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Giving small speakers' audio quality a shot in the arm

Posted: 02 Sep 2010     Print Version  Bookmark and Share

Keywords:audio quality  small speaker audio  handset speaker 

Modern smartphones are slick and powerful machines. Their size varies between different models but generally speaking, state of the art devices pack lots of features into an approximately 110 x 60 x 15 mm package.

Once the display and the circuit boards are taken into consideration, not much room is left for the speakers. Now, think about a home theatre and how much room a subwoofer takes. Most of you may think that these are two completely different applications and should not be even compared. This is true to a certain extent. In fact, while they are indeed wildly different applications, the content running on them is becoming more similar every day. Faster standards for mobile communications (3G, 3.5G, 4G) and their supporting networks have enabled audio and video download and playback on handsets. Customers expect that higher bandwidth comes with higher audio and video quality.

The problem is that improving audio is not easy. Handset manufacturers face various constraints, with two of the main ones being the size of the cell phone case, and how compressed the audio file is. Let's examine them.

Case Size—Speakers translate electrical energy into sound waves by moving their diaphragm back and forth. The diaphragm pushes the air, creating sound waves that our ears interpret as sounds. Given the above-mentioned size limitations, there is not much room left for movement. Only very small speakers with a small diaphragm and short excursion can be used.

In a world of static integrated circuits, speakers, with their need to move, are kind of 'pesky.' Small speakers cannot reproduce audio that well, and bass frequencies are the most affected. Obtaining good audio quality from a small portable consumer electronic device is a challenge that can be solved only by a cross-functional team of industrial, mechanical and electrical designers. Electrical engineers have a tool at their disposal: audio processing algorithms.

Compressed audio—Audio is often compressed to reduce the size of the file to be downloaded. Size reduction is achieved by using encoding algorithms (i.e., MP3). The smaller size means a loss of information and, therefore, of audio quality. Also, in this case, audio processing algorithms can help.

Audio processing algorithms
A variety of algorithms are available today to process the audio signal and improve the listening experience.

Basic processing is given by equalisation and filtering that change the amplitude of different frequency bands to overcome the shortcomings of speakers. By looking at the frequency response of speakers, we can determine what can and cannot be reproduced and set equalisation curves accordingly. The goal is to obtain sound with pretty much constant amplitude, no matter what frequency is played through the speaker.

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Figure 1: Speakers have frequency responses that change with the intensity of the audio signal.

Basic equalisation nowadays is pretty common; most audio converters available on the market have it. Unfortunately, in some cases this is not sufficient to improve audio quality. In fact, speakers have frequency responses that change with the intensity of the audio signal (Figure 1).

To compensate for this effect, dynamic filters should be used. These are filters whose poles and zeros change to take into account a speaker's frequency response changing with the signal amplitude. To implement dynamic filters, DSP-like processing capabilities are needed. The vast majority of low-power audio converters do not have enough horsepower to do it.

Another interesting algorithm is bass boost. This algorithm improves the reproduction of bass frequencies by exploiting the psychoacoustic principle of the missing fundamental.

If we look at the frequency response of small speakers, we can see that they have a bass response with a 3dB point that can be in the hundreds of Hz. This means that such a speaker will not be able to reproduce lower frequencies well. Driving the speaker with these low frequencies is not only worthless (since they cannot be reproduced anyway), it's also pernicious. The low frequencies force the speaker to move in a way it cannot and by doing so create additional distortion for higher frequencies as well.

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