introduction
With the continuous miniaturization of communication products, the cost and space constraints of the system are also increasing. The volume of the speaker becomes smaller and smaller, and the distance between the microphone and the speaker continues to shorten, making the echo more difficult to eliminate. In complex use environments such as commercial areas, cars, and subways, environmental noise has seriously affected the quality of calls. How to realize the full-duplex hands-free function in small systems such as PDAs, notebook computers, hands-free mobile phones, etc., and ensure a sufficiently large volume has become a problem.
FM1083 is a low-power chip for echo cancellation, noise suppression, and far-end pickup of sound signals introduced by Fortemedia. This article will introduce the design features of small hands-free communication products, the main functions and typical applications of FM1083.
Design Points
The mold housing, microphone and speaker are related to the hands-free performance of the entire system. There are two ways to generate echo: acoustic propagation and mechanical vibration. From the point of view of continuity of frequency spectrum, echo can be divided into two types: linear and nonlinear. In the mold design stage, consideration should be given to minimizing the linear echo picked up by the microphone and reducing the nonlinear vibration of the speaker and the enclosure. The microphone should be designed with an independent cavity, pay attention to the cooperation of the front and rear covers of the housing, otherwise the echo will increase. The loudspeaker should be fully fixed, and damping gaskets should be added between the loudspeaker and the enclosure.
FM1083 supports one unidirectional or omnidirectional microphone and two array microphones. The array microphone is composed of omnidirectional and unidirectional microphones. In applications that use a microphone, the microphone should pick up the far-end (FE) sound, that is, the difference between the echo and the near-end (NE) speech signal, as small as possible. When the signal of near end SN picked up by the microphone is 10mVpp, the signal of echo SE picked up by the microphone is 40mVpp, the loop gain is 20dB, the amplitude of the line output is 100mVpp, and there is no echo cancellation Under the circumstances, the echo will reach 400mVpp, unable to communicate at all. In order to eliminate the echo, the echo must be at least two orders of magnitude smaller than the useful signal, that is, the echo must be reduced by more than 40dB (the echo will be less than 4mVpp), so as not to affect the useful signal. The system with FM1083 and two array microphones can eliminate echoes up to 65dB, making communication clear and smooth. For the application of two microphones, the sound of the far end (FE) picked up by the main microphone M1 and the reference microphone M2 should be kept at a certain difference, and the echo picked up by M1 and M2 must be different from the signal of the near-end (NE) speech. Value, the signal difference of the near-end (NE) speech picked up by M1 and M2 should be as small as possible. The frequency spectra of the echoes picked up by M1 and M2 cannot overlap in the band.
The main functions and features of FM1083
FM1083 is specially designed for mobile handheld devices. The two built-in DC / DC converters provide power for CODEC and DSP. The power consumption during 1.8V operation is less than 30mW. The digital circuit part supports a wide voltage of 1.8 to 3.6V. Built-in class D Power amplifier, differential output can directly drive the speaker. The multimedia codec interface (CODEC Host Interface) is directly connected to the Bluetooth 1.1 and 1.2 interfaces. With two microphone inputs, built-in programmable gain input and output amplifier (-6dB ~ + 26dB), adapt to microphones with different sensitivities. When picking up at the far end, the distance between the microphone and the user is 0.6m, and the maximum can reach 5m. The analog signal supports pass-through mode to improve fidelity. The range of echo cancellation is 45 ～ 60dB. It supports dynamic range compression and nonlinear echo cancellation. It is suitable for the overload of small speakers at high power output. The minimum distance between the microphone and the speaker can reach 40mm, and the noise suppression is 13 ~ 18dB, which is suitable for noisy environments. The sampling rate is 8kHz, supports 4.096MHz and 13MHz clocks, and has three operating modes with default / EEPROM / UART microprocessor control.
FM1083 interface
FM1083 supports USB1.1 slave mode, and can transmit audio data with the computer through the USB interface, as a USB hands-free device for voice communication.
The EEPROM interface supports 256B to 1KB of low-voltage EEPROM. In the absence of a microcontroller, most variable parameters of the FM1083 can be stored.
The audio data codec interface supports master mode and slave mode. You can select internal and external clock sources. When the internal clock is selected, the frame frequency is 8kHz, and the data encoding format is 16-bit linear PCM (Pulse Code ModulaTIon, pulse code modulation) or Bit zeros fill PCM, or 8-bit μ-law or A-law coding.
The URAT interface is used to program the EEPROM or the microcontroller to directly control the FM1083. The interface baud rate at 4.096MHz is 300 ～ 230400b / s, and at 13MHz is 9600 ～ 38400b / s. The system structure of FM1083 is shown in Figure 1.
Signal flow
The signals picked up by the main microphone 1 and the reference microphone 2 are amplified by a programmable gain amplifier, analog-to-digital conversion and high-pass filtering, and then sent to the speech processor for processing (linear echo cancellation, nonlinear echo cancellation, VAD detection, noise suppression processing, and then the microphone Automatic gain control, side tone cancellation and microphone volume setting), the output 16-bit voice data undergoes digital-to-analog conversion, and the microphone signal is output from the line output terminal for single-ended output.
The far-end voice signal is single-ended input from the line input, converted into 16-bit voice data after analog-to-digital conversion and high-pass filtering, and sent to the voice processor for processing (side cancellation, automatic gain control, echo reference signal extraction, dynamic range control) , After the output signal undergoes digital-to-analog conversion, the built-in Class D amplifier is driven to drive the speaker directly in a differential manner.
In the application of USB, the input signal of USB is used as the signal of the line input, and it directly drives the speaker after being processed by the voice processor. The microphone input signal is processed by the voice processor and sent to the computer via USB. The process is the same as the analog signal input / output.
When Bluetooth is used, the signal of the microphone is processed and sent to the Bluetooth chip in the format of 13-bit or 16-bit linear PCM through the audio data interface. The Bluetooth chip sends the received sound data to the FM1083 in the same format, and finally outputs it after the power amplifier .
Small hands-free system
As a small integrated hands-free system, it is characterized by simple lines, versatile functions, and easy portability. The entire system occupies only 80mm & TImes; 55mm of space. The system takes FM1083 and CSR MBT-4107 Bluetooth module as the core, built-in 600mA lithium battery and USB 200mA charging circuit. The typical application is shown in Figure 2.
High frequency board dielectric material has the following characteristics
1. It has the characteristics of small signal transmission loss, short transmission delay time and small signal transmission distortion.
2, has excellent dielectric properties (mainly DK, DF). Moreover, this dielectric property (DK, DF) maintains its stability under environmental changes in frequency, humidity, and temperature.
3. High-precision control with characteristic impedance (Zo).
4, has excellent heat resistance (Tg), processability and adaptability.
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