one: in this way via OTG you can easily access HiRes files up to DSD formats. Several companies have already started offering adapters and cables in OTG standard.

N.B. Obviously it is not strictly necessary to use the OTG connection to connect a mobile device to an external DAC. Let's say that OTG opens up new possibilities for dialogue and file transfer between different components, without the use of a computer as an intermediary.

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A crash course on OTG - USB connection On-The-Go

How to turn your mobile device into an audiophile device