6. Digital Wireless Systems

As other links in the audio chain have been converted to the digital domain it is of interest to look at the impact of digital technology on wireless transmission systems. Digital techniques can be applied to professional wireless in several ways, each offering potential benefits. The first level of application has been the use of digital control circuits for various tasks such as frequency selection, diversity antenna switching and most display functions. Nearly all frequency-agile wireless systems benefit from the use of digital controls and digital displays.

The next digital application level employs DSP (Digital Signal Processing) to replace traditional analog companding circuits. An audio DSP circuit is used in the transmitter to optimize the input signal for transmission and a complementary audio DSP is used in the receiver to optimize the output signal. The radio transmission path is still in the analog domain. Benefits may include increased audio dynamic range, decreased companding artifacts, and wider frequency response.The highest level of digital implementation uses a fully digital transmission path.

The input signal is digitized in the transmitter and remains in the digital domain until the receiver output. It is even possible to output a digital signal from the receiver to subsequent digital equipment. Potential benefits of an all-digital wireless approach include both improved audio quality and improved radio transmission. However, the technical requirements are not trivial and the inevitable compromise between performance and cost requires some difficult decisions. In concept, fully digital wireless transmission is simple. Add an analog-to-digital (A/D) converter at the input of the transmitter. Transmit the resulting digital information to
the receiver.

Demodulate the digital information and add a complementary digital-to-analog (D/A) converter at the output of the receiver. The ultimate limitation lies in the amount of digital information that must be reliably transmitted for acceptable audio quality. In general, information transmission techniques (wired or wireless) must balance bandwidth limitations with hardware (and software) complexity. Bandwidth refers to the range of frequencies and/or amplitudes used to convey the information. In audio, a frequency range of 20- 20,000Hz and an amplitude range (dynamic range) of 120dB is perhaps the ultimate goal.

However, a frequency range of 300-3000Hz and a dynamic range of 30dB are sufficient for telephone-quality speech. As expected, high fidelity audio equipment tends to be more complex and costly than telephone equipment. In analog FM radio systems, audio fidelity is greatly dependent on allowable deviation, which is related to RF bandwidth: wider deviation increases occupied bandwidth. Walkie-talkies use less bandwidth than wireless microphones. Even so, bandwidth limitations necessitate the use of companders to achieve acceptable dynamic range in most high quality analog wireless systems.

The bandwidth required for a high fidelity digital wireless system depends on the amount of digital information transmitted and the transmission rate. In practice, the bandwidth is limited by physical and regulatory requirements. This effectively constrains the amount and rate of information that can be transmitted. Ultimately, the fidelity and reliability of a digital wireless system is limited by these same bandwidth restrictions. A digital representation of an analog audio signal is generated by sampling (measuring the amplitude of) the audio waveform at some rate. The rate must be equal to at least twice the highest audio frequency desired. The resolution (accuracy) of the amplitude measurement must be sufficient to handle the desired dynamic range. The resolution is given in "bits". 8-bit audio is considered moderate fidelity while 16-bit audio is considered high fidelity.


Example of microphone wireless.

The bit rate of a digital signal is the resolution multiplied by the sampling rate. CD audio is 16 bits x 44.1KHz for a bit rate of 705,600 bits per second or 705.6K bits-per second. In the simplest form of digital transmission, the theoretical occupied bandwidth of such a signal would be equal to the bit rate. That is, to transmit CD-quality audio would require a bandwidth of 705.6KHz. In "real world" systems the occupied bandwidth would be even greater. Based on allowable deviation limits it is not possible to transmit such a signal. By comparison, cellular telephones use 8-bit resolution with a 6KHz sample rate.

By using special "coding" techniques the occupied bandwidth is only 30KHz. The resulting audio quality difference is obvious. In digital signal transmission it is possible to send morethan one bit per cycle by coding the bits into "symbols". The symbol rate is equal to the bit rate divided by the number of bits transmitted with each symbol. The
theoretical occupied bandwidth of a coded digital transmission is then equal to the "symbol" rate. For instance, a digital coding scheme that transmits two bits per symbol will have only half the occupied bandwidth of the CD example above.

It is further possible to reduce the bandwidth by using
compression schemes similar to those used in MiniDisc and MP3 recording devices. However, these are "lossy" techniques that eliminate some of the audio information. Nevertheless, when done properly, the audio quality can be quite good. Finally, the "reliability" of the digital signal transmission is also affected by the integrity of the radio path. Dropouts, interference, and multipath can cause loss of digital data. Extra bits are usually added to the signal for error correction, though this increases bandwidth slightly. One issue that is important in any digital scheme is latency, which is the signal delay that occurs whenever a signal passes through certain digital processes.

These include the A/D or D/A converters, the coding and decoding devices and any DSP that is applied in the analog signal path. Latency must be kept to a minimum to avoid distraction to the user and possible interference with non-delayed signal paths. The latency that is typical of cellular telephone circuits would be unacceptable in a live performance setting. Traditional analog transmitters and receivers use a moderate amount of bandwidth. Complex transmit/receive technologies are required to transmit digital information in a comparable bandwidth. Since spectrum is limited and increasingly crowded, successful digital transmission systems must have not only high audio quality but high bandwidth-efficiency as well.