Streaming audio: How to optimize battery life and music quality

Bluetooth 1.2-equipped headphones for receiving audio streams from MP3 players are arriving on the market to less than rave reviews. Designs are criticized for being heavy (due to large batteries), producing "tinny and shallow" sound, and chewing through batteries in a matter of hours.

It's not surprising that Bluetooth 1.2 (BT 1.2) struggles to reproduce good quality sound and is heavy on the batteries in a wireless headphone application—it was never designed for streaming audio content. It was designed for file transfer between one master and up to seven slaves in a "piconet" or Wireless Personal Area Network (WPAN), a task it handles extremely well.

BT 1.2's lack of bandwidth is its downfall in a streaming audio application. While a nominal 1 Mbit/s data transfer rate is acceptable for wirelessly linking voice calls from a mobile phone to headphones, it's inadequate for carrying CD-quality audio.

In addition, Bluetooth's relatively complex packet structure—a pre-requisite for ensuring compatibility—makes heavy processing and synchronization demands that eat into the meager power budget of battery-powered devices.

The latest Bluetooth 2.0 plus Enhanced Data Rate (BT 2.0+EDR), boasting a nominal 3 Mbit/s data transfer rate, promises to resolve the bandwidth restriction. However, challenges remain when employing this technology specifically to audio.

Fortunately, Bluetooth is not the only proven wireless technology around. For niche applications such as relaying CD-quality audio from an MP3 player to a headset, specialist designs are commercially available.

One example is an RF chipset specifically designed for audio streaming and developed by Nordic Semiconductor. Dubbed the nRF24Z1, it is rated at 4 Mbit/s yet consumes power at half the rate of a comparable BT 1.2 chipset. These attributes make it a compelling alternative to Bluetooth in most wireless audio applications.

The product does not have to be ratified to meet BT's IEEE802.15.4 standard (although it must meet relevant local rules such as the FCC's and Europe's ETSI regulations.)

However, the transceiver does operate in the global 2.4GHz ISM band and does have its own form of adaptive frequency hopping to prevent clashes with other devices on this crowded band.

Losing the wires
A typical wired MP3 player schematic is illustrated in Figure 1. The audio source—typically flash memory or mini-hard disk—outputs digital audio, via an MP3 decoder in either I2S or S/PDIF format.

A microcontroller supervises the audio source and controls playback characteristics such as volume control or bass boost delivered from a combined DAC/amplifier.

Click here for Figure 1
Figure 1: Typical MP3 solution with wired connection.

The wired connection between the MP3 player and the headphones can be dispensed by adding an RF link between the audio source and DAC/amp as shown in Figure 2.

Unlike a wired system's fixed connection between the MCU and DAC/amp, the wireless system requires an additional control data channel alongside the audio channel (otherwise volume control is restricted to the headphones only, with other control buttons remaining on the player). Both MP3 player and headset need batteries.

Click here for Figure 2
Figure 2: Batteries are required for both the MP3 player and the headset.

There are two methods for wirelessly relaying streamed audio content from a portable music player to headphones.

One method is to simply relay the compressed MP3 data across the link. The de facto "good quality" compression standard for MP3 is 192 kbit/s, well within the capabilities of BT 1.2's bandwidth. Even the "lossless" proprietary standards, such as Apple's own, require only 320 kbit/s; again no problem for Bluetooth.

However, there are downsides to this technique. First, the headset will require the necessary electronics (DSP, DAC/amp and batteries) to decompress the MP3 stream, this adds weight, bulk, complexity and cost to the headphones.

Second, the quality of the reproduced sound will be a function of the headset, no matter how good the player happens to be. And finally, the MP3 player couldn't then be used with conventional wired headphones.

The second technique is to decompress the MP3 data in the player and stream the uncompressed audio information to the receiver in the headphones. This mimics the process in a conventional wired MP3 player/headphone combination and is perhaps the most practical configuration—reducing the complexity, weight and power consumption of the headphones.

This does, however, demand an RF link with much greater bandwidth.