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Texas Instruments Analog Microphone and ADC System in Far-field Application Application notes
Application Report
SBAA395 – June 2019
Analog Microphone and ADC System in Far-field
Application
Sungjin Max Roh
ABSTRACT
Far-field audio applications such as AI speakers and soundbars, AI TVs, and other voice-activated
products need the following:
• Wide DR (Dynamic Range)
• High AOP (Acoustic Overload Point)
• Low THD (Total Harmonic Distortion) even for very loud signal
• Low equivalent input noise
• Small form factor with low power consumption
All of these electrical specifications have led to the voice commander getting precise voice command
recognition without any unexpected errors. This application report discusses and shows the benefits and
strengths of analog microphones with Texas Instruments’ TLV320ADC51x0 system in far-field audio
application.
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Contents
Near-Field and Far-Field ....................................................................................................
Constituents in the Far-Field Application .................................................................................
Understanding of Digital and Analog Microphone .......................................................................
Quantization Noise Density for Each Microphone .......................................................................
Dynamic Range in Far-Field................................................................................................
Design of Any Microphone with the TLV320ADC51x0 .................................................................
Conclusion ....................................................................................................................
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2
2
4
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6
8
List of Figures
1
Near-Field and Far-Field from Sound Source............................................................................ 2
2
Digital PDM Microphone for 1-bit Output ................................................................................. 3
3
Analog Microphone with ADC for Multi-bit Output ....................................................................... 3
4
Quantization Noise Density................................................................................................. 4
5
DRE Block Diagram ......................................................................................................... 5
6
THD+N Performance Measurement with DRE Enabled ................................................................ 6
7
THD+N Performance Measurement with DRE Disabled ............................................................... 6
8
Internal Block Diagram of the TLV320ADC5140 ........................................................................ 7
9
Analog Microphones System Block Diagram ............................................................................ 7
10
Digital PDM Microphones System Block Diagram
11
......................................................................
Combination Microphones System Block Diagram ......................................................................
8
8
List of Tables
1
Constituents of Far-field Application....................................................................................... 2
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Near-Field and Far-Field
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Trademarks
1
Near-Field and Far-Field
1.1
Definition
Near-field and far-field are components of the physical distance from the sound source, as shown in
Figure 1. It depends on how far away a listener is from the sound-projecting object. At different distances,
the acoustic energy the wavelength emits is heard differently. It is a major issue to comprehend these
differences and design measurements precisely. Figure 1 implies that up to the second wave length is
near-field, while from the second wave length is a range of far-field.
Sound
Source
Near Field
0 ~ 2 wavelength
Far Field
2 wavelength ~ infinity
Figure 1. Near-Field and Far-Field from Sound Source
The essential idea within the range of a far-field is that 6 dB of sound pressure decreases for each
doubling of distance away from the sound source. For the most of the voice recognition system, the
commander stands within the range of a far-field.
2
Constituents in the Far-Field Application
For a more accurate voice-activated system, far-field applications must obtain these elements, especially
when used for a voice recognition application.
Table 1 explains constituents that must be used for a better voice recognition system.
Table 1. Constituents of Far-field Application
APPLICATION NEEDED
• Crystal clear
• Distortion-free for audio capture
Wide dynamic range microphone and ADC
Microphone close to speakers in loud, noisy environments
Far-field recording
• High AOP (Acoustic Overload Point)
• Low THD (Total Harmonic Distortion even for very loud
signal)
Low EIN (Equivalent Input Noise)
• Small form factor
• Low power consumption
Portability
3
SYSTEM SPECIFICATION NEEDED
Understanding of Digital and Analog Microphone
A microphone is a kind of transducer that converts sound pressure into electrical signal. There are two
kinds of microphones in collection with the sound waves. One of them is digital type, and the other is
known as analog type. This section contains the representing format of microphones, a digital PDM (Pulse
Density Modulation) microphone, and an analog microphone with ADC.
2
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Understanding of Digital and Analog Microphone
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3.1
Digital PDM Microphone System
A digital PDM microphone works 1-bit stream as output with the direct output of the Sigma-delta
modulator. The sample rate of the PDM is typically between a few hundred kHz to 3.072 MHz. This
Sigma-delta modulator needs a decimation filter so that the PDM data can process further. The
decimation filter is implanted in either the codec, or the DSP where the PDM microphone is connected.
This output from the filter sends out data at a lower sample rate of 16 and 48 kHz. This is a weaker point
than an analog microphone with an ADC system shown in Section 3.2. However, a PDM microphone does
have some advantages over an I2S microphone. The PDM microphone typically requires only two signal
lines (PDM clock and PDM data) for audio interface, while I2S requires three or four lines (bit clock, word
clock, audio data, and occasionally master clock). This leads to a beneficial idea where the hardware
engineers are easily able to minimize the physical interface line.
Modulator Clock
3.072 MHz
MIC
1-bit
ADC
PGA
PDM
Interface
1-bit
PDM
Output
Figure 2. Digital PDM Microphone for 1-bit Output
3.2
Analog Microphone with ADC System
An analog microphone with an ADC system based on Texas Instruments’ TLV320ADC51x0 uses multi-bit
modulation ADC to project audio data with a range higher than 24-bit, along with I2S (or PCM, TDM, DSP)
interface, as shown in Figure 3. This is unlike the PDM microphone, which uses the 1-bit modulation with
PDM interface. For the analog microphone with ADC to project as shown in Figure 3, the I2S audio signal
of ADC output requires one or two more interface lines than the two of the PDM signals. The analog
microphone has advantages in in-band (20 Hz to 20 KHz) and out-of-band (above 20 KHz) for
quantization noise characteristics (further explained in Section 4). This is because while the digital PDM
microphone uses a 1-bit ADC modulator with a 3.072 MHz clock, an analog microphone with ADC (Texas
Instruments’ TLV320ADC51x0) uses multi-bit architecture with a modulator clock of 6.144 MHz, which
allows it to have superior qualities in quantization noise density.
Modulator Clock
6.144 MHz
DRE
MIC
PGA
Multi-bit
ADC
Digital
Filters
> 24-bits
I2S/PCM/
TDM/DSP
Output
Figure 3. Analog Microphone with ADC for Multi-bit Output
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Quantization Noise Density for Each Microphone
4
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Quantization Noise Density for Each Microphone
The 1-bit modulator in the digital PDM microphone has architecturally high quantization noise that limits
the PDM microphone SNR and dynamic range compared to what can be achieved with an analog
microphone along with multi-bit ADC. The TLV320ADC51x0 uses a multi-bit modulator which has in-band
quantization noise up to 15 times lower, and better, than the typical 1-bit modulator. Moreover, the total
quantization noise of the TLV320ADC51x0 multi-bit modulator is 30 times lower than the total typical
quantization noise of a 1-bit PDM modulator. The high quantization noise in in-band has worse voice
recognition, and every 6 dB improvement in microphone and system performance can be improved by the
record distance and doubled sensitivity.
The higher total quantization noise of a 1-bit PDM modulator drastically limits the maximum high amplitude
signal that it can record with low distortion compared to a multi-bit ADC. This limitation makes the digital
microphone the incorrect choice to be used for soundbars, HDTVs, professional speakers, and so forth,
where the microphone is required to record very loud speaker output with low distortion so that the echo
cancellation can be achieved with good quality, and voice command detection successfully is enabled.
Figure 4 shows the typical quantization noise density difference in all bands for the 1-bit PDM modulator
and multi-bit analog modulator.
Figure 4. Quantization Noise Density
5
Dynamic Range in Far-Field
5.1
DR
Dynamic range (also defined as DR, DNR, or DYR) describes the ratio between the level at which the
greatest sound converted with very low distortion to the softest sound in the system. In other words, the
audio field uses dynamic range to depict the ratio of the softest sound to the loudest sound level in the
musical instrument system. Therefore, dynamic range works as the SNR (Signal-to-Noise Ratio) for the
case when the signal is the loudest possible in the system.
4
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Dynamic Range in Far-Field
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5.2
DR in Microphone
As shown in Section 5.1 for dynamic range, better SNR, lower THD (Total Harmonic Distortion), and better
AOP (Acoustic Overload Point) create high dynamic range. Here, AOP is the point at which the
microphone can record the loud sound with distortion as high as 10%. The microphone equivalent input
noise (EIN) is used to define microphone SNR to the point at which the microphone no longer effectively
projects the difference between the actual sound signal level and microphone self-noise level. It no longer
works effectively as a sound pressure sensor at this point.
5.3
DRE in the TLV320ADC5140
The TLV320ADC5140 device integrates an ultra-low noise front-end PGA with 120 dB dynamic range
performance with a low noise, low-distortion, multi-bit delta-sigma (ΔΣ) ADC with a 108 dB dynamic range.
The dynamic range enhancer (DRE) is a digitally assisted algorithm to boost the overall channel
performance. The DRE monitors the incoming signal amplitude and adjusts the internal PGA gain
accordingly and automatically. The DRE achieves a complete channel dynamic range as high as 120 dB.
At a system level, the DRE scheme enables far-field, high-fidelity recording of audio signals in very quiet
environments, and low-distortion recording in loud environments. This algorithm is implemented with very
low latency (a few µs loop time), and all signal chain blocks are designed to minimize any audible artifacts
that can occur resulting from dynamic gain modulation.
The block diagram in Figure 5 describes the complete signal chain performance with a high-performance
analog microphone.
As shown in Figure 5, if the system uses the high performance analog microphone with effective dynamic
range performance as high as 114 dB, along with a low-cost, 108 dB ADC without DRE, then the overall
signal chain dynamic range gets limited to 106.8 dB (114 dB–7.20 dB) due to the 108 dB ADC. However,
once the DRE scheme of the TLV320ADC5140 is enabled, an overall signal chain dynamic range of
112.97 dB (114dB–1.03dB) can be achieved in a very economical manner, even after using a low-cost,
108 dB ADC.
Mic SNR = 70 dB
Mic 1% THD = 138dBSPL
Mic DR = 114 dB wrt
2Vrms
Mic o/p noise = 4 µVrms
PGA SNR = 120 dB wrt
2Vrms
PGA Gain = 0 dB
PGA i/p referred noise =
2 µVrms
Mic + PGA noise =
4.46 µVrms
ADC SNR = 108 dB wrt
2 Vrms
ADC noise = 7.96 µVrms
Mic + PGA + ADC noise =
9.12 µVrms
Digital Gain = 0 dB
Final o/p noise = 9.12 µVrms
SNR degradation = -7.20 dB
V2 n-PGA_IN
™¨
Modulator
MIC
V2 n-Mic
Digital
Filters
PGA
DRE
PGA SNR = 120 dB wrt
2Vrms
PGA Gain = 24 dB
PGA i/p referred noise =
2 µVrms
Mic + PGA noise =
70.61 µVrms
ADC SNR = 108 dB wrt
2 Vrms
ADC noise = 7.96 µVrms
Mic + PGA + ADC noise =
71.06 µVrms
Digital Gain = -24 dB
Final o/p noise =4.48 µVrms
SNR degradation = -1.03 dB
Figure 5. DRE Block Diagram
5.4
DRE Performance in TLV320ADC5140
Figure 6 and Figure 7 show the THD+N measurement results with and without DRE. The measurement
graph with DRE enabled (DRE threshold set to -36 dB) shows DR improvement of 12 dB and it also
improves the record distance and sensitivity by four times.
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Design of Any Microphone with the TLV320ADC51x0
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-60
-70
Channel-1 : DRE enabled
Channel-2 : DRE enabled
Channel-3 : DRE enabled
Channel-4 : DRE enabled
THD+N (dBFS)
-80
-90
-100
-110
-120
-130
-130
-115
-100
-85
-70
-55
-40
-25
-10
Input Amplitude (dB)
0
THD+
Figure 6. THD+N Performance Measurement with DRE Enabled
-60
-70
Channel-1 : DRE disabled
Channel-2 : DRE disabled
Channel-3 : DRE disabled
Channel-4 : DRE disabled
THD+N (dBFS)
-80
-90
-100
-110
-120
-130
-130
-115
-100
-85
-70
-55
-40
-25
-10
Input Amplitude (dB)
0
THD+
Figure 7. THD+N Performance Measurement with DRE Disabled
6
Design of Any Microphone with the TLV320ADC51x0
6.1
Structure of the TLV320ADC51x0
Texas Instruments’ TLV320ADC51x0 is four analog microphones, eight digital microphones, or a
combination of both that an audio analog-to-digital converter can design as the input with four internal
Sigma-delta ADCs and eight digital microphone interfaces. This enables the hardware designer to have
access to flexible design structures, especially supporting the microphone array design.
6
Analog Microphone and ADC System in Far-field Application
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GPIO1
IN1P_GPI1
IN1M_GPO1
ADC
Channel-1
PGA
IN1P_GPI2
PGA
ADC
Channel-2
IN1P_GPI3
IN1M_GPO3
PGA
ADC
Channel-3
IN1P_GPI4
IN1M_GPO4
PGA
ADC
Channel-4
IN1M_GPO2
MICBIAS
Programmable
Microphone Bias
Audio Clock Generation
PLL
(Input Clock Source ± BCLK,
GPIOx, GPIx)
8-channel Digital Microphone Filters
Multifunction Pins
(Digital Microphones Interface,
Interrupt, PLL Input Clock)
SDOUT
Digital Filters
(Low Latency LPF
Programmable
Biquads)
Audio Serial
Interface
(TDM, I2S, LJ)
and
BCLK
Dynamic Range
Enhancer (DRE)
FSYNC
I2C or SPI
Control
Interface
Regulators, Current Bias and
Voltage Reference
SHDNZ
SDA_SSZ
SCL_MOSI
ADDR0_SCLK
ADDR1_MISO
IOVDD
Thermal Pad (VSS)
DREG
AREG
AVSS
AVDD
VREF
Figure 8. Internal Block Diagram of the TLV320ADC5140
6.1.1
Design Example 1: Only Analog Microphone System
A differential analog microphone can be formed up to four multi-channels. In this case, the output signal
changes the front-end PGA into gain control, then uses high-performance, multi-bit sigma-delta AD to be
converted by ADC. After this process, it is sent to the decimation filter.
GPIO1
8-channel Digital
Microphone Filters
Multifunction Pins
(Digital Microphones Interface,
Interrupt, PLL Input Clock)
PGA
ADC
Channel-1
PGA
ADC
Channel-2
PGA
ADC
Channel-3
PGA
ADC
Channel-4
Figure 9. Analog Microphones System Block Diagram
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Conclusion
6.1.2
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Design Example 2: Only Digital Microphone System
The digital PDM microphone can be formed up to eight multi-channels. In this case, the PDM output from
the microphone goes through PDM interface to be sent to the decimation filter.
GPIO1
8-channel Digital
Microphone Filters
Multifunction Pins
(Digital Microphones Interface,
Interrupt, PLL Input Clock)
PDM mic
PDM mic
PGA
ADC
Channel-1
PDM mic
PDM mic
PGA
ADC
Channel-2
PDM mic
PDM mic
PGA
ADC
Channel-3
PDM mic
PDM mic
PGA
ADC
Channel-4
Figure 10. Digital PDM Microphones System Block Diagram
6.1.3
Design Example 3: Analog and Digital Microphone Combination System
The system can be formed as desired with analog and digital PDM microphones combined. In this case,
each input goes through design example 1 and 2, and is gradually sent to the decimation filter.
GPIO1
8-channel Digital
Microphone Filters
Multifunction Pins
(Digital Microphones Interface,
Interrupt, PLL Input Clock)
PGA
ADC
Channel-1
PGA
ADC
Channel-2
PDM mic
PDM mic
PGA
ADC
Channel-3
PDM mic
PDM mic
PGA
ADC
Channel-4
Figure 11. Combination Microphones System Block Diagram
7
Conclusion
By using the advantages of the analog microphones with Texas Instruments’ TLV320ADC51x0 system
that brings in the far-field application, quantization noise density due to the modulation improves, which
brings up the height to the DR system. This leads to the near perfection of the voice recognition system
that far-field application requires. The TLV320ADC51x0 is an analog or digital microphone input used for
an audio analog-to-digital converter that can be designed to be accommodated as needed.
8
Analog Microphone and ADC System in Far-field Application
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