CMPE 650 – High Speed Digital Design

Final Project

Audio Acquisition, Storage and Playback

Jerome Stanislaus

Kirit Chatterjee

Spring 2011

Introduction:

The goal of this project is to design a PCB to capture audio signal through a

playback from the card through a speaker

and analyzing using MATLAB.

The project had a problem that the input from the microphone picked up a lot

going to the ADC. Also, the 2 LSBs of the ADC are stuck at 01 and hence

from these, the other parts of the project worked fine as expected. The audio capturing is tested by pr

low frequency SINE and TRIANGULAR at the input of the ADC and stored it to the SD Card. The

using DAC and SD Card are tested using a WAV file present inside the SD card

Materials Used:

IC PIN Package

TPA6111A2 8 SOIC

TLC7524 16 SOIC

OPA344 8 SOIC

TLC0820A 20 SOIC

ATMEGA328P 28 DIP/SMD

LD1117S33CTR 4 SOT-223

Table 1. List of ICs used and their function

Block Diagram & Theory:

Figure 1 is a simple block diagram of the whole system.

The goal of this project is to design a PCB to capture audio signal through a microphone, store it in an SD card and

playback from the card through a speaker. This also involves sending the acquired data through USB to the laptop

The project had a problem that the input from the microphone picked up a lot of noise and it gets amplified before

going to the ADC. Also, the 2 LSBs of the ADC are stuck at 01 and hence we had constant quantization error. Apart

other parts of the project worked fine as expected. The audio capturing is tested by pr

low frequency SINE and TRIANGULAR at the input of the ADC and stored it to the SD Card. The

DAC and SD Card are tested using a WAV file present inside the SD card.

Function

150-mW Stereo Headphone Audio Amplifier

8-Bit, 0.1 us MDAC, Parallel Input, Fast Control Signalling fo

Low Power, Single-Supply, Rail-To-Rail Operational Amplifiers

MicroAmplifier

8-Bit, 392 kSPS ADC Parallel Out, Microprocessor Peripheral, On

Track-and-Hold, Single Channels

Microcontroller with Arduino BL

3.3 V regulator

Table 1. List of ICs used and their function

Figure 1 is a simple block diagram of the whole system.

Figure 1: Block Diagram

microphone, store it in an SD card and

the acquired data through USB to the laptop

of noise and it gets amplified before

quantization error. Apart

other parts of the project worked fine as expected. The audio capturing is tested by providing a

low frequency SINE and TRIANGULAR at the input of the ADC and stored it to the SD Card. The audio playback

Bit, 0.1 us MDAC, Parallel Input, Fast Control Signalling for DSP

Rail Operational Amplifiers

Bit, 392 kSPS ADC Parallel Out, Microprocessor Peripheral, On-Chip

The microphone picks up audio signal and is amplified by the audio amplifier. The output of the audio amplifier is

converted to digital data by an 8-bit ADC and sent to the microcontroller. The microcontroller stores it into the SD

card in WAV format. After this, the ADC circuit is switched off and the microcontroller retrieves the stored data

from the SD card and sends it to the DAC. Here, the digital data is converted back to analog signal which is then

amplified and is played back through the speaker. The microcontroller is the master control of the circuit and is

responsible for switching on and off each part of the system while working.

The data is captured at a sample rate of 25 KHz. The audio signals range from 20 Hz to 20 KHz and human voice

frequency ranges from 300 Hz to 3.4 KHz. Hence the sampling rate of 25 KHz is a good choice. Since the data sent

to the SD card is serial, the data transfer rate is greater than 200 KHz. Hence from this data, it can be calculated

that the delay required per sample is 1/25000 = 40 µs.

Procedure Followed:

• First, a decision regarding which project to choose was taken. This project was chosen because it seemed

to a feasible choice considering the time frame that we were supposed to do it in.

• Then the circuits were designed and implemented using the Cadence Design Entry CIS tool. The design

spanned 4 pages, each covering a part of the project.

• After the circuits were successfully designed, we designed a 2 layer board using the Allegro PCB Editor

tool. We wanted to see whether we could successfully implement it on a 2 layer board instead of a 4 layer

one. All the routing was done manually.

• The board was fabricated “in-house”, drilled and soldered.

• Testing of the board took time comparable to the design of the same. For testing, we followed these

steps:

Test microcontroller

Test SD card.

Test DAC conversion and play audio file from SD card.

Test ADC conversion

Play sound into microphone and test the whole system.

Circuits & Discussion:

1.Audio Capturing

The ADC circuit is shown in Figure 2.

Figure 2: Microphone-ADC Circuit

The connector on the left is where the microphone is plugged in and the high pass filter feeds it into the

operational amplifier. The amplified output is fed into the ADC and converted. The signal from the microphone

picked up a huge amount of noise that is being amplified and hence makes it improbable to expect the ADC to give

sensible conversions on the audio input. The LED (D3) is fixed just as an indicator to see if the circuit is switched on

or not. The output of the amplifier is fed to a speaker and found that it picked up so much of noise and still could

hear the audio with low gain.

The bits D0 to D7 (as seen on the right) are the data ports connected to the microcontroller along with the CS and

RDB signals. The ADC interface is “parallel” that makes things easier and the CS pin is for selecting the chip before

reading the data using RDB pin. The timing diagram for the ADC is given below.

Figure 3. Timing Diagram for ADC

The CS pin should be pulled low before reading any data from the ADC. After a delay of 1 µs, the RD pin is pulled

low. Now, after 1 µs the data will be ready to read and this data is taken by the microcontroller. Since the sampling

requires 40 µs delay, we have ample time for the above process. This is repeated every 40 µs and stored it in SD

card.

2. Microcontroller

Figure 4 here shows the microcontroller connections.

Figure 4. Microcontroller Circuit

The switch was implemented to facilitate the control (switching on/off) of the ADC and the DAC circuits separately.

The LEDs again are used as indicators here. The switch SW2 is a push button switch that is used to reset the

microcontroller whenever required. All the references are given to Vcc and Gnd as and when required. The bit

lines, D0 to D7, are connected to the ADC and DAC circuits. The control over the bits is handled with software.

CONN6 is for external connections like power supply, serial transmission to USB, etc.

The microcontroller has arduino bootloader and I programmed using Arduino ISP. The software for the

microcontroller is written using C (modified for arduino) and flashed using Arduino IDE. Arduino is an open source

and is available online.

The interface to the DAC and ADC are design by ourselves and the interface to the SD card is achieved by FAT File

system library found in the arduino forum [3]. The software contains 2 buffers of size 256 bytes. In case of audio

acquisition, the data is read from the port D0-D7 and stored in the buffer1. Once the buffer 1 is full, the controller

starts filling the buffer2. During this time, the contents of buffer 1 is written to the SD card. Once buffer2 is filled,

the data is filled in buffer1 and during this time, the data in buffer 2 is written to the SD card. This process is

continued till the audio acquisition is complete. The 25 KHz signal is generated using timer interrupt present in the

controller. The timer is set to interrupt once in every 40 µs to achieve the required interrupt. So every time the ISR

is called, the data is read from the ADC or written to the DAC depending upon the operation we are doing.

An addition to this circuit is Figure 4 which was made because the whole thing couldn’t fit on one Design Entry CIS

page.

3. SD Card Interface

Figure 5. SD Card Circuit

The figure here shows the SD card connections from the microcontroller. The 8-pin connector on the right is where

the SD card is inserted. The data is controlled as before by the microcontroller. The SD card runs in 3.3 V and hence

a voltage divider circuit is used for all the input/output lines to/from the SD card. The 4 lines to the SD card are

given as:

Pin Function

DI Serial Data In

SCK Serial Clock

DO Serial Data out

CS Chip Select

Table 2. Ports for SD Card

4 .Audio Playback

Figure 6 shows the DAC circuit.

Figure 6: DAC Circuit

This circuit is designed right to left. The bits come in on the right side and the DAC converts them to audio. An

audio amplifier IC is used here to amplify the audio signal. The resistor and capacitor values were calculated as per

the IC datasheets. As seen on the amplifier, the gain is set to 1 and the resistance is calculated using the inverting

amplifier formula:

A = -Rf/Rin

A = -1

Rin = 20 KΩ

Rf = -A * Rin

Rf = 20 KΩ

From the data sheet [2], we found that to filter frequencies above 100 Hz, 68 µF capacitor is used.

The internal circuitry of the amplifier consists of two separate operational amplifiers to facilitate stereo

amplification and output. We decided that mono audio would suffice for our purposes. The Junction at the end is a

modified BNC part since the speaker’s part was not available in the tool. Even the DAC was designed as a stand-

alone part. Again, as seen in the ADC the data lines are coming from the microcontroller.

The interface to the DAC is “parallel” and it runs from the microcontroller. This is shared between ADC and DAC.

There are 2 other control pins for the DAC which are CS and WRB. CS is to select the chip and WRB is to write the

data to the DAC. The timing diagram of the DAC control pins are shown below:

Figure 7. Timing Diagram for DAC [2]

The CS pin should be pulled low before writing any data to the DAC. After a delay of 1 µs, the WRB pin is pulled

low. Now the DAC is ready to read the get the data from the controller. Once the data is placed in the pins, the

WRB is taken high. Now the DAC output is proportional to the input given. The after 1 µs delay the CS is taken high.

Since the sampling requires 40 µs delay, we have ample time for the above process. This is repeated every 40 µs

and stored it in SD card.

Working

So, overall the circuit works but there are a few problems. The microphone is picking up a lot of noise and thus the

ADC conversion is faulty. The microcontroller circuitry works perfectly and is able to manipulate the data. The DAC

circuitry works fine and converts audio to a near ideal output.

To test the ADC and the data capturing capability of the microcontroller, the input to the ADC is disconnected from

the audio input from the microphone. A sine or triangular wave of amplitude 1 V and an offset of 1 V is fed to the

ADC input. The frequency of the signal is set to 100 Hz to 1 KHz and tested. The microcontroller could read the

data from the ADC and store the data into SD card as explained before. The check the data from the SD card, the

data is sent serially to the COM port of the laptop and by using Matlab, the data is captured and plotted. The

captured signal is shown below:

Figure 8. Analog Data capturing with Triangular Input

When zoomed in, it is found that the LSB 2 bits are held constant at 01

trying to find the source of the issue, it is seen that the data read from the ADC is fine and it is also fine when seen

from the microcontroller. We suspect that the error may be due to the serial transmission thr

are not sure about the problem.

Figure 9. Zoomed In View of the above triangular input showing quantization error

Figure 8. Analog Data capturing with Triangular Input

When zoomed in, it is found that the LSB 2 bits are held constant at 01 and this gives a quantization error. When

trying to find the source of the issue, it is seen that the data read from the ADC is fine and it is also fine when seen

from the microcontroller. We suspect that the error may be due to the serial transmission through the USB and we

Figure 9. Zoomed In View of the above triangular input showing quantization error

and this gives a quantization error. When

trying to find the source of the issue, it is seen that the data read from the ADC is fine and it is also fine when seen

ough the USB and we

Figure 9. Zoomed In View of the above triangular input showing quantization error

Since we tried to multiplex the data lines to DAC and ADC using same pin and tried to disconnect the power source

of ADC/DAC while applying input to the IC, Dr. Robucci felt that the chip might have burnt. But so far the ICs are

working fine but for the 2 LSB bit errors. Later after the project demonstration, we found that there is no need to

disconnect the power source from any of the ICs and we can turn on/off the IC using CS pin fed to the ICs. So this

will fix the major problem in the board design and still satisfy the function of working.

Now to test the DAC and audio playback, we took a wave file of 25 KHz sampling rate and 8 bit resolution mono

audio and stored it in the SD card. Since the file system interface to the SD card is FAT, it was easy to store the data

into the SD card using a laptop and read it using the microcontroller. The song is then played back as explained

before and the song is listened using the earphones. Though it had little bit of noise due to low resolution of 8 bits,

the playback was very good and satisfactory.

The layout of the entire board was done in Allegro PCB Editor and Auto route was not used. This is shown in Figure

8.

Figure 10: Board Layout

The board was fabricated in the lab by using the ART files in Figures 9 and 10.

Figure 11: Top layer Figure 12: Bottom layer

Figure 12. Final PCB of the project

Conclusions

The project worked well under the circumstances. The operational amplifier picked up a lot of noise from the

microphone and thus the ADC conversions became moot. Hence we fed some external analog signal to the ADC

and able to capture those signals with little quantization error due to the 2 LSB bits getting stuck at 01. The rest of

the board performed well and as expected. The DAC output was fine and didn’t require extra testing. We were

able to play a song out through it successfully. Overall, the project performed as expected and we have a

successful output.

References

[1] Adv LinCMOS High-Speed 8-Bit A-to-D Converters Using Modified Flash Techniques (Rev. A)

http://www.ti.com/lit/gpn/tlc0820a

[2] 8-Bit Multiplying Digital-to-Analog Converters

http://www.ti.com/lit/gpn/tlc7524

[3] SD card interface with Arduino

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1206874649/8