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By: Morten Christiansen, Technical Marketing Manager, ²ÝÁñÉçÇø

The ?¡± phone jack was invented more than 100 years ago to connect people using a new invention called the ¡°telephone.¡± Today, the modern variant¡ªthe 3.5mm phone jack¡ªis widely used. As modern mobile phones are used for more than phone calls and do not have room for multiple connectors, a new approach for audio connectivity is needed, so product designers are retiring the 3.5mm jack.

As designers determine their next move, they consider the multiple proprietary and standard USB audio protocols already in use. This article describes the features and advantages of the latest USB Audio Device Class (ADC) 3.0 specification, which will help the industry standardize on one protocol and accelerate consumer adoption. ADC 3.0 enables high quality digital headsets with a local digital signal processor (DSP) for advanced functionality like adaptive noise cancelling, user-defined equalization, and hot-word detect. Most importantly, compliant ADC 3.0 implementations enable significant system power savings, which are critical to low-power mobile applications. ADC 3.0 is expected to be the new standard for phones using the USB Type-C^TM connector. 

Analog Headset Power

Analog audio solutions, typically using a 3.5mm phone jack, are generally low-power, but use power in bursts. Figure 1 shows that audio processing for voice calls or MP3 playback is typically bursty, and allows functional blocks to enter lower power modes between bursts of activity. Some manufacturers have implemented even more highly optimized power profiles for audio playback than shown.

Figure 1: Power profiles for mobile phone with analog headset

USB Audio Power Challenge

Unlike analog headsets, USB audio headsets use isochronous transfers. Isochronous transfers provide the guaranteed bandwidth which is required for audio streams, but at the cost of a higher power consumption. The power profiles to support a USB headset are shown in Figure 2. The system includes two new functional blocks (USB Host and USB Device), and both operate continuously in high power mode. Some implementations could even require the application CPU to be continuously in high power mode to feed the USB host with audio bursts to the speakers and receiver and continuously process samples from the microphone.

Figure 2: Power profiles for mobile phone with legacy USB audio headset

The additional power required to support legacy USB audio is of little concern for a headset used with an always-plugged-in desktop or laptop with a high capacity battery. However, legacy USB audio cannot replace the 3.5mm phone jack in mobile phones and other portable, battery powered products, as it would drain the battery too quickly.

USB Audio Power Save Solution

Isochronous transfers for legacy USB audio occurs every 1ms for Full Speed USB. The bus is idle between transfers, but cannot enter Suspend L2. The USB 2.0 LPM L1 specifications defined a new power state Link Power Management (LPM L1) for High Speed USB. The combination of High Speed USB bursting and LPM L1 Suspend offers significant power savings opportunities. Figure 3 shows the power saving events of the new audio specification as a function of time.

Figure 3: Concept for audio bursting and LPM L1 for power saving

ADC 3.0 Proof of Concept

Proof of concept was required to allow the ADC 3.0 specification to progress. However, proving the power savings of ADC 3.0 was challenging because LPM L1 is not widely used nor supported today. Figure 4 demonstrates the successful proof of concept using ²ÝÁñÉçÇø DesignWare? USB IP.  Transaction 13713 shows the host sending the LPM request, which is accepted by the device. Packet 81813 shows the bus in the LPM L1 suspend power state for 3.5ms. Packet 81814 shows the host resuming the device. Resume signaling lasts 86us in this example, and this value is configurable to account for different device wake-up times. Packets 81815 and 81816 are two timer ticks. Transfer 6194 shows the host reading isochronous data from the device. Packets 81819 and 81820 are two timer ticks, but the last timer tick is not needed and can be removed in future implementations. Finally, transaction 13175 shows a new host LPM L1 request and acknowledgement before packet 81824 shows the bus in LPM L1 Suspend for another 3.5ms as the cycle repeats.

Figure 4: Proof of concept for LPM L1 power save

USB Audio Device Class 3.0 Power Save Solution

The resulting power profiles based on proof of concept system are shown in Figure 5. This system¡¯s LPM L1 Suspend residency is 87.8% (3.513ms of a 4ms service interval).  PHY power consumption is important to consider alone, as it is typically much higher than controller power consumption. LPM L1 power is typically less than 1% of active or idle power and using LPM L1 can result in PHY power savings of 86%. This enables USB Audio Device Class 3.0 USB headset solutions that are power-competitive with legacy analog headset solutions.

Figure 5: Power profiles for mobile phone with USB Audio Device Class 3.0 headset

In addition, designers can save power by putting the CPU and/or DSP system in deep sleep between USB bursts. Furthermore, implementation dependent optimization is possible by scheduling multiple audio bursts in advance. This allows the CPU or DSP subsystem to be in deep sleep for longer periods of time at the expense of added audio latency. Note that this might or might not be acceptable, depending on the application.

²ÝÁñÉçÇø USB IP Supports ADC 3.0

To implement ADC 3.0 in low-power products like phones, tablets or laptops, designers will need an ADC 3.0 compliant host controller with Hardware Controlled Link Power Management capability. ²ÝÁñÉçÇø¡¯ DesignWare xHCI USB 1.1 controller was used for the LPM L1 proof of concept. The ADC 3.0 specification describes some important and required changes to device controller implementations for an ADC 3.0 compliant headset. In addition to the controller, designs supporting ADC 3.0 require LPM L1 capable PHYs. ²ÝÁñÉçÇø PHYs in both advanced process nodes for hosts and mature, low power process nodes for headsets support LPM L1 using proprietary extensions to the PHY¨Ccontroller interface. ²ÝÁñÉçÇø¡¯ lead customers are already designing ADC 3.0 compliant headsets using our industry leading LPM-capable USB IP. 

Additional ADC 3.0 Features

One well-documented limitation for analog headsets is that audio quality is limited by the choice of ADC, DAC and audio processing in the phone, tablet, etc. Device manufacturers must carefully balance cost, power and quality. Whatever choices the device manufacturers make, cannot be un-done and better quality, more features, lower power or lower cost are only possible by choosing another product. When device manufacturers implement ADC 3.0, there are no analog to digital converters or digital to analog converters in the device. Device manufacturers can concentrate on optimizing power since features, cost and quality have been removed from what they influence.

ADC 3.0 headsets support up to 24bit/192kHz audio format for high audio quality. Headset manufacturers can add a local digital signal processor (DSP) to implement features like microphone beamforming, active noise cancellation, user-defined equalization, hot-word detect, and more. Of special interest is active noise cancellation (ANC), as shown in Figure 6. 

Figure 6: Power profiles for active noise cancellation in ADC 3.0 headset

Multipurpose headsets designed for both convenience and quality is possible; manufacturers can design a combined wireless Bluetooth and wired USB headset that charges the battery when connected to a phone or USB charger. Bluetooth is used for convenience and phone calls, USB is used for listening to music, watching videos or just cancelling ambient noise.