Smartphone audio is important, and here’s what it takes to build the best

This probably sounds obvious, but audio quality is an extremely important factor in a premium smartphone. In fact, audio quality is becoming increasingly important for mid-tier phones as well. From making calls to playing your favorite songs to streaming videos, audio is at the heart of the mobile experience.

While headphones used to be the heart of the smartphone audio experience, it’s mostly a hardware problem that has been resolved. Although the shift from the 3.5mm analog headphone jack to the USB-C port poses new problems, Bluetooth headphones and wireless audio codecs offer a wide range of quality. Increasingly, users are tuning in to podcasts, videos, and more with their phone speakers. According to a survey conducted by Dxomark, most consumers use their smartphone speakers for podcasts, music and films at least intermittently.

Good audio quality makes a difference to your daily mobile use, but it is all too often overlooked. Between your phone’s tiny microelectromechanical (MEMS) microphones, mono and stereo speakers, and audio over USB-C or a headphone jack, there’s a lot to do and many corners that some manufacturers cut off. No two smartphones ever sound alike. What should audio customers look out for when planning their next smartphone purchase?

OEMs are investing in better mobile audio

When it comes to speakers, there are a few important things that separate excellent sound from your basic experience. Low distortion, high peak volume, large dynamic range with clean bass and crisp high frequencies for outstanding special precision ensure a first-class experience. A loudspeaker that sounds just as good at high volumes as it does at lower volumes is also an important requirement in the high-fidelity audio room.

With this in mind, the audio amplifier chip is a key component in the speaker system of the smartphone. The job of the amplifier is to provide power to the speaker and to move the speaker driver back and forth to stimulate the air and create sound. Any amplifier worth the money will allow the speaker to reproduce the entire audible frequency range with high fidelity, ensuring that the highs are well defined and the lows are free from distortion to match the spatial characteristics and timbre of the sound maximize. Also, if the amplifier is pushing the speaker to produce a louder and more dynamic sound, it must ensure that the speakers are kept in safe operating conditions. An intelligent amplifier therefore requires robust loudspeaker protection algorithms.

A high quality amplifier produces superior dynamics, clear yet powerful bass frequencies and a clean, transparent sound under all battery conditions.

At the same time, the pursuit of higher volume and sound quality puts additional strain on a phone’s battery. The amplifier chip must deliver power without introducing clipping or other distortion artifacts that often occur at more demanding low frequencies. It also needs to ensure that it can deliver enough power at all battery levels and handle situations where battery capacity may be lacking.

Indeed, distortion is often a symptom of insufficient power to the speaker. Mismanagement of power consumption and output, as well as speaker protection algorithms, can result in a “booming” or “pumping” noise where the speaker has difficulty reproducing clear audio and attenuates the overall sound.

A high quality amplifier, paired with a good loudspeaker, of course, produces superior dynamics, sharp yet powerful bass frequencies and a clean, transparent sound under all battery conditions. That’s all you want in a high-end smartphone experience.

How an amplifier improves the phone’s audio

While it is easy to point out the best theoretical audio quality, it is not that easy to achieve in the real world. This is how smartphone speaker amplifiers actually work to improve audio quality.

Newer, smarter classes of amplifiers are a key component. Power amplifiers are divided into “classes” based on their transistor and circuit design, each with its own advantages and disadvantages in terms of linearity, distortion, and energy efficiency.

Traditional Class A (or push-pull), B, and AB amplifiers don’t do very well in a smartphone form factor. They are just not efficient enough for battery operated devices (less than 80% energy efficient). As a result, most smartphone amplifiers are Class D and use pulse width modulation to deliver high power to the speaker more efficiently. In other words, audio signals are converted into on / off pulses that are filtered into an analog signal at the speaker.

However, high-end, high-power amplifiers can improve the smartphone form factor. Power Envelope Tracking (Class H) amplifiers dynamically adjust their power rails to further improve energy efficiency, especially at high listening volumes. At low volumes with small signals, the power rail levels are reduced to save battery life and wasted heat. The rails are dynamically increased at louder volume to ensure sufficient power to drive the speaker and avoid distortion.

With great power comes great responsibility. Because the audio amplifier is capable of drawing high levels of current from the battery, a state-of-the-art amplifier solution must be “system aware” so that it only draws power when it can and delivers it to the speaker when needed. Likewise, a high performance amplifier should contain speaker protection algorithms to ensure operation within safety limits. When implemented correctly, such an amplifier will provide high quality sound that does not change with the state of charge of the battery.

Smarter amplifier solutions also use built-in digital signal processing (DSP) functions to manage battery power and connected speakers. By monitoring current and voltage, smart amplifiers can deliver high performance while ensuring the safety of the speakers, reducing distortion and reducing the load on your phone’s battery.

Built-in DSP functions are also used to optimize the audio system and remove the drawbacks always associated with small smartphone micro-speakers. Equalization and psychoacoustic improvements compensate for the frequency response of small loudspeakers, which often lack bass reproduction functions. Frequency response correction and the creation of harmonics to get our brains to hear suppressed fundamental frequencies further improve speaker sound. Often times, these algorithms can be run on the amplifier IC, bypassing the computation time spent on a phone’s processor or on-board DSP.

Where can I find this technology in smartphones?

Information about the inner workings of smartphone audio technologies is often difficult to come by. While some of the above functions can be performed on a smartphone’s main SoC, high-end companies often turn to external amplifiers and audio codecs to provide additional innovative features.

Many of these speaker quality features can be found on chips like the Cirrus Logic CS35L40 and the CS35L41. These class H amplifiers include battery management, speaker protection, frequency correction, and the aforementioned psychoacoustic processing functions. IC solutions for Cirrus Logic amplifiers can be found in a number of high profile smartphones, including the Samsung Galaxy S, Note and Fold flagship series, the Huawei P30 Pro, and other Huawei phones (US trade ban), to name but a few to name a few.

Cirrus Logic customers occupy the top 10 positions in Dxomark’s audio playback ranking, including several phones from leading smartphone OEMs.

Dxomark also listed the Xiaomi Mi 10 Pro as one of its best phones for audio playback of all the smartphones tested in 2020. The phone speaker is powered by the Cirrus Logic CS35L41. The review found the phone to have excellent timbre and spatial reproduction, as well as accurate bass reproduction. Further awards are the maximum maximum volume, understandable music at low volume and the natural volume between the maximum and minimum settings.

Other Cirrus Logic customers also hold the top 10 in Dxomark’s audio playback ranking, including several phones from leading smartphone OEMs. High quality speaker audio isn’t limited to just premium products, you usually pay a little more for the best technology out there.

Audio quality is often overlooked on modern smartphones, but there are still plenty of improvements to be made if you know where to look. As speakers play an increasingly important role in our daily experience, a high-end amplifier and speaker pair can make all the difference in your next smartphone.