For a recent trip to New Orleans, I invested in a quality pair of noise cancelling headphones to help me drown out the chatter. I am thrilled with my new purchase but I’m concerned I won’t be able to use them when mobile phone developers remove the traditional 3.5mm audio jacks from their phones in exchange for using USB Type-C™ connectors. This change will allow for thinner phones and cost savings due to using one less connector.
The USB 3.1 specification supports both charging and data-transfer capabilities, as well as connections to analog audio headsets, by multiplexing four analog audio signals onto pins on the USB Type-C connector. The analog audio signals are the same as those used by traditional 3.5mm headset jacks. This makes it possible to use existing analog headsets with a 3.5mm-to-USB Type-C adapter if the phone will support analog audio. Figure 1 and Table 1 illustrate the pin mappings of a USB Type-C connector that support analog audio.
A simple solution exists that would enable mobile phone designers to easily remove these 3.5mm audio jacks and remain backwards compatible with the existing analog audio signals.
Figure 1: USB Type-C connector pinout
Table 1: USB Type-C connector analog audio pin assignments
The pin mappings in Table 1 look straightforward, but a few challenges will arise when trying to support this type of pinout in your system:
Analog audio signals traveling through the 3.5mm jack are typically alternating current (AC) signals centered at 0V. The negative voltage portion of the analog audio signal can be damaging to USB PHYs. The best way to resolve this issue is to place a USB and audio switch like the TS5USBA224 on the D+ and D- pins of the USB Type-C connector. As Figure 2 shows, the device can route the analog audio signals to the codec and the USB signals to the USB PHY when appropriate.
Figure 2: Analog audio signal routing from USB Type-C connector
The second challenge with supporting the four analog audio signals on a USB Type-C connector is the connector’s reversibility. For the left and right signals on the D+ and D- pins, you can easily accommodate this reversibility feature by shorting the D+/D- pins to their respective D+/D- pins on the top and bottom of the USB Type-C receptacle. Handling the microphone and ground signal on the SBU1 and SBU2 is not as simple as shorting the two pins together. You will need a crosspoint switch like the small TS3A26746E (manual switching) or an audio jack detection crosspoint switch like the TS3A226AE (automatic switching) to route the microphone and ground signals to the codec appropriately.
It is possible to support the same analog audio signals in a 3.5mm jack by solving many of the challenges that arise in multiplexing analog audio signals onto a USB Type-C connector.
Would you like to see more mobile phones remove their 3.5mm jack, but still be able to use your favorite headphones? Log in to comment or visit the TI E2E™ Community Signal Switches forum.
"when mobile phone developers remove the traditional 3.5mm audio jacks from their phones in exchange for using USB Type-C™ connectors. This change will allow for thinner phones and cost savings due to using one less connector."
Why can't they just make a thinner headphone connector? Like a magnetically-attached linear connector that detaches safely if you pull too hard. Why do we have to put our audio through all these nonlinear analog switch ICs and thin pins and breakable USB/Lightning connectors?
I would suspect the cell phone makers explored the option of making a thinner headphone connector, but as we know these types of engineering decisions are complex and are never bounded by one consideration like form factor. This is the fun part about engineering. We have the challenge of deciding how to weigh the pros and cons of many, sometimes unrelated, considerations like, form factor, mechanical durability and cost in order to make a decision.
Take the challenge of supporting two different audio jack configurations CTIA/AHJ and OMTP that have the microphone and ground signals in two different places. Cell phone makers decided to solve this issue by implementing semiconductor solutions like the TS3A226AE audio jack switch. The TS3A226AE can automatically detect the location of the microphone and ground signals of an audio jack that was inserted into the phone and automatically switch the microphone and ground signals to their appropriate location inside the phone. Unfortunately this solution requires audio signals to be routed through nonlinear analog switch ICs.
As you can see in the post above, the same issue of detecting the location of the microphone and ground signals will be present in the new USB Type-C connector. The USB Type-C connector is reversible and cell phone engineers will still be faced with the challenge of detecting the location of the mic/ground signals and switching them to their appropriate place inside the phone. I think cellphone manufactures will continue to solve this problem by using audio jack switches like the TS3A226AE or new products that integrate the functions of both the TS5USBA224 and TS3A226AE in one IC.
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