Today's growth of HD content creation and digital audio recording/transmission has generated the need for large-scale, multichannel wireless microphone systems with improved sound quality. To meet this challenge, Sony developed a new digital audio wireless transmission technology.
In a conventional analog wireless microphone system, a compander (compressor/expander) system allows for wide dynamic range transmission. A compander is composed of complex analog circuits that provide high-quality sound and response characteristics. However, an analog system requires use of advanced techniques to keep performance levels constant, because analog circuits are subject to variable performance.
In contrast, a digital audio wireless transmission system doesn't use a compander system, and it avoids such fluctuations. In short, digital audio wireless transmission offers significant advancements in high-quality audio signal transmission.
Digital wireless transmitter
An analog audio signal is first amplified and then digitized in the A/D converter. In the DSP, three types of processing are performed: digital encoding, channel coding and modulated signal generation. In the codec, data reduction has been applied to the digital audio signal input from the A/D converter, and it is converted into a low transfer rate digital stream. Subsequently, the channel coder adds the synchronization and error-handling data necessary for wireless transmission and then encrypts the data. Next, the digital modulator generates a ?/4-QPSK modulation signal for digital wireless transmission from the channel-coded digital stream. The modulation signal is converted to analog through the D/A converter and carried to the RF block. In the RF block, the modulated carrier wave is amplified to the adequate transmission power level and transmitted to the receiver.
Digital wireless receiver
In the RF block, the received signal is digitized through the A/D converter. Similar to the transmitter, three main processes are performed in the digital signal processor: digital demodulation, channel decoding and digital decoding. After receiving the A/D-converted RF signal, the digital demodulator reproduces the digital stream that was channel-coded at the transmitter. The channel decoder then performs synchronization, decryption and audio data abstraction.
Consequently, the codec decompresses the low transfer rate audio signal that was data reduced in the transmitter and regenerates the digital audio signal. Finally, the digital audio signal is output, either as an analog or digital audio signal.
In a wireless system, audio signals must be transmitted within a limited wireless bandwidth. Transmitting the highest quality audio in this limited bandwidth is a major issue for wireless microphone development. Low-latency audio transmission is equally critical for microphone applications. To transmit high-quality audio within limited bandwidth, Sony developed a digital audio codec with good sound quality and low latency. (See Figure 1.)
The new one-chip digital modulator and demodulator enables up to 12 channels of simultaneous multichannel operation in a 6MHz bandwidth.
The left side of Figure 2 shows an analog system. In analog systems, the desired/undesired (D/U) signal ratio becomes the SNR of the demodulated audio signal. The higher the undesired signal level, the higher the noise will be in the modulated signal. Typically, an analog wireless system requires a 40dB D/U signal ratio.
In the Sony digital wireless system (on the right side of Figure 2), transmission errors won't occur as long as the D/U signal ratio reaches 20dB. For this reason, the digital system is far more tolerant of the undesired signal of the analog transmission system (by more than 20dB) and has an advantage compared with the potential risk of intermodulation interference.
Innovative remote control
A variety of transmitter settings can be wirelessly controlled from the receiver. This is convenient because transmitter setting changes can be made easily, even after the transmitters are attached to on-camera talent.
When the receiver is used with the Sony PDW-700 XDCAM HD camcorder, users can monitor the status of the digital wireless microphone through the camera's viewfinder. They can also wirelessly control the settings of the transmitter via the camcorder menu. This wireless remote control makes use of 2.4GHz IEEE 802.15.4 communication technology.
With the development of these and plans for future technologies, Sony continues to enhance the digital audio wireless transmission system.
Karl Kussmaul is pro audio senior product manager for the broadcast and production systems division of Sony Electronics.