Some people distrust wireless audio systems. They believe that the sound quality is compromised, and that the convenience of wireless systems must somehow carry a price of lower fidelity. Their fears are understandable; we are all familiar with the low-fi sound quality of cellphone calls. Sometimes the voice on the other end is barely intelligible; surely music over a wireless system must have similar problems.
However, those fears are not justified by the facts, at least not in the case of wireless technologies like Apple’s AirPlay, DTS’s Play-Fi, and CSR’s aptX for Bluetooth. The technology used by cellphones is very different from that used in these wireless audio systems. Whereas audio transmitted by over cellular networks can be shaky, wireless audio is quite robust. A wireless audio system can provide the same outstanding fidelity as a wired system. In fact, the wireless part of a wireless audio system can be completely transparent, and absolutely will not affect the sound quality of the audio signal. This is because of lossless coding.
When a digital audio signal is conveyed from one point to another, it must be coded, that is, placed in a format that can be transmitted and received. Many wireless audio systems use lossless coding to convey the data signal. With lossless coding, the data signal arriving at the receiver is identical to that leaving the transmitter. This is in contrast to technologies such as MP3 where the audio signal, to a greater or lesser degree, is audibly degraded by the non-lossless coding.
With lossless coding, the bits of the audio signal are “packed” in a way that allows them to be efficiently transmitted, then they are “unpacked” at the receiver. In this way, the bits at the wireless speaker are identical to those at the source. Lossless coding uses an algorithm to analyze digital audio data “words” and remove redundancy; this can make the words shorter (for more efficient transmission) but the exact bits can be restored later.
For example, consider the twelve 20-bit data “words” below, taken from an actual audio file:
Sample number Binary value
1 00000000010000110000
2 00000000011000010000
3 00000000011001100000
4 00000000010011110000
5 00000000001000110000
6 11111111111011100000
7 11111111101111010000
8 11111111100111110000
9 11111111100110100000
10 11111111101100010000
11 11111111110111010000
12 00000000000100100000
The lossless coding algorithm observes that in each sample the four least-significant bits (rightmost) are all zeros. At the transmitter, this can be coded more efficiently (by right-shifting the data). In addition, the nine most-significant bits (leftmost) are all 1s or 0s; this can be coded more efficiently (by coding one bit and using it to code the others). With techniques such as these, the file size in this example can be reduced by 60% for efficient transmission. Because the amount of data is reduced, the transmission is more robust. More important, at the receiver, these mathematical changes are exactly reversed, and the original data restored, bit for bit. The fidelity of the audio signal is absolutely not affected by the coding.
Lossless coding enables fidelity, but but other factors must also be present to assure true high fidelity. For starters, as we’ve noted, the WiFi network must be reliable. If a whole-house wireless audio system is envisioned, then the WiFi signal must be strong throughout the house. If necessary, a WiFi range extender (also known as a wireless repeater) can be added to increase coverage. While that does entail some effort, the wireless benefit goes far beyond audio. Installing special audio cables through a house only lets you convey audio, but a good WiFi network can give you the audio, video, internet, and much more.
Once you have a robust WiFi wireless network setup that can reliably transmit lossless audio throughout your house, what other factors must you consider to obtain top-quality audio fidelity? We’ll address those factors in the next post and introduce you to the new world of Hi-Res audio.
