Tag: A10-TX-US

A significant challenge for engineers designing a wireless microphone system is addressing multipath interference. The Audio Ltd A10 digital wireless microphone system devotes significant signal processing resources to achieving robust performance in the presence of both short range and long range multipath. This note briefly explains what multipath is, why it is a problem, and some of the techniques used to mitigate it.

What is Multipath Interference?

Multipath interference occurs when an RF signal from a transmitter arrives at a receiver via two or more routes. Typically there is a direct path plus a number of indirect paths caused by reflections. Walls, people, set pieces, and other objects in a room can cause reflections indoor. Nearby buildings and vehicles can cause reflections outdoors.

In general, the total length of each signal path will be different. Therefore, the time delay associated with each path will also be different, which in turn causes the phase of each received signal to be different. If signals arrive approximately in phase, received signal power increases. If signals arrive out of phase, the signal is attenuated.

Flat and Frequency Selective Fading

As a transmitter or receiver moves, the relative phase of the multiple signals changes, causing the total received power to fluctuate. This is known as fading. As well as being affected by the path differences, the signal phases are also affected by the transmitted frequency. If the path difference between the various signals is short, the entire signal tends to be affected by this fading. This is sometimes referred to as flat fading.  But for longer path differences, the phases of the received signals vary across the signal bandwidth. The signals can therefore interfere constructively or destructively at different frequencies in this bandwidth. The effect is to induce a slope in the system frequency response, or even to create multiple peaks or notches in the received signal spectrum. This is known as frequency selective fading.

The path length difference at which frequency selective fading starts to become significant depends on the signal bandwidth. With a path length of  c/4B (where c is the speed of light and B is the RF signal bandwidth), there is the possibility of a severe slope across the band. For a typical wireless microphone system with 200 KHz RF bandwidth this corresponds to 375 meters. However, path lengths of reflections can be several multiples of the direct path length (if there is one), so frequency selective effects can start to be seen at much shorter operating range – well under 100 meters.

In practice, for typical indoor operation, the primary effect is flat fading. Outdoors, particularly in an environment like a sports stadium or a city street, both types of fading may be encountered.

How Multipath Affects Wireless Microphones

Considering frequency selective fading first, the effect on an analogue FM system is to introduce audio distortion or, in severe cases, signal drop-outs. A simplistic digital system is also impaired by this fading—prone to digital errors and signal drop-outs as the receiver or transmitter moves.

However, it is possible to equalise the channel frequency response and apply error correction techniques such that a digital system has excellent immunity from frequency selective fading. The advanced digital signal processing used in the Audio Ltd A10 Digital Wireless System benefits from the additional signals present in multipath reflections.

Flat fading is in some ways worse than frequency selective fading. It can effectively cause a complete loss of signal—no amount of signal processing can correct for this.

Spatial Diversity

The most common way to manage the effects of flat fading is to use a diversity antenna topology, where more than one antenna receives signal. In sophisticated systems such as the A10 digital system, its advanced diversity system can help with both flat fading and frequency selective fading.

As a transmitter or receiver is moved, the phase of the received signals vary. The probability that the signals received at an antenna at a particular location sum and cancel to nearly zero is quite small. But in a typical room there may be numerous “dead spots” with severely attenuated reception.

If two antennas are placed at different locations, the chances that both are at a signal null is much lower, as long as the antennas are far enough apart to effectively de-correlate the signals. Ideally, the antennas should be as far apart as possible, though a minimum distance of half the wavelength of the transmitted signal frequency is generally effective. With a transmitting frequency of 500 MHz half a wavelength is roughly 30 centimeters.

Another way of de-correlating signals is to orientate the receiving antennas at 90 degrees to each other. The two antennas receive different signal polarisations (reflections tend to scramble signal polarisation randomly). In this case the antennas can be co-located.

Implementing Antenna Diversity

How do we make use of two or more antennas in a wireless system? In principle, multiple antennas could be used at the transmitter, or receiver, or both. It is typical for portable wireless microphone systems to use a single antenna on the transmitter and diversity antennas at the receiver because of the need to minimise transmitter size and power dissipation.

A simplistic diversity implementation is to use an electronic switch to connect one of two antennas to a single receiver. The receiver monitors the signal quality, and when signal at one antenna falls below some threshold, it switches to the other antenna. There are two disadvantages to this simple approach. Firstly, the receiver has no knowledge of the signal quality from the unused antenna; it may make a false choice and switch to an antenna with worse signal quality. Secondly, particularly in an analogue system, it is difficult to make the switching between the two signals completely inaudible.

A better, and widely used, approach is to have two complete receiver chains. This is sometimes referred to as ‘true diversity’. As with the simple receiver, this can be used in an analogue or a digital system.

In the case of the analogue system, both receiver chains make some measure of signal quality (often measuring signal strength). At a point in the demodulation process, usually immediately after the FM demodulator, the output from the better signal chain is selected and sent to the final output. A slight improvement on this is to perform a cross-fade between the audio from the two chains. The signals are weighted according to the signal quality.

Diversity Switching in Digital Systems

In a digital system, the diversity switching is usually implemented on data streams before audio decoding. At the expense of a small delay in the data streams, it is possible to make this switching completely seamless, and effectively anticipate the loss of signal of one stream or the other before it happens. This is sometimes known as selection diversity, and is highly effective.

However, digital receivers can use additional strategies to improve performance. The A10 Digital Wireless System carries out two additional processing steps, 1) it calculates a weighted sum of the two signals and, 2) the combining ratio varies with frequency.

Even if the signal from one of the antennas is below the signal / noise decoding threshold of the data demodulator, it still contains some useful information. Instead of performing a hard switch between the data streams from the two receivers, a weighted sum of the two signals is calculated. The weighting depends on an estimate of the channel signal / noise ratio (the approach is known as maximal ratio combining). This gives theoretically optimum reception in a non-frequency selective (flat) channel.

However, to improve reception given a frequency selective channel, a further step is possible. Instead of having a single combining ratio for the whole channel, the combining ratio varies with frequency across the channel to take account of the frequency selective fading. This combining is then theoretically optimum for both frequency selective and non-frequency selective channels.

What is the practical consequence of this? It turns out that this approach results in a 3 dB improvement in receiver sensitivity compared to selection diversity even without any multipath.  Given the presence of flat fading or frequency selective fading, the improvement can be substantially greater, depending on the severity and nature of the fading.

Conclusion

Managing multipath interference is an essential feature of any wireless microphone system. The A10 system from Audio Ltd uses several advanced signal processing techniques to maximize robustness under the most difficult reception conditions.

The A10 Digital Wireless System has a significant advantage over analogue FM wireless and other digital wireless systems in its ability to reject interference. This arises because of three main reasons.

1. The A10 uses two cascaded SAW filters. These precision RF filters are excellent at rejecting out-of-band signals. The SAW filters used in the A10 Digital Wireless System can closely match the signal bandwidth of the system because the A10 topology is tolerance of the group-delay-distortion caused by these filters. FM wireless systems generally require wider filters than would be ideal for interference rejection because of their susceptibility to group delay and other effects.
2. The A10 Digital Wireless System can operate in lower signal-to-noise and signal-to-interference environments than an FM wireless system. For some types of interference, this difference can be up to 10 dB or more.
3. Perhaps most significantly, a digital system operates perfectly up to the point of signal loss, when it then mutes. When a digital system is working, it works with no signal degradation. FM systems, on the other hand, degrade gradually, from either noise or interference, and operate with audible artifacts present when in a degraded state. While this may be acceptable in a communications-only wireless application, it is not acceptable for critical production applications. Many forms of interference affect the recovered audio in a particularly objectionable way. This means that analogue systems require a much higher signal-to-interference ratio for acceptable operation than a well designed digital system.

What does this mean for the user? The most obvious advantage is that a digital system can successfully operate on or near a frequency that is exhibiting RF energy (whether noise, carrier, etc.) from an unrelated system in closer proximity than an analogue system. But even more important is its immunity from various non-linearities and spurious signals is greatly enhanced.

These spurious signals arise from imperfections in the transmitter and receiver design. For example, a transmitter placed close to a second transmitter will, to some extent, act as a mixer. The two signals will combine and result in a distorted “wanted” signal and radiation of “unwanted” signals on other frequencies. This is intermodulation. Multiple signals can also mix in the receiver. Depending on the receiver design, interference may present at frequencies when there really isn’t any. A great deal of skill in designing transmitters and receivers is in minimising these effects, but they can’t be eliminated completely.

The result is that, for analogue systems, it is necessary to publish tables (or a spreadsheet, or computer program) showing allowable combinations of frequencies. The more systems in simultaneous use, the more frequencies that become unavailable because of unwanted spurious signals.

Digital systems like the A10 have no consequential intermodulation and thus no IM products with which to contend. Because of it’s much greater tolerance of interference, the A10 Digital Wireless System is immune from this problem. The interference is still there, but it has no effect on the transmitted audio. This results in more efficient use of available spectrum and greatly simplifies the deployment of multiple channel systems. A good approach to choosing frequencies for such a deployment is to simply spread them evenly amongst those available.

In summary, the A10 Digital Wireless System exhibits both enhanced performance compared with analogue FM systems, and is also much simpler to deploy in multi-channel environments.

Analogue FM wireless system use a compandor circuit in the signal chain to maintain dynamic range. A compander first compresses audio at the transmitter by a fixed compression ratio before RF modulation. At the receiver the signal is then expanded by the same ratio after demodulation. Companders help overcome the noise limits inherent in narrow bandwidth FM radio signals.

This analogue companding process can produce audible artifacts, including ‘pumping’ effects at low levels and garbled high-frequency transients. Some analogue wireless systems use digital companding circuits to address the limitations of analog companding, though these systems still transmit analogue audio. These “hybrid” systems send audio through an analog-to-digital converter and digital signal processing at the transmitter and receiver to address the limitations of analogue RF. Hybrid systems introduce >3 ms of latency, from transmitter to receiver. 

The all-digital topology of the A10 Digital Wireless System eliminates all companding. Audio is converted to a digital data stream at the transmitter and that digital data stream is received, without signal loss, at the receiver. The signal sent over an A10 Wireless link is not modulated audio, but bits. The end-to-end latency of the A10 Digital Wireless System is a category-leading 2 ms.