The antenna signal strength meters, or “Receive Signal Strength Indicators” (RSSI) commonly seen on wireless receivers are not always the most helpful. Sometimes you might have full bars of RSSI but still experience audio dropouts or poor performance. The RSSI is a visual representation of how well the antennas on the receiver are picking up a wireless microphone. But a crucial piece of information is missing: the amount of in-band interference nearby that could be affecting your wireless system’s performance.
Enter the Link Quality Indicator (LQI), sometimes called a Quality Meter or simply a “Q” meter. The Q-meter measures the difference between the signal from the wireless mic transmitter (what the RSSI measures) and any in-band interferers or in-band noise. This difference is called the carrier-to-interference ratio, or C/I ratio for short.
In-band interferers are usually low-level RF signals caused by other transmitting sources in the vicinity, such as TV towers or other nearby wireless microphones. For Sound Devices wireless transmitters, in-band interferers are any unwanted signals within about +/- 200 kHz from the frequency the wireless mic is tuned to.
Luckily, a digital wireless system can handle stronger interference than analog wireless systems. Analog wireless systems depend on absolutely pristine spectrum for good audio. Much like listening to a record album which has any dust on it, when listening to analog wireless, it is easy to hear any in-band interferers. Audible artifacts can sound like static noise or distortion in the audio, so it is crucial to predict possible in-band interference. Frequency coordination is used to avoid creating intermodulation artifacts when wireless transmitters operate near each other.
With digital wireless systems, the audio is represented as a digital bitstream sent over the air, much as a CD is to a record album. The audio is perfect until it is not there at all. Depending on the modulation type, a digital wireless transmission may only need 20dB of C/I ratio to achieve perfect audio over the air. For this reason, absolutely pristine spectrum is not needed for digital wireless transmission. Low-level interference will not create audible artifacts. In fact, in the USA, licensed users are allowed to operate digital wireless microphones indoors on top of low power DTV and with adequate C/I these systems can still achieve perfect audio quality.
Users of analog wireless microphones are likely familiar with how sound quality degrades when reception is poor. With a digital wireless microphone, audio quality will not degrade, but range will decrease when reception is poor. This is where the Q-meter is useful.
|Perfect, very low probability of audio dropout.|
|Good, but not perfect. Either some interference happening or signal getting weaker.|
|Beware, quality is dropping, audio dropouts are possible.|
|Drop-outs definitely occurring – consider taking remedial action.|
|Very weak – audio cutting in and out.|
While the Q-meter may be useful on its own, the best use of the Q-meter is in conjunction with the RSSI meter.
|If the RSSI is high and the Q-meter is high, this indicates that all is well – the best of all worlds.|
|If the RSSI is high, but the Q-meter is low, this indicates that there must be high in-band interference.|
|If the RSSI is lower, but the Q-meter is high, this|
indicates that all is well, and the user is operating in very clean RF spectrum.
|If both the RSSI and the Q-meter are low, this indicates that the receiver is operating on a clean frequency, but the signal from the transmitter is very faint.|
The Q-meter gives digital wireless system users additional information about the RF spectrum so they can quickly adapt to interference and avoid dropouts or poor performance.
But what settings should be adjusted for better Q-meter readings? Watch this video with Sound Devices RF Applications Engineer Gary Trenda to learn more: