Master Clock Circuit

Using crystal circuit to generate clock pulse for the chip, I'm using a technique called Schmitt- Triggered inverter.  So the if the input of the inverter goes low and therefore the output goes high. And the transition is very sharp contain many different frequency harmonics but only the one with the same frequency as the crystal will pass through. So the Schmitt-Triggered  inverter compares the input signal with two thresholds instead of one as in simple inverters or gate. Therefore, the input has to be higher than one threshold for the output to change and lower than the other one for the output to change back. As shown in figure below when there is a low on the input of the inverter and a high at its output, the capacitor on the input of the inverter is charged through the feedback resistor. When the capacitor is charged high enough that the input voltage of the inverter passes the higher threshold then its output drops to zero. Then the capacitor will start discharging through the same resistor and when its voltage drops below the lower threshold of the inverter and its output jumps up again and this will happen continuously and resulting in a square wave. The oscillating frequency of this circuit depends on how fast the capacitor charges and discharges and therefore is a factor of the resistance and capacitance value, while it is also dependent on the threshold levels. Below is the circuit that used 2 MHz crystal to generate master clock for the chip. The biggest problem for this circuit is that the oscillating frequency is not stable so I had to find the best combination of the resistor and capacitor to make it stable. So we get the clock frequency that we desire for the master clock.

Research on components

Since this project requires ADC and DAC for data conversion. There are many converter that can be used in this project. I've found this PCM codec which utilizing A/D and D/A conversion architecture. Since this chip contain both A/D and D/A therefore it is easy to implement it and learn it. Because of the chip is used for voice communication the sample rate is very low. And also the price is very cheap. Since this chip requires three different clocks such as Master clock, Bit clock and Frame clock, therefore this project will require clock generation circuit to supply the clock. In order to get the Master clock circuit it is necessary to find out how we're going to achieve it. The best solution would be using crystal to build crystal oscillator circuit. Since there are different techniques to build a crystal circuit and therefore further research needed to be done. Then the driver circuit for LED also needed for transmit the data. Also need to decide which LED we're going to use. For the transmitter side the chip and LED we're going to use for this project are shown below.

                                                                     
                                                                       PCM codec

                                                                     Transmitter LED

Presentation For Project Proposal

To be able to present my project proposal successfully I totally understand that I have to do more research on the topic that I'm going to talk about. In my presentation, I'm planning to talk about the outline first and then explain the outline of the presentation. The main topics of my presentation going to be a thing like - what do I want to do for my project? Why do I want to do the project? And how do I achieve that?

What do I want to do for my project?

- Transmission audio signal using optical fiber

- Frequency range 20Hz-20kHz

- With line level Input voltage

- Improve design for XLR

- Comparison performance with other method

- Investigate (TDM) Multiplexing

- Investigate modulation techniques (AM, FM, PCM)

Why do I want to do the project?

- Gain new knowledge

- Build multichannel transmission ( In the future)

- Learn advantage of using fiber optic

How do I achieve that?

- Research

- Literature Review

- Design, build and test

- Perform audio test

- Investigate possible of using XLR

All these topics will be delivered and explain on the day. In order to successfully deliver the presentation I believe I got to do a lots of practice.

Fiber Optic Technology

Since the nature of this project will be using the fiber optic cable to transmit audio signal, it is essential to understand how fiber optic transmission works and also the construction of the fiber cable, how light propagate in an optical fiber, dispersion characteristics, various connector types, bandwidth, operation of light emitting diodes etc. in order to understand all these topics I'm totally aware of that an intensive research needed to be done. Optical fiber uses light pulse to transmit information from one point to another. So light will act as a carrier and carry modulated signal using light pulse through optical cable and at the receiver end the photodiode will detect the signal and convert it back to electrical signal. So the fiber is a waveguide and basically guide the light to the end to the receiver. The most important of light propagate within the fiber is Snell's Law, which is from basic physics. So the Snell's law is all about refraction and the bending of light as it goes from one medium to another. There are different types of optical fiber cable such as single mode, multimode and plastic optical fiber. It is necessary to decide which type of cable to use for this project and chosen cable is a plastic optical cable because of the price. And also it can be bended up to 20mm with no change in transmission. But the downside of the plastic fiber is that the transmission reduces as the distance increase. Single mode fiber cable is much narrow, which confine the optical signal to a straighter path with less reflections and therefore reduces light dispersion and result cleaner signal. Multimode optical fiber is mostly used to transmit over short distance because it can propagate many mode and also available in graded index. The graded index is the fiber core that has a reflective index, which reduces the intermodal dispersion.

PCM Modulation

Many transmission link like telecommunication nowadays using digital transmission because of its accuracy. For this project we're going to using PCM modulation technique to do the transmission. PCM modulation is mostly used in telecommunication to transmit voice signal. So PCM modulation will include sampling and quantization as shown in the Figure. PCM is a technique of converting analog signal to digital which can be transmitted over a medium like fiber or Ethernet. Then this signal will be converted back to analog signal. There are three steps involves in pulse code modulation such as sampling, quantization and coding. Sampling process is where the signal is sampled regularly at uniform intervals and the result of the values is called samples. Quantization is the process of converting the samples into discrete digital values.

FM Modulation

FM modulation is the process of the frequency of carrier wave is changed with the intensity of the signal is called frequency modulation. The amplitude and phase of the carrier wave remains constant in frequency modulation. Only the frequency of the carrier wave is change with the signal. The analog sound in frequency modulation signal is that the signal to noise ration at the receiver can be improved. And also the frequency of an FM transmitter without signal input is called the resting frequency or centre frequency and this is the allotted frequency of the transmitter.

AM Modulation

There are three different types of modulation techniques can be used for this project such as AM, FM and PCM modulation. These can be digital or analog but we have to be careful with the bandwidth. Because the analog system can have a limitation and therefore the digital system is more suitable for this project. Amplitude modulation is where the amplitude of high frequency carrier wave is changing with the intensity of the signal and it is much simpler to design. But have a Bandwidth limitation which is not the best solution for this project.

Planning For Writing Proposal

As all the necessary research has done for proposal and it is time to start planning and writing the proposal document. Since the proposal document well structure therefore it is essential to think and plan the document logically so it will allow the reader to read easily. Below is how I'm going to structure my proposal document. 

Chapter1: Introduction
          1.1: Motivations
          1.2: Aims
          1.3: Objective
       1.3.1: Literature Review over Fiber Optic Technology
       1.3.2: Investigate Various Modulation Techniques
       1.3.3: Design, Build and Test Optical Fiber Transmitter and Receiver Circuit
       1.3.4: Perform Audio Test in Studio
       1.3.5: Investigate Possible of Using XLR
       1.3.6: Investigate Multiplexing
Chapter2: Literature Review
         2.1: Optical Fiber For Professional Audio
         2.2: Modulation Techniques (AM, FM, PCM)
         2.3: Time Division Multiplexer (TDM)
Chapter3: Methodlogy
Chapter4: Possible Solution
Chapter5: Chosen Solution
Chapter6: Project Action Plan
Reference List
         

Meeting

Met supervisor and discuss all the option and that we could do to implement the system. Things like ADC/DAC, LED driver and also type of fiber cable. How we're going to design the circuit and also the project plan. Got a lot information and guidance from supervisor things like what research need to be done as well as how to go and do it. We both agreed that to do the project step by step and so we started to design the project. Since the natural of my project involve a lots of different part and circuit therefore need to break down into part by part. And also we've decided to do research on different modulation techniques and when is done, we'll then decide which technique we're going to implement. Next step will be doing the research and also research on what ADC/DAC we're going to use for the project.

Researches

There are many way of doing transmission such as AM, FM and PCM technique. So it is necessary to do a intensive researches so we can understand the weakness and strength of the technique. And also it is necessary to decide whether to use digital or analog and also find out their differences. Nowadays, the industry mostly used digital transmission because of their accuracy. Since we are using fiber cable to transmit signal it is also necessary to understand how the optical fiber technology work. Things like the construction of the fiber cable, how light carry signal within the cable, the LED and the photo-detector all these must be understand in order to build a transmission link. It is also essential to do intensive literature review on a topic such as Fiber optic technology, various modulation techniques (AM, FM, PCM) and so on. So the next step to do will be meeting supervisor to discuss and decide how to implementing the system.

Introduction

The purpose of  this project is to design, build and testing an audio signal transmission optical link. Since we know that an optical communication is not a new technology and was started since 1960 and the technology gradually improved within the communication industry. It becomes key role in world communication services. Any transmission link such as long distance will involve optical fiber. Optical fiber has more advantages than copper wire for instance low attenuation, immunity to electromagnetic interference and so on. So as the development of plastic fiber optic cable improved as well as with the decreased of cost, it attracted many audio industry to consider using optical technology. Currently, in live sound industries as well as large-scale system like stadium, convention centers, theme park use Ethernet technology to do audio networking. Some how there still a limitation with Ethernet for long distance transmission therefore the Ethernet technology still needs optical fiber to support long haul transmission if the destination is more than 100 meters. My aim of this project is to design, build and test a basic one channel low cost optical transmission system that can transmit an audio signal through a optical fiber cable.