Welcome to the tenth installment of All About Wireless. In this issue, we will examine the stereo multiplexing process and discuss techniques for maximizing the performance of IEM systems.
How do IEM Systems Work?
IEM systems are typically required to transmit stereo audio signals. To achieve this, the RF carrier is encoded to transmit two audio signals via a process referred to as stereo multiplexing (MPX). In a stereo IEM system set to operate in mono, the stereo input is summed to mono at the transmitter, then transmitted and demodulated as a standard FM signal. When the transmitter is set to operate in stereo, Sum (L+R) and Difference (L-R) signals are transmitted along with a 19kHz pilot tone. The 19kHz pilot tone serves to inform the receiver that the transmission is encoded in stereo.
The MPX encoding scheme specifies that a 38kHz sub-carrier is Amplitude Modulated by the Difference signal. The Difference signal is therefore transmitted above the audio frequency range. The 38 kHz sub-carrier is suppressed, but both the upper and lower AM sidebands are retained. These sidebands are then combined with the Sum signal and used to frequency modulate the carrier.
A mono receiver presented with an MPX encoded stereo transmission will only demodulate the L+R mono signal from 30Hz – 15kHz. All other components above 15 kHz are suppressed by the receiver.
A stereo receiver is able to demodulate the MPX encoded carrier to output discrete left and right channels. The 19kHz pilot tone, initially used to notify the receiver that the transmission is a stereo MPX encoded signal, is doubled in the receiver to synthesize the original 38kHz sub-carrier. This is then used to demodulate the double sidebands to obtain the Difference signal. The left audio channel is equal to the electronic sum of the Sum and Difference components. The right audio channel is equal to the electronic difference of the Sum and Difference components. In other words, (L+R) + (L-R) is equal to 2L (left audio channel), and (L+R) – (L-R) is equal to 2R (right audio channel).
The frequency response of a stereo multiplexed transmission is fixed at 30Hz – 15kHZ. IEM transmitters feature internal filtering to ensure audio signals above 15kHz are attenuated prior to modulation to ensure they do not modulate the 19kHz pilot tone. Accidental modulation of the pilot tone can result in distortion and degradation of the stereo image, or in severe cases, muting of the IEM receiver.
Ensuring Mix Integrity
The key to ensuring mix integrity in IEM systems is to respect the frequency response limitations of the MPX process. Excessive gain applied to high audio frequencies at the mixing console is further exaggerated by the pre-emphasis EQ applied in the transmitter. This can severely degrade the integrity of the pilot tone, regardless of the transmitter’s internal filtering. Inserting notch filters at 16kHz on the console output busses will increase the slope of the transmitter’s internal filter, helping to maintain pilot tone integrity.
The pilot tone is not the only characteristic of stereo multiplexing that needs to be considered. The upper harmonics of widely panned audio signals may aggravate the L-R Difference signal sidebands, especially if the audio signal features sharp transients. This too may cause degradation of the stereo image and frequency response may also be compromised. In extreme cases, dynamic interactions between one channel and the other can be detected. For this reason, limiting the stereo width of dynamic high-frequency audio sources can make challenging signals easier to encode and faithfully demodulate.
These phenomena are commonly misdiagnosed as hardware faults, RF interference, or dying batteries. In many cases, these anomalies are actually the sound of stereo multiplex transmission breaking down at its fundamental operating principle. If source audio is allowed to modulate the pilot tone, stereo reception and the resultant sound quality will be poor. If transient high-frequency signals panned hard left and right are prominent in the mix, stereo imaging and frequency response can be compromised.
Many audio engineers have discovered that switching the IEM receiver to mono mode seems to solve these problems. The pilot tone is not required for demodulation in mono, so switching to mono merely hides these problems. Providing artists with a coherent stereo IEM mix by mixing appropriately for MPX is a better solution. That said, the use of the mono operating mode may be useful in congested RF environments where the stability of a stereo multiplexed transmission may be compromised by interference.
In terms of RF noise, IEM systems consisting of 3 or more channels require active combining in order to use a single transmit antenna. Connecting individual antennas directly to each transmitter is not recommended as this can enhance the power level of intermodulation products, increasing the risk of interference. Passive combining of IEM transmitters is also not usually recommended as this can lead to the generation of intermodulation products at disruptive power levels. Active combiners are recommended as they are designed to combine IEM transmitters while suppressing the power level of intermodulation products and keeping the signal noise floor to a minimum.
Another common issue with IEM receivers is that most models feature a single receive antenna. Unfortunately, the lack of a diversity receiver makes the system more susceptible to multipath dropout. Some manufacturers market their IEM receivers as diversity systems, but it’s the earphone cable that is sometimes used as the second antenna.
The diversity performance of such systems is questionable as the earphone cable is not usually tuned appropriately to act as an antenna. Thankfully, the Shure PSM 1000 system features a belt pack IEM receiver with two tuned antennas for true diversity performance. The RF performance of a true diversity IEM receiver is much more stable, especially in congested RF environments.
Whilst most manufacturers have now migrated towards digital modulation schemes for wireless microphone systems, the majority of IEM systems currently available are still analog. In the future, if extremely low latency systems can be realized, we may see manufacturers move towards digital modulation schemes for IEMs as well.
Until then, Shure has implemented a hybrid approach whereby PSM receivers feature a digital audio architecture in conjunction with the analog MPX transmission scheme. The hybrid approach allows Shure to offer the benefits of high quality digital audio processing with no sacrifice in system latency.
In our next post, we begin examining some key differences between analog and digital wireless microphone systems. We compare bandwidth requirements and the impact of noise in each system.
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