We’ve always been huge fans of USB audio – external audio devices connected to a computer via USB – particularly on the PC, simply because it takes the sensitive audio circuitry away from the interference commonly generated by CPUs and most digital components to give you cleaner, higher-quality audio.
But with the introduction of native USB audio support in Lollipop, interest in mobile USB audio is starting to grow – and not just for playback.
USB audio recording
Over the last ten years or so, there’s been a boom in high-quality portable digital audio recording devices.
Brands like Zoom and Tascam are churning out models that record at up to 96kHz/24-bit stereo for under $300.
Meanwhile, smartphones, with considerably more horsepower to play with, are typically stuck offering recording from a single built-in MEMS (micro electromechanical system) microphone somewhere in the corner of the chassis.
The beauty of USB audio recording is that it takes away the actual recording box as the arbiter of quality and leaves it in the hands of the USB audio capture device. The recording box simply becomes a digital stream recorder, which is a much simpler task.
We might live in a digital world, but audio recording is still very much an analog endeavour and depends on the process of converting real-world analog audio into digital samples.
It has to start with low-noise, low-distortion microphones capturing the changes in air pressure that represent sound as an analog signal. You then need low-noise, low distortion preamplifiers to increase the amplitude of the analog signal so that it’s at a useable level.
Finally, you need high-quality, fast analog-to-digital converters (ADCs) to convert that analog signal into some digital format. Without those analog components, ADCs are useless.
USB audio playback
Not surprisingly, getting high-quality audio playback is the reverse – starting with accurate digital-to-analog converters (DACs) to turn the digital stream back into an analog approximation; then low-noise, low-distortion preamplifiers or power amplifiers to feed the output to a sound system or headphones, respectively.
The key to much of this conversion process (recording especially) is ‘low noise’ – doing this within the confines of a smartphone or tablet, where you have much more going on besides, is not easy.
The thing Google did with Lollipop/5.0 is provide ‘native’ support, so it’s built into the basic OS. That doesn’t stop you adding it yourself to older devices.
The key is searching for apps that incorporate their own USB audio device driver software.
In that situation, you only need a USB-OTG or USB Host port and at least Ice Cream Sandwich/4.0 (although there are reports it may work going as far back as Honeycomb/3.1).
Ultimately, it comes down to the age-old caveat ‘your mileage may vary’.
To see how this all works in practice, we ran tests on a number of low-cost DACs we purchased online, looking at how they perform with Lollipop/5.x, as well as older Android releases.
DACs – Burr-Brown PCM2704C
Like almost all successful brands, BB began life in a garage, grew quickly and, now years later, still cranks out audiophile-quality tech (its OPA-series of low-noise, low-distortion audio op-amps, for example, are legendary).
About a decade ago, BB was taken over by US giant Texas Instruments (TI) and today, churns out CD-quality USB DACs, one of the earliest models still available being the PCM2704C.
It’s been superseded these days but the Windows 7-certified PCM2704C is still available by the truckload online and has excellent references with 98dB signal-to-noise ratio (SNR) and total harmonic distortion (THD) as low as 0.006% when used at line-level.
With a typical 32-ohm headphone load, that THD rises to around 0.025%, which still isn’t bad. If the PCM2704C has any perceived downside, it’ll be that it only supports up to 48kHz/16-bit samples rates, which is low by today’s 192kHz/24-bit peaks.
However, it still matches most music audio formats and more importantly, 48kHz/16-bit is generally as high as Android’s compressed audio codec support goes, so unless you’re playing PCM samples, anything more is mostly a waste.
The chip’s power consumption is between 23 and 35 milliamps (23-35mA).
This is the DAC inside the popular MUSE X5, but you can find it in many no-name clones, either as a bare board or in a small aluminum chassis.
Unlike the X5 and its clones, the one shown here not only has a 3.5mm headphone/line-out port, it also features S/PDIF coaxial and optical outputs.
Either way, it’s arguably the cheapest way to get S/PDIF audio out of an Android device.
In fact, we had no trouble getting this one to work on our Lollipop/5.1 ROM’d Samsung Galaxy S3 with all apps – it connects straight in via a USB OTG cable.
The low-power consumption also means you shouldn’t have any trouble bus-powering it via USB-OTG.
Maximising battery life
That said, clearly anything you plug into your Android device that’s bus-powered is going to draw its power from your device’s battery, lowering battery life. So to maximise that battery life, flick your Android device into airplane mode while recording.
Not only will it turn off all radio communications (improving audio quality), it should also help ensure you get no interruptions or dropouts during recording.
And as with PC recording, don’t try to play games or do other demanding tasks at the same time – for the same reasons.
Generic USB Audio #1
Search on eBay and you’ll find the later by the truckload, starting for as little as $1.23 for this tiny USB dongle – and that includes shipping!
The DAC in this one is just a blob on the circuit board we’ve identified as a C-Media CM108AH, rated by the company as having a 94dB SNR and 0.03% THD with a 32-ohm load. It also includes a single (mono) 16-bit microphone input.
Again, this one worked perfectly on all our Android devices, including the Lollipop’d Galaxy S3. In practice, we found the SNR of our test unit closer to 70dB and the power consumption around 30mA.
One thing to keep in mind, chip specs don’t necessarily always translate into real-world performance – you still need good physical layout design to ensure you’re not introducing interference nasties into the audio path.
But I prefer this module to my PC’s integrated motherboard audio – and by a long way.
Break the headphone socket of your Android device? This is one very low-cost way to still get audio from it, but ultimately, you get what you pay for (more or less).
Generic USB Audio #2
But just to show that not everything always goes according to plan in APC Labs, the second one we purchased cost us $7, but included basic on-board switchable EQ, volume control as well as separate microphone and line-in ports.
Lollipop had no trouble finding this one – it even provided full control of Android audio apps via the on-device buttons, operating audio levels, play/stop and next/previous track control.
The only problem is, it didn’t output anything.
On a Windows PC, it works perfectly and even more surprisingly, USB Audio Recorder PRO on the same device quite happily recognises it, connects to it and plays perfectly.
The other disappointing thing was that the line-input isn’t stereo – it’s only mono (single-channel). Still, for converting old audiobook tapes into digital, combine this with USB Audio Recorder PRO, plug in your cassette player and it should do the job nicely.
We ripped the cover of this one and again, it features another blob for a chip, but we assume it isn’t a C-Media CM108AH, given it worked intermittently with Android. Power consumption was also around the 30mA mark.
Legacy USB DACs
Both of these devices provide line-level stereo input with an SNR approaching 90dB and they work well, but only via USB Audio Recorder PRO.
By the same token, Creative’s Sound Blaster Play dongle happily worked via USB Audio Recorder PRO and natively on Lollipop.
But USB Audio Recorder PRO works with a large range of more modern audio interfaces, including Behringer’s UCA202/UCA222. And if you need studio-quality microphone recording, a number of balanced-XLR microphone interface devices are also supported, including Lexicon’s Alpha/Lambda/Omega series, plus M-Audio’s M-Track mic/MIDI combo unit.
There’s an extensive (but not exhaustive) compatibility list for USB Audio Recorder PRO here.
Two other things. First, make sure you plug in your USB audio device before you launch the USB Audio Recorder PRO app; otherwise, the device won’t be detected and won’t work.
Second, the app isn’t guaranteed to work with every device, so grab the demo, available as a sideload from eXtream.
Setting audio levels
One of the best features of USB Audio Recorder PRO is its mixing console that lets you control inputs and outputs using linear/slider controls.
It works extremely well, if somewhat inconsistently – depending on the device, you’ll find you can move some stereo channels as a locked pair, but in other devices, they’ll be separate right/left channels, making it difficult to keep both tracking when you change levels.
But the eXtream team has cleverly incorporated the peak-program meter (PPM) display into the mixing console, so you can monitor recording levels as you would with a genuine digital audio recorder.
Digital Audio Workstations
Apple was brilliant to get GarageBand onto the iPad, making it the default tool for a generation of aspiring musicians.
But Android isn’t devoid of such tools with options such as n-Track Studio Pro and Audio Evolution Mobile Studio (from the makers of USB Audio Recorder) that enable you to mix multiple channels of audio, turning your device into a portable digital audio workstation.
Audio Evolution Mobile Studio features eXtream’s own USB device drivers, so it works on (almost) anything from Android 4.0 up, while n-Track Studio Pro also has its own USB compatibility list and installs on a minimum of Android 2.3.
Locating USB device specs
Head to Google Play and install Root Browser – it’s a file manager that delves down a bit deeper than most, but is free and doesn’t need root access.
Android lists or ‘enumerates’ all devices connected to the OS through the ‘proc’ virtual filesystem, including USB audio devices starting with Lollipop/5.0.
Make sure your USB sound module is connected to your Android device and launch Root Browser.
Tap the little ‘home’ icon at top-left, scroll down, choose the ‘proc’ folder and now scroll down again until you find ‘asound’. Once this folder opens, you’ll likely find two subfolders – ‘card0’ and ‘card1’.
We can’t guarantee it but ‘card0’ is usually your device’s internal sound module and ‘card1’ will be the USB device. Now depending on how your version of Android enumerates things, you’ll see a series of files.
Open them as ‘text files’ with your favourite text editor and you can read the data. For example, ‘stream0’ in our case provides details on the current run status and specs of our PCM2704C DAC with info on available sample rates.
It might seem as though only the audiophile brigade would be interested in the benefits of USB audio, but with many of us throwing down hundreds of dollars on high-end headphones, there’s already demand for high-end USB audio products.
Adding native USB audio support to Android will only grow that further.