Today's appliances finally have the ability to incorporate the higher-end audio solutions typically reserved for only the highest of high end appliances. Thanks to the reduction in the price/performance ratio, algorithms typically handled by specialized ASICs or high end microprocessors, are now possible with 8- and 32-bit microcontrollers with headroom to spare for additional machine control tasks. High-end audio has also come into the mainstream feasibility by piggybacking on the volumes associated with the numerous MP3 and other portable audio players.
The adoption of audio is enhanced by the trend of appliances with more sophisticated user interfaces that have among other things, LCD displays, touch screens and touch panels. With these additional upgrades in the users visual experience, sounds such as tones, dings, and buzzers do not often fit the overall user experience.
Features such as verbal instructions, polymorphic tones, as well as music are now available to marketers and designers at a reasonable cost. This article will discuss some of the various techniques for putting the audio data in formats that can both compress, as well as enhance the audio to account for last minute machine console "musicality".
The typical system requirements of incorporating audio in appliances incorporate the following key components:
In this article we will concentrate on the techniques used in the Decoding portion of the stream and how it ultimately relates to the memory requirements.
Uncompressed audio offers the best sound reproduction possible with the theoretical best sound reproduction using a sampling rate dictated by the Nyquist criteria which states:
Fs > 2*B
where Fs is the sampling frequency and B is the maximum frequency found in the signal.
If an audio signal with a maximum frequency component of 15kHz is used, the number of samples needed to reproduce the exact waveform would be thirty thousand samples/second. Now even in today's market of low-cost memory, the space needed would be tremendous for user manuals or even instructions if numerous languages are stored.
The size required to store large audio files can be reduced by various compression techniques. In this paper, we will cover the following popular options:
• Linear & Compressed PCM
• MPEG/Psychoacoustic Coding
Each method has its place in the incorporation of audio in appliances. For instance, sounds that have a limited duration may not require much data space, but must be rendered appropriately if they are not to sound "tinny" or muffled. In general, beeps and buzzers require the lowest compression techniques due to their size.
Music quality is very noticeably reduced if an appropriate compression technique is not used. In this case, Psychoacoustic coding is often used to reduce the size of an audio file, while maintaining the listening quality. The scale of necessary audio quality is shown below.