Fault depth analysis of six core technologies of wireless microphone

Fault depth analysis of six core technologies of wireless microphone
Wireless microphone is the core equipment of audio system in commercial scenes such as conference room, stage performance and live broadcast with goods. When most users encounter problems such as broken sound, noise, howling and pairing failure, they simply replace the battery and re-align the frequency, but ignore the underlying technical incentives such as RF circuit, power loss, spectrum interference and sound field feedback. In this paper, from the three dimensions of hardware principle, environmental interference and system collocation, six high-frequency technical fault sources and standardized solutions are dismantled to help users quickly locate and solve audio abnormal problems at one time.
1. Intermittent mobile radio reception and long-distance intermittent mute: multipath signal fading is the main reason for invisibility.
Many people think that the sound break is only caused by the wall and metal occlusion. The measured data show that nearly 70% of indoor signal jams come from RF multipath reflection cancellation, not linear occlusion.
1, the underlying principle
The UHF wireless microphone RF signal will generate reflected waves when it touches the wall, glass, metal cabinet and LED screen. When the direct signal and the reflected signal reach the receiving antenna, the phases are offset, forming a signal blind area. Even if the microphone and receiver are unobstructed, a large area of hard reflective building materials will still cause short-term sound interruption.

2. Three easily overlooked incentives
① The antenna placement is not standardized: the dual antennas of the receiver are placed in parallel and horizontally, and the diversity receiving gain is directly attenuated by 9dB；; Double antennas should be placed vertically, with a distance of ≥15cm, away from the metal cabinet and large screen for more than 1.5m.. ② Battery voltage drop: When the voltage of two alkaline batteries is lower than 1.25V, the power supply of the internal phase-locked loop is insufficient, and the RF transmission power is greatly reduced, and the nominal transmission distance is directly reduced by 70%; Mixing old and new batteries will aggravate the voltage difference and cause frequent frequency drift. ③ Spectrum crowding of peripheral wireless devices: Wi-Fi6 routers, Bluetooth devices and monitoring bridges continuously release broadband clutter, which raises the environmental noise floor. When the signal-to-noise ratio of the receiver is lower than the critical value, the squelch circuit automatically cuts off the audio.

3. Standardized processing flow
Replace new batteries in the same batch; Adjust the cross placement of receiver antennas; Lock the blank and clean frequency points by using the automatic spectrum scanning function of the receiver; Raise the height of the receiver and reduce the reflection interference of ground acoustic waves.

Second, the infrared frequency matching fails, and there is no sound after the pairing is completed: physical contact and output gear are easy to step on the pit.
After-sales statistics show that 92% of the abnormal pairing is not caused by the damage of the RF motherboard, but by the loss of the infrared synchronous link and the wrong setting of the output port, so it is not necessary to directly judge the equipment failure.
1. The core reason of infrared synchronization failure
Fingerprints and dust are attached to the infrared window of the microphone and receiver, and the infrared light attenuation exceeds 40%; When synchronizing, the distance between them is more than 20cm and the offset angle is more than 15, so the photosensitive element cannot receive the synchronization signal; The metal reed of the battery compartment is oxidized, the synchronous instantaneous voltage drops suddenly, and the power of the infrared module is insufficient. Solution: Wipe the infrared window with an anhydrous alcohol cotton swab and clean the battery contacts. When synchronizing, the microphone is operating close to the infrared window of the receiver.

2. Two misunderstandings: successful pairing but silence
① Incorrect switching of output gear: the dialing codes on the back of the receiver are divided into LINE output and MIX output. The live sound card and MIXer are only compatible with mix mix files, and the volume will be extremely low or even silent when switching to LINE files; ② The squelch SQ threshold is too high: when the squelch knob is set to the maximum position, the slight signal fluctuation will be judged by the circuit as noise cutting off the human voice, and the stable radio reception can be restored after being adjusted to the middle position.

Third, continuous buzzing current floor noise and fine rustling noise: grounding loop and switching power supply ripple are the main reasons.
The whole sound reinforcement system appears low-frequency humming, and 80% of the scenes are not the failure of the microphone itself, but the mismatch of the power supply system of the whole audio equipment.
1. Noise principle of grounding loop
When the mixer, microphone receiver, computer and power amplifier are plugged into different sockets, there is a potential difference between the zero ground wire of the power grid, forming a loop current, which is mixed into the audio channel through the audio signal line to produce a continuous low-frequency current sound; Air conditioner, LED driver and printer switching power supply release high-frequency ripple, which will form fine rustling noise.

2. Scenario solution
Fixed meeting room: all audio equipment shall be connected to the power outlet with filtering function, and the grounding terminals of the equipment shall be connected to the ground. It is forbidden to share the power supply circuit with high-power electrical appliances. Outdoor live broadcast scene: the receiver and sound card are powered by independent lithium batteries, the mains ground loop is disconnected, and the double-layer shielded cannon audio line is used to reduce electromagnetic coupling interference; Optimization of equipment parameters: the input gain of the receiver is controlled within 60%, and the gain is not pushed up to avoid synchronous amplification of RF noise floor.

Fourth, the high-frequency harsh howling can not be suppressed: distinguish between acoustic feedback howling and radio frequency interference abnormal sound
There is a common misunderstanding in the industry: confuse RF interference noise with space acoustic feedback howling, and use feedback suppression function to deal with RF noise, which will aggravate the distortion.
1, spatial acoustic feedback howling (sound box and human voice cycle resonance)
Production conditions: the microphone picks up the amplified voice of the speaker, and the sound waves are cyclically superimposed and amplified to form a sharp high-frequency howling. Long-term high-power howling will damage the power amplifier and tweeter unit. Debugging scheme: the microphone and speaker are separated diagonally by more than 3 meters, and the microphone is prohibited from facing the speaker; Turn on the built-in digital feedback suppression DFS； of the device; Add carpets and sound-absorbing curtains indoors to weaken the sound wave reflection of hard walls; 30% margin is reserved for channel gain, and the volume fader is not fully pushed.

2, radio frequency interference abnormal sound (no fixed tone, intermittent buzz)
Features: There is still noise when the speaker is turned off and the earphone is monitored. The noise is weakened when it is far away from the router and on the big screen. The feedback suppression algorithm is ineffective for this kind of noise, and it can only be solved by frequency spectrum switching and antenna layout optimization.

Five, the battery life is greatly reduced, and the power is automatically cut off after 1-2 hours of use: RF load loss is often ignored.
With the same microphone, the battery life gap of different users can reach 4 times, and the core difference lies in the load power consumption of RF module:
1. Under the occlusion and strong interference environment, the transmitting chip automatically increases the RF power to maintain the connection, so that the power consumption of the whole machine is increased by 2-3 times and the power consumption speed is doubled; Radio frequency low-power standby in open and undisturbed environment, and the battery life is greatly extended.
2. The temporary mute is not turned off, and the transmitter continues to scan the channel circularly, so the standby power consumption has not decreased significantly. It is not necessary to suggest that the whole machine be powered off directly in the short term, instead of just mute.
3. The discharge platform of Ni-MH rechargeable battery is low, and the power is insufficient when the voltage is lower than 1.05V; Brand-new alkaline batteries are preferred for outdoor performances, and rechargeable batteries can be selected for indoor short-term use.
Six, the simultaneous use of multiple microphones, cross frequency, mutual frequency scramble: lack of channel planning.
Dual-channel and quad-channel wireless microphones are turned on at the same time, and the microphones are disconnected from each other, and the root cause is the chaotic frequency point planning:
1. When multiple devices are turned on, automatic frequency scanning is not performed, and adjacent overlapping frequency points are automatically occupied, and radio frequency signals are mutually suppressed;
2. Multiple sets of wireless receivers are stacked at close range, the antenna spacing is insufficient, and the signals interfere with each other. Standard operation: each set of equipment performs spectrum scanning independently and locks non-overlapping frequency points; Multiple receivers are scattered, and the antennas are staggered. Each set of equipment is reserved with a spacing of more than 30cm.
Most audio faults of wireless microphone can be solved by five standardized processes: power supply detection → RF antenna optimization → frequency spectrum scanning and frequency switching → audio link verification → sound field power supply debugging; After completing the above operations, it is still impossible to improve, and then judge the hardware failure of the equipment motherboard. It is suggested that equipment testing and environmental interference investigation should be completed in advance before project installation and activity performance, so as to avoid abnormal radio reception from the source and ensure the stable operation of audio system.