Avnera has released a set of application-specific standard product (ASSP) chips aimed at the audio market. Avnera’s chips, implemented in CMOS, transmit and receive stereo audio (close to CD quality) over the 2.4 GHz wireless ISM band assigned internationally for industrial, scientific and medical uses. Avnera claims a typical range of 45 feet, with longer range if an optional external power amplifier is used.
Figure 1. Avnera implements wireless audio with a transmitter chip and a receiver chip.
As shown in Figure 1, the AudioMagic AV71x1 are wireless transmitter chips, paired with the AV71x2 receiver chips. The transmitter sends stereo audio at a sample rate up to 48 kHz, with close to 16-bit quality. Within the AV71x1 family, the AV7111 offers an I2S input, and the AV7101 includes a USB input (both interfaces are shown in the figure). To facilitate integration with other audio hardware which may be capable of handling higher-quality audio, the Avnera transmitter chips accept audio at sample rates up to 96 kHz, and samples up to 32 bits wide.
The AV71xx components are intended for audio applications. Avnera claims “>90 dB SNR digital-in-to-analog-out.” In an uncompressed digital audio signal, each bit contributes about 6 dB to signal-to-noise ratio, so the Avnera figure of “>90 dB SNR” implies “>15 bits”. Standard CD quality is 16 bits at a sample rate of 44.1 kHz. Thus, the Avnera may not quite reach CD quality. Still, in most consumer settings such as a home-theater-in-a-box in a normal living room, the difference between 15 and 16 bits of precision will probably be imperceptible.
Avnera offers a second series of chips, called the AV61xx, intended for speech applications. In addition Avnera offers complete PCB modules intended for consumer audio devices. For example the AVMD7111-A transmitter module accepts analog audio and produces the wireless signal.
Previous attempts at wireless transmission of high-quality audio, including infrared and digital, have suffered from poor reliability, for example interference, dropouts, clicks and pops, and so on. Avnera’s proprietary solution employs several strategies to overcome these problems. One of these is dynamic channel selection: Avnera splits the 2.4 GHz ISM band into 40 narrow subchannels. The Avnera transmitter and receiver implement an algorithm to dynamically determine which of those subchannels are currently free, and switch to available subchannels with no loss of audio fidelity. If an interfering device enters the picture, then the Avnera chips change channels. When the Avnera transmitter and receiver are close to each other, the transmitter reduces power, to lengthen battery life and reduce possible interference with other devices in the 2.4 GHz band, such as wireless telephones. The chips implement two antennas, and the antenna with the best signal is chosen dynamically. Should the RF path fail momentarily, the Avnera receiver implements error correction and error hiding to minimize the effects of data loss. In addition, in the transmitted signal individual audio samples are spread in time, so that the effect of a brief data loss is reduced. Although none of these techniques is by itself new, the combination of techniques may well be new, and is appropriate for these applications.
Avnera envisions these ASSPs being used for products such as wireless microphones, wireless VoIP headsets, cordless telephones, wireless headphones, gaming headsets, and wireless speakers. For example, the home-theatre-in-a-box market faces the challenge of consumers needing to run extra wires to the back speakers. Since the distances involved in the typical living room are relatively small, and the speakers and transmitter stay in a more or less fixed position, wireless transmission to the rear speakers makes sense. Products using Avnera chips are already on the market, including the RocketFish Wireless Rear Speaker Kit;
the Polycom CX400 cordless VoIP phone, connecting to a PC with a USB connector and integrated with Microsoft Office Communicator 2007; and the AWD210 wireless headphones from Acoustic Research.
BDTI conducted informal tests of the A-R AWD210 wireless headphones provided by Avnera. These headphones provided clean sound for a non-audiophile product. They were easy to set up and use. A small transmitter box connects to a computer via a USB connection. The headphones feature on-off and volume controls on the left earpiece. In a home setting, the headphones received a clear signal when the listener was in motion, with no audible clicks or pops or dropouts. The signal traveled well through some walls and floors. When the audio did fail, for example due to increasing distance from the transmitter, it tended to fail suddenly and completely. The headphones did interfere with the operation of a Panasonic 2.4 GHz cordless phone, although the phone had no effect on the headphones. A microwave oven (which operates in the same frequency range) did not affect the headphones.
Some technologies transmit audio across a wireless link in a compressed form, such as MP3. But this can create problems if the source material was already compressed in a different format, such as the AAC format used in the iPod. In this scenario, the AAC-encoded recording must first be converted to MP3 format before transmission, i.e., transcoded. Transcoding can lead to degradation in sound quality. Avnera, like many manufacturers of wireless audio devices, sends uncompressed digital audio to avoid this problem.
By sending an uncompressed digital stream, the Avnera chips enable system developers to implement features that might otherwise not be practical. For example, Dolby Headphone renders 5.1 channel sound in stereo headphones. Effective operation of Dolby Headphone requires a stereo stream that has not been modified. For example variable phase shifts between the left and right channel, which can occur with some transmission schemes, are not acceptable.
As shown in Figure 1, some of the Avnera chips feature a reverse audio channel. This can be used for voice audio in headset applications, for example in gaming.
Wireless consumer audio is a field with demanding specifications, highly critical listeners, and an unforgiving environment: if data is dropped, the speaker cone keeps moving, and the listener hears a click. In this market, ASSPs often make sense, because potential volumes are high, and system designers need low-cost, low-power solutions. Avnera’s offerings are worth considering for short-range wireless audio applications.
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