Digital audio out cable to headphone jack
Ever since the first rumors that popped up late last year that Apple will be dropping the headphone jack from its next iPhone, the tech world has gone into a tizzy. Since then other manufacturers have added fuel to the fire, launching models without headphone jacks to preemptively attack the next iPhone and do the tech equivalent of the "First!!!" comment on YouTube.
By now you must have read a dozen thought pieces on how dropping the beloved analog headphone jack is a bad idea, and at the same time others have been writing about how the Lightning port (or USB Type-C on other devices) is the future for all our audio needs.
But before we throw our hat in either of those rings, it's important to assess both of them and see what advantages and disadvantages they offer. Not that it's going to change the course of what companies are planning to do and it certainly won't stop Apple from dropping the headphone jack from the next iPhone, if it so wishes, but as a consumer you should hopefully be more educated and when the time comes would vote with your dollar.
Before we start with the pros and cons of either analog or digital audio output, we should learn a bit about how audio is processed on your phones. Like your computer, the modern smartphone has something called the audio codec IC, which is responsible for processing all the sounds on your smartphone. It includes a digital to analog converter (DAC), analog to digital converter (ADC) and an amplifier, among other things. Every audio that goes in or comes out of your phone passes through this little chip, which is separate from the main SoC and takes up a tiny footprint on your phone's motherboard (about a couple millimeters in either direction).
The DAC processes the sound that exits the phone, including music and phone calls. The amp amplifies the analog signal so it's audible through the loudspeaker, earpiece, or your headphones. The ADC processes the analog signal received by the mics on the phone. There is a ton of signal processing going on - things such as applying audio effects on your music to noise suppression during phone calls from signal received from multiple microphones, it all happen inside this one little chip.The Qualcomm WCD9335 audio codec (in dark blue) on the Galaxy S7 edge. Image courtesy iFixit.
The majority of the phones on the market come with a single audio codec IC. Think of it as the onboard sound card on your computer. These are good enough quality for most users and they keep getting better every year. A lot of the phone manufacturers use Qualcomm audio codec, since it's more economical if the phone also uses a Qualcomm SoC but some also use Cirrus Logic (notably Apple), or Wolfson. These three are the most common suppliers of audio codecs on your phones, with Qualcomm being the preferred choice these days.
However, some companies go above and beyond with a higher quality component. Think of this as an aftermarket sound card for your PC. This audio codec resides alongside the standard part (so now there are two) and usually only handles music playback duties, with calls and other boring stuff still being handled by the standard chip. These chips usually boast a higher sample rate conversions and also include a more powerful amplifier that are better suited for higher bit-rate audio and high impedance headphones. Some companies go even further and include a separate amplifier for even more powerful output. The LG V10, the Lenovo Vibe X3, the ZTE Axon, and some of the HTC BoomSound phones are known to have an additional DAC/amp unit inside.
So this is how audio has been traditionally sent to your ears. The SoC will send the digital data to the audio codec, which would process it and convert it to an analog signal, which would then go to the amplifier, which would then send it through the headphone jack (or the speaker). Your headphones would then just have to intercept the amplified audio signal and use that to drive the voice coil, which then moves the drivers, which produces the sound.
But USB audio is different. In this case, the SoC sends the digital data straight out through the USB port (Lightning, microUSB, Type-C, what have you) and there is zero processing done on the device side. Of course, since neither your headphones (nor you) can process the digital signal, it has to be converted to analog at some point. This is why every device connected to the USB port for audio purposes has to have its own DAC/ADC built-in. It processes the digital signal, which otherwise would have to be done on the phone and then sends it to the amp, which also has to reside on the USB device, so it can power the speakers. These parts take power from phone port itself.
If you take the example of a USB Type-C headphone pair, such as the ones LeEco now sells with its new phones, the DAC/ADC and the amp are actually placed somewhere within the cable, usually near the USB port itself. On some headphones, it could be placed within the microphone module, or sometimes within the driver units. This is not a new practice and this is how Bluetooth headphones have always operated, as they too receive pure digital signal from the device and then have to process it. They even have the burden of dealing with the wireless signal and have extra circuitry for that.
So now we have the basics out of the way, let's look at each of our contenders.
The standard 3.5mm TRS (or tip, ring, sleeve) jack has been around for decades now. The best thing about it is that it just works. You plug it in and that's it. You never have to worry about whether or not it is compatible because in most cases it is. Higher end audio equipment does use the larger 6.2mm jack but on phones and other portable devices it has always been 3.5mm. You also don't have to worry about whether it's right side up or not and can plug it in the dark in your sleep. There are few things as ubiquitous and user friendly as the standard TRS jack out there.