Mitigating TDMA Noise in Microphone Lines
The number of wireless devices operating at the same time in the typical home, office, or public space is increasing rapidly. For instance, an average home might have two or three smartphones and a similar number of smart speakers, as well as laptops and other connected IoT devices, all functioning simultaneously.
With multiple wireless devices operating in the same environment, communication waves from cellular or Wi-Fi sources can intrude into the other nearby devices' microphone lines, causing interference and creating noise in the audible bands. This phenomenon is known as time division multiple access (TDMA) noise and can manifest as unpleasant sounds emanating from speakers.
What is TDMA noise?
Have you ever been in a conference meeting and accidentally placed your smartphone near to the speakerphone and heard an irritating, humming noise? Similarly, if you put your phone on the kitchen counter near an FM radio at home, the same thing happens. Moving the phone away from the humming device reduces the problem. But what is it, and why does it happen?
Figure 1: Mechanism for TDMA noise generation
In the past, this had become a severe problem in GSM cellular phones that use the TDMA system (figure 1). GSM communication transmits signals in intermittent bursts; these signal bursts occur at intervals of 4.165ms, which translates into generating a frequency tone of 217Hz that falls within the audible range. As this frequency is in the audible range, if left unaddressed, each burst will be picked up by the microphone line and heard, which is why this phenomenon is generally referred to as TDMA noise.
How to Suppress TDMA Noise
Inserting noise suppression filters in the microphone lines will prevent TDMA noise; however, the properties of these devices must not have a negative impact on the signals or audio quality.
With voice-activated applications, such as a smart speaker, voice becomes the interface for connecting to the application. A key component in these devices is a micro-electromechanical system (MEMS) microphone, which acts as the sound sensor. Unfortunately, the sound inlets in these structures can act as entry points for electrostatic discharge (ESD) generated by human bodies.
Figure 2: Functions of ESD notch filters
ESD countermeasures are, therefore, also required (figure 2). Here, the AVRF series of ESD notch filters is recommended. These multilayer chip varistors protect against transient over voltages associated with ESD.
AVRF ESD noise filters also have a specific parasitic capacitance designed to work. The AVRF series of ESD notch filters are produced using multilayer chip varistor technology; they also have a parasitic capacitance, which can be targeted to specific values. This enables them to provide both protection from transient overvoltages associated with ESD and work with the inductance of the MAF noise suppression filters (LPFs) to filter noise sources associated with wireless communication. Cellular, Bluetooth, or WiFi band interference are examples of this effect. Filter circuits that use a combination of MAF products and AVRF ESD notch filters will, therefore, suppress TDMA noise levels further (figure 3).
Figure 3: Comparison of effectiveness in TDMA noise suppression in microphone lines using the MAF series and ESD notch filters, and a combination of both.
In mobile devices, their built-in antennas are often placed near the microphone lines. This causes the antenna and microphone lines to couple together, reducing the sensitivity of cellular, Wi-Fi, and Bluetooth communication signal reception.
Figure 4: Coupling of antenna and microphone lines
One method of improving reception sensitivity is to isolate the lines from each other (figure 4). The MAF series in the microphone lines will increase the impedance of the coupled circuit.
Just as with TDMA noise suppression, combining the MAF filters with AVRF ESD notch filters will further improve reception sensitivity while providing ESD countermeasures (figure 5).
Figure 5: Reception sensitivity improved when MAF noise suppression filters are combined with AVRF ESD notch filters
The low DC resistance (Rdc) of the MAF series allows audio engineers to connect two types of filter in series. For example, for a mobile device with both cellular and Wi-Fi communication, a filter for cellular band countermeasures and a filter for Wi-Fi countermeasures can be used in combination.
There are numerous combinations of MAF products and AVRF ESD notch filters for microphone lines to suppress TDMA noise and improve reception sensitivity. Using sample kits with the available product range will help audio engineers during development and prototyping by speeding up the process and enable them to optimize their design to the fullest.