Digital Encoding and Compression
The vast majority of consumer music today is on CD and therefore audio quality is often measured by the quality of CD music. Music is stored digitally on a CD with the left and right channels both encoded using 16-bit samples and a sampling rate of 44.1k samples/second. This results in a total data rate of 1.411Mb/s.
Of course, music is increasingly being distributed electronically. Electronically distributed music is CD quality music (i.e. 16-bits, 44.1ks/s) that has been compressed using high quality compression algorithms that can reduce the digital file size and the download data rate (e.g. 192kb/s or 256kb/s) without a detectable loss of quality.
Well-known compression formats include MP3, WMA, AAC, and others. These formats use lossy compression that targets a specific compression ratio (e.g. 4:1, 10:1, etc.) or data rate (e.g. 128kb/s, 192kb/s, etc.) and eliminates as much information as is necessary to achieve that amount of compression. Perceptual encoding is used to minimize the audibility of the information loss. Relatively high compression ratios can be achieved at the expense of audio quality and at the cost of complexity and power consumption of the circuitry performing the encoding and decoding.
Bluetooth is an example of a wireless technology that uses lossy compression to transmit audio and SMSC's Kleer technology is an example of a wireless technology that transmits audio losslessly. The figure below shows the impact on audio quality of the two methods. Bluetooth compression results in a noise floor that is at least 40dB higher than lossless, with a corresponding reduction in SNR and dynamic range.
Note: Multi-tone test performed using Audio Precision test equipment comparing digital audio input to digital audio output after radio transmission. BT results based on off-the-shelf stereo headphone.
An argument often used to rationalize the use of lossy compression in digital wireless audio applications is that the music itself is usually stored using lossy compression and therefore the wireless transmission compression is not causing any additional reduction in audio quality. There are several problems with this argument.
The first problem is that the audio quality associated with compression formats used for wireless transmission is not as good as that associated with digital storage compression. This is because the compression used for wireless transmission must take into consideration the power consumption and latency associated with real-time compression and decompression.
The second problem with this argument is that combining digital storage compression in series with wireless transmission compression (sometimes referred to as transcoding) may reduce the audio quality more than either compression method individually. The third problem with the argument is that portable audio devices are evolving towards using lossless storage compression as the cost of storage comes down.
Portable audio devices are becoming the primary storage vehicle for entire music collections that are then played in the home and car, as well as on the portable device. This is driving a demand for higher quality (lossless) storage, in which case lossy wireless transmission will be the limiting factor in audio quality.