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Wireless Primer: The Key Issues Of Digital Audio Transmission
Issues of sound quality, data rates and more
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Digital is a buzzword that many presume solves all the technical issues we face today.

More and more digital equipment, such as mixing consoles, audio signal processors, and the like, are used for several applications, as a digital audio signal chain offers many advantages.

A digital signal on a wire (i.e., fiber optic cable) is easier to handle than on a copper wire because 48, 64, or more audio channels can be transported on one thin fiber optic cable. If an audio signal is already in the digital domain, it makes sense to keep it in this domain as long as possible.

As for digital wireless transmission, a digital wireless system is beneficial when the sound, occupied RF spectrum, and battery lifetime is as good or even better than an analog system. On top of this, latency (time delay between input and output) is always a very important topic to keep in mind.

Let’s start with sound and the related data rate.

The best sound can be expected if there is no audio data compression used in the wireless system. This will lead to a very high data rate.

• Minimum for 20 kHz audio and approximately 110 dB dynamic range: 18-bit 48 kHz = 0.864 Mbit/s

• Necessary overhead (framing, channel coding) leads to even higher data rate (factor approx. 1.5 to 1.296 Mbit/s)

When transmitting this high amount of data, it is no longer possible to use a simple and robust digital modulation scheme like FSK (Frequency Shift Keying) ASK (Amplitude Shift Keying) or PSK (Phase Shift Keying), because these concepts will be not able to fulfill the spectrum mask, 200 kHz of occupied RF spectrum, defined by the FCC. Even if this constraint didn’t exist, greater occupied RF spectrum could inhibit large multichannel systems.

To improve this, it is necessary to use a more complex modulation scheme with narrow filtering. The amplitude and the phase of the transmitted signal must be very precise when usmg this approach.

Behind every point of the constellation diagram, a digital word is deposited, which the receiver has to pick up and transfer back into an audio signal. This requires a very linear RF amplifier. This is a current-hungry device. The unwanted effect is reduced battery life of transmitters and portable receivers. By driving the RF amplifier with a better efficiency, the occupied RF spectrum will increase in an unwanted way.

If the data rate described above can be reduced, the modulation scheme can be simplified and the amplified RF can be used in a more efficient way to conserve battery power and increase operational time.

Constellation diagram of a 16 GAM modulation. (click to enlarge)

To reduce the amount of digital data, a compression algorithm has to be defined. This algorithm will add some latency to the whole data= transmission process. low latency is especially important during a live performance on stage.

If the total latency in a PA system, including contributions from digital mixing consoles, effects, etc., is greater than 10 ms, the timing of the band will be thrown off.

Further, if streaming video is projected to accommodate a large audience the picture and sound will be out of sync.

New audio data compression algorithms show good performance with a very low latency’, However, audio compression would introduce the possibility of audible artifacts (at least with awkward signals).

As technology improves, there will be solutions to the obstacles described above and digital will become available for wireless transmission.

The key questions for a digital system at this time are:

—Is data compression used?

—What RF spectrum is necessary and how will this impact multichannel systems?

—What is the latency of the system?

—What is the battery lifetime?

Volker Schmitt is a senior engineer for Sennheiser US, and Joe Ciaudelli also works with Sennheiser US and has a history of providing frequency coordination for large multi-channel wireless microphone systems used on Broadway and by broadcast networks.


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