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How does the transmission of wireless audio work?

Of course wired is better than wireless. But by how much?

The sound we humans can hear with our ears is basically a modulation in air pressure. The airwaves push on our eardrums and the resultant movement is converted to electrical signals sent to our brain, which we sense as sound. Microphones sense sound in the same way (mechanical vibrations to electrical signals), and you may have noticed, it is the exact opposite of how a speaker works. Understanding the sensed sound is a higher level brain function. The mechanical analogue of recognizing sound would require a whole book on Fast Fourier Transforms. (For those interested, there is a great book titled “Who is Fourier?”). For now, it will suffice to imagine sound as vibration, and keep in mind that the information of any sound is contained in the patterns of the aforementioned vibrations. This chapter is dedicated to the science and technology behind audio information magically travelling and transforming from a file of bits and bytes on a server halfway across the earth to comprehensible airwaves in your inner canal.

Transmission Mediums

Sound, being nothing but oscillations, needs a medium in order to traverse across space. Just before we hear anything, with the exception of bone conduction, the medium is always air. That is why there is the eerie silence of outer space, which has no medium. Most broadly, transmission media can be categorized into two types, pertaining to the usability of the technology – Wired and Wireless.

Wired media needs very little explanation as it is just a conducting material that transmits electrical signals from one end to another. Though it is a physical media, it should be noted that the actual sound vibrations are not being transmitted directly (as is the case for a cup-string-cup telephone). The sound waves are encoded into electrical signals, the kind that (usually) a speaker can understand. Then again, there is the whole analog vs digital issue, which will be covered in a subsequent chapter. The audio cables consist of two or more individual wires, one for the ground, one for the voltage and further wires if it is a stereo connection or has a mic. The important part of the working of a wired transmission medium is at its ends (the connectors), and those too will be covered in a subsequent chapter on connectors. For now, let us move on to the second category of transmission medium, the kind that really looks like magic – wireless transmission.

Wireless Audio

Whether you have an age-old MotoRokr bluetooth handset or a spanking new Apple Iphone 7, the sound your phone is playing reaches the device in your ears through the air, i.e Wirelessly. Again, note that though the medium is the same (air), this isn’t the actual sound waves travelling but the encoded version of the audio information. To illustrate, let us travel even further back in time to visit one of the first major wireless mediums mankind has ever known – the Radio.

AirPods
Apple’s Wireless Airpods

Surfing the radio waves

If you identify as a millennial, or even as a millennial’s progeny, somewhere in your mind may lie a decaying memory that goes something like the following. You are speedily running your finger over the serrated edge of a submerged gear shaped dial, paying close attention to the shifting sound of white noise.

*click*

You spin the dial backwards, slowly. Like a whale from the depths of the ocean breaching the surface to spout, a voice or a song gradually emerges from the meaningless noise. Before you know it, the clarity of the station is submerging in the noise once more. Again, you reverse the direction of spin and find the sweet spot.

Radio
If you owned one of these, perhaps you should start saving up for retirement.

Of course, these days wherever there is radio (FM or AM) there is mostly also automatic station selection. Radio waves are electromagnetic waves of a comparatively high frequency (3kHz to 300GHz). They travel large distances via line of sight, as well as by reflection from the ionosphere. However, unlike the electrical signal going into a speaker, the pattern of a radio wave doesn’t directly correspond to the sound wave. Instead, the information is encoded onto a carrier wave via modulation of either its amplitude (AM) or frequency (FM). The pattern of the modulation is usually what corresponds to the audio signal. FM radio has lower noise and affords better quality than AM radio, however we even have digital radio which can transmit CD-like quality over FM radio.

AirPlay and DLNA

Back in the realm of the everyday individual’s personal electronics, almost everything around the house is connected to the WiFi. If you own an Apple device that stores your media, and have an Apple TV connected to the HDMI IN port of your media playing system, you’re using their proprietary AirPlay protocol to stream your media over the home network. While this is smart and convenient, you are limited to the Apple ecosystem. The most common and easiest to use non-Apple alternative is the DLNA (Digital Life Network Alliance) standard. Chances are, your home theatre will be DLNA compliant, and if so it will automatically show up as a playback device on your Android/PC under the “Play To” menu. The speed and bandwidth of WiFi allows you to enjoy high quality audio without having to plug in any cables.

Airplay

Bluetooth

Whether it’s listening to music in your portable speaker, or talking on the phone whilst retaining the mobility and freedom of your hands, the most ubiquitous wireless audio technology of today (at least on the road) is definitely Bluetooth! Named after Danish king Harald Bluetooth, this wireless communication technology was invented in the labs of Ericsson Mobile in Lund, Sweden. It was designed for short range data transfer and uses Ultra High Frequency (UHF) radio waves to communicate over the 2.4GHz – 2.485GHz band. Bluetooth networks are Personal Area Networks and therefore generally limited to a range of about 10 meters. Devices need to have a Bluetooth chip to communicate with each other, and the data transfer rate, range, and bandwidth are much lower than that of WiFi, but Bluetooth scores big on convenience and mobility, and doesn’t need an intermediary router.

Bluetooth

Ask any audiophile for recommendations on a good pair of bluetooth headphones and all you may get in response is a scoff and maybe a chuckle. While it is good enough for calls and does manage decent quality audio in the car (the credit goes to the amplifier), Bluetooth is a lossy transmission protocol. Bluetooth uses its own compression protocol and codecs to ensure that the packet sizes fit the data bandwidth. The receiver pieces back the packets to play back the compressed audio. Bluetooth physically doesn’t have enough bandwidth to transmit high-quality audio, and even if it did, it would consume more power. The key CSR aptXvariable at play here is the codec, which is most commonly SBC (low complexity SubBand Coding). Newer codecs like A2DP and AptX promise better (CD-quality) audio, using a method called time domain ADPCM (Adaptive Differential Pulse-Code Modulation). Basically, they enable smaller size samples, so that a higher sampling rate can fit in the same bandwidth, resulting in better quality audio.

Bluetooth v5 is the latest specification soon to be released that doubles the speed and increases the range up to four times. Although the upgrades are aimed towards IoT devices, it also means better audio quality for all!

Kleer

KleerDeveloped in 2006 specifically for wireless audio, Kleer is a proprietary wireless transmission protocol. It enables transmission of lossless audio, as it offers a maximum data bandwidth of 2.37Mbps. Compare this with the fact that CD-quality 16-bit 44KHz stereo audio requires only 1.42Mbps. Kleer also has other advantages like a lower radio bandwidth requirement and hence lower battery consumption, along with the ability to transmit to up to 4 different receivers simultaneously. Unfortunately for all its advantages, Kleer hasn’t proliferated the market and support remains limited.

This article was first published in November 2016 issue of Digit magazine. To read Digit’s articles first, subscribe here or download the Digit app for Android and iOS. You could also buy Digit’s previous issues here.

Sahil Dawka