To USB or Not to USB


USB Audio Device Class 3.0

Welcome to what I call the second Wednesday of this week, or what you would call Thursday.  Here’s a blog on USB Audio Device Class 3.0

USB 2.0 and Audio work well together. The highest audio quality data streams with 7 channels of audio at the highest bit rates easily fit into the USB 2.0 bandwidth available.  What wasn’t perfect was the power consumption of the existing USB audio standard.  USB headsets for PCs have been popular for gamers. They have not been popular for mobile phones.  Mobile phone users use the 2.5 analog headphones

In fact, we have USB audio which is so slow USB 2.0 is more than sufficient to to transmit super high quality USB 2.0


Consumers love their analog headphones. Cheap headphones in all colors with the 2.5mm connector.  The problem for consumers: their favorite phone makers flattened their phones and  will remove the 2.5mm headphone connector so it won’t be an option.   Phone makers expect consumers to user Bluetooth wireless headsets.  Despite the fact, Bluetooth headsets can be purchased for under $15, 2.5mm headphones are still super popular.

So the phone makers and the USB-IF used the new.


The Audio Device Class 3.0 focuses on power savings by decreasing the amount of time the audio and USB circuitry are on. By maximizing the off time, power is saved.  Essentially data bursts from the source (a phone) to the USB headphones.


Old 3.5mm analog headphones were designed over 100 years ago for telephone switch boards. They started out mono, and became stereo, and then 2.5mm for the phone form factor (primarily). I couldn’t tell you the exact power consumption.  The 2.5mm receptacle is always on, and always consuming power. This means that it’s transmitting all the time when you are transmitting audio. It never turns off.  It’s gone now because it’s simply too big for today’s phones(and probably hard to waterproof) so it doesn’t really matter how much power it used.


USB headphones today (and yesterday) typically use the Standard A connector to connect into a PC. This means they don’t and weren’t designed to plug into a phone or tablet. The audio quality should be higher than a straight analog. This is because the pure audio is digitally transmitted through the USB lines and then unpacked near your year for better clarity. The downside of this kind of USB audio is that it’s on all the time again. The USB continues to transmit and never turns off. It consumes power all the time.   It’s super easy to use USB audio headphones because both Microsoft and Apple have standard drivers. As long at the headphones are compatible, they standard drivers load and they work.  For this reason, I only buy Microsoft headphones these days.  (Also they are reasonably priced and have good sound quality for music or conference calls).


To save power on mobile devices, the USB-IF created the Audio Device Class 3.0 (ADC 3.0).  the primary benefit to this standard is that the audio transmits in bursts. This means the USB analog (and digital) silicon turns on, transmits the data, and turns off. As a result the USB parts are only on about 10% of the time. This means power could be reduced 90% or more.   The USB port on the host (the mobile phone) and the device (the headphones) turns on only when data is ready to transmit.   As mobile phone makers try to extend battery life further, a 90% reduction in any area is an advantage.   In addition, because the data is transmitted digitally, it is /or can be high audio quality.


You can see in the graphic on the left, the always-on of USB audio (or an analog 2.5mm headphone). On the graphic on the right shows how the USB ADC 3.0 headphones operate by transmitting in bursts and shutting down until it’s time to burst data again.


The ADC 3.0 digital headphones should deliver a higher audio quality than the analog 2.5mm headphone jack. Since this it Type-C, it fits on today’s mobile phones.   A standard ADC 3.0 driver for Linux and other mobile operating systems should make ADC 3.0 headphones compatible with all sorts of phones.  HOWEVER, it does allow manufacturers if they want to offer addition customized features if using the the earphones and mobile phone from the same company. It’s possible there could be even greater power savings.  We will see.

It should be possible to develop ADC 3.0 headphones or earbuds for Type-C at prices comparable to low energy Bluetooth and with better battery life.  It’s only a matter of time before we have $10 then $5 USB ADC headphones.  They will probably cost a bit more for now.


There’s more Synopsys has done for USB Audio. You’ll need to contact us if you want to know more.


And from XKCD