Android devices & USB audio

Previous millennium – USB basics – USB host & USB peripheral

The Universal Serial Bus (USB) is introduced in 1996.

USB is a master-slave communication bus: USB communication can only take place between a master device and a slave device.A master, or USB host, is a computer.A slave, or USB peripheral, can be a mouse, a keyboard, a DAC, a telephone, a smartphone, a tablet …

USB was designed to be easy to use: plug-and-play!

When a user connects a USB peripheral (for example a USB DAC) to a USB host (for example a PC), the USB host’s operating system (for example Windows or Linux) detects the USB peripheral and loads the appropriate software driver (for example a USB DAC driver or USB audio driver).

Current millennium – USB On-The-Go (OTG): a supplement to the USB standardMore and more smart peripherals, like smartphones or tablets, have all the features of a computer.For example, Android smartphones or tablets are Linux-based computers.

Android relies on Linux for core system services such as security, memory management, process management, network stack, and driver model. Thus Android handles all the things that Linux is really good at such as a vast array of device drivers, which take the pain out of interfacing to peripheral hardware.

As legacy computers, legacy peripherals like Android smartphones or tablets having all the features of a computer need to interconnect with peripherals. This slave-to-slave communication functionality is not supported by the master-to-slave communication USB model.To address this need, the USB OTG concept is added to the USB standard in 2009. USB OTG retains the legacy master-to-slave or USB host-to-USB peripheral communication model.A USB OTG device is a dual-role USB device, sometimes a USB host, sometimes a USB peripheral:

. a USB OTG-capable Android device operating as a USB peripheral can interwork with a PC operating as a USB host or as an A-device. a USB OTG-capable Android device operating as a USB host can interwork with a USB DAC always operating as a USB peripheral or as a B-device.

A USB OTG software program and USB OTG hardware components are added at the USB level. A USB OTG device is configured to USB host mode or USB peripheral mode by USB OTG software/hardware measuring the value of the ID resistance of the USB interface.

When a (regular) Micro-B plug of a USB cable is inserted into a USB OTG-capable Android device, the ID resistance is greater than 10Ω, and the Android device is configured to USB peripheral mode.

When a (functional) Micro-A plug of a USB cable is inserted into a USB OTG-capable Android device, the ID resistance is less than 10Ω, and the Android device is configured to USB host mode.

To be exhaustive, an initial USB host and an initial USB peripheral can exchange role if necessary by using the Host Negotiation Protocol.

USB powering

A portable rig: Samsung Galaxy S III smartphone + Apex Glacier USB DAC/amp + JH Audio JH 13 Pro IEM

When the Android device is in USB host mode, it acts as a USB host and powers the bus, i.e. a bus-powered USB DAC/amp like an Apex Glacier in bus-powered mode.

The Apex Glacier can also be configured in self-powered mode: it draws power from its own batteries.

An Accessory Charger Adapter (ACA)-compliant Android device operating as a USB host is also able to charge at the same time.It draws power from an external ACA-compliant battery, or from an ACA-compliant USB DAC.

ACA is described in the USB Battery Charging 1.2 specification.The ACA software program is present in the Linux kernel source code of the Samsung Galaxy III.https://github.com/sgs3/GT-I9300_Kernel/blob/master/arch/arm/mach-exynos/p10-battery.c#L248

A USB OTG device is configured to USB host mode supplying or drawing power, or USB peripheral mode by ACA/OTG software/hardware measuring the value of the ID resistance of the USB interface.

According to the value of the ID resistance. the Android device should act as a USB host, interwork with the USB DAC acting as a USB peripheral, and supply power to the bus-powered USB DAC . the Android device should act as a USB host, interwork with the USB DAC acting as a USB peripheral, and draw power from an external battery. the Android device should act as a standalone USB peripheral, and draw power from an external battery. the Android device should act as a USB peripheral, interwork with a USB host, and draw power from an external power source.

Jelly Bean-announced USB audio: yet another Android fragmentation?

At the announcement of Android 4.1 Jelly Bean in June 2012, Google introduced a new type of USB DAC behaving as USB host, that can’t interwork with PC or Mac also behaving as USB host.

According to Google, this gives Android devices that do not have host capabilities the ability to interact with USB hardware.

These brand new USB hardware or peripherals have to act as USB host. These possible new peripherals incompatible with PC or Mac divide the Android peripheral ecosystem. Divide and delay! There is only one Jelly Bean-announced USB DAC available for the time being: GROM USB.http://www.head-fi.org/t/595071/android-phones-and-usb-dacs/2310#post_9458311

It feeds and justifies the attacks of Android foes for their favourite target: Android fragmentation!

Standard Android USB audio: a New Kid in USB audio Town – Hi-Fi for the masses?

The standard USB audio, compatible with PC or Mac, will be boosted by numerous Android devices, the most numerous computing machines due to the free and open source Android.

More and more Android devices can interwork with standard USB DAC:http://www.head-fi.org/t/595071/android-phones-and-usb-dacs/1935

A lot of human beings already have the important part of their Hi-Fi systems: the source!

The sources can be:. cheap or subsidized Android smartphones:http://smartphone.geekbuying.com/. cheap Android tablets:http://www.geekbuying.com/Search?keyword=tablet&c=1146&sort=3. cheap ($30+) Android mini PC:http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=Android+mini+PC&rh=i%3Aaps%2Ck%3AAndroid+mini+PC

A digital audio system is composed of:

. A storage module: Music is stored in file as a series of bits 0 & 1 with different encoding formats (MP3, WAV, FLAC).. A player: The player decodes the bits 0 & 1 to PCM digital audio signals.. A DAC (Digital to Analog Converter): The DAC converts the PCM digital audio signals to the original analog audio signals.. An amplifier: The amplifier amplifies the analog audio signals.

. A speaker: The speaker restitutes the music.

Like a PC or a Mac, an Android device is a digital audio system.

Android devices with USB audio enabled can output the PCM digital audio stream to an external standard USB DAC & amplifier, potentially much more efficient than the internal DAC & amplifier.