Charles Limonard at Dialog Semiconductor explores overcoming the limits of physics in ultra-thin smartphones with psychoacoustic bass boost
Consumers are increasingly using smartphones as their main music storage and consumption device. This is leading to a need for more sophisticated audio codecs, featuring a dedicated Digital Signal Processor and advanced new software that provides psychoacoustic bass boost functionality. This gives consumers the perception of enjoying a richer, deeper bass in the music that they’re listening to than the speakers in today’s ultra-thin smartphones are physically able to produce.
An audio codec IC provides a switching and routing hub for all audio signals. Codec is short for Coder-Decoder and generally refers to the analogue to digital converter (ADC) and digital to analogue converter (DAC). Audio can come from several sources, including the microphone that’s built into the handset or a wired or Bluetooth headset and speech during a call, which is handled by the phone’s baseband processor.
A variety of outputs are needed for the phone’s earpiece speaker, loudspeaker, wired or Bluetooth headsets, headphones or speakers and speech delivered to the baseband processor during a call.
When you’re listening to music on your phone with a wired headset, for example, and you get an incoming call, you need to either stop the music and route to the ringtone signal in speakerphone mode or mix the two. If you answer the call, you may want to use your headset, but not broadcast the conversation through the speakerphone.
When downloading music, you can often choose the bit-rate of the music. A higher bit-rate will sound better, but takes more storage space for a given song. The music could have a bit rate of 256 kilobytes per second (kbp/s) and the ringtone 64kbps. These two digital audio signals cannot simply be added together. They need to be converted to the same kbp/s sample rate.
When combining audio signals from different sources, the two clocks of these signals can be out of synchronisation. Asynchronous Sample Rate Conversion (ASRC) technology within the codec or DSP engine makes it possible to synchronise these audio signals using sample rate conversion.
One key area of innovation relates to psychoacoustic bass boost to improve the loud speakers in ultra-thin smartphones by addressing the problem of a lack of bass.
Simple physics dictate that as the surface area of a speaker cone gets smaller, the speaker’s ability to reproduce low frequencies is reduced as it isn’t able to move the necessary amount of air to reproduce those low frequencies at audible volumes.
Many smartphone speakers have a low cut off frequency of a couple of hundred Hertz, but the low bass notes are well below this in most music and will not be reproduced by the speaker, which is unappealing to the ear.
The solution is simple and ingenious; there are ways to ‘trick’ the brain, into hearing frequencies that physically cannot be reproduced by a speaker.
We apply the overtones and undertones of the low frequencies to the mix. The brain then fills in the missing fundamentals, allowing the listener to perceive low frequencies. Psychoacoustic bass boost solutions, then, can enable small speakers to deliver big bass for a more enjoyable listening experience.
A DRC (Dynamic Range Compressor) algorithm in the DSP can be used to amplify soft signals and reduce overly loud signals, which can damage the speaker, the result is that the ‘average’ audio level is higher and therefore sounds louder. DRC is used during commercials on TV. Your ears perceive it as being louder but the peak signals are the same as during non-commercial time. It also helps when wearing a headset in noisy environments: it is hard to hear soft passages and loud passages might be too loud. DRC makes it ‘easier on the ears’.
Many other algorithms are in use in today’s modern smartphones, including five-band equalizer, wind, noise and babble reduction, echo cancellation, stereo widening effects and 3D sound effects. Real Hi-Fi is already delivered from a smartphone and personal audio is still a field of strong innovation so expect much more to come.
For audio codec makers the trend is clear: more analogue and digital inputs and outputs, stronger DSP engines with more horsepower while minimising power consumption. This will enable consumers to enjoy the superior sound performance they want from their smartphone.
Dialog Semiconductor
