|ENGINEERING TRIPOS PART II A and EIST I|
|EIET LABORATORY||ENGINEERING AREA ACTIVITY|
Areas 5,6,7 and 8
|INVESTIGATION OF THE DESIGN OF CD PLAYERS|
SUBTOPIC: DIGITAL TO ANALOG CONVERTER
The biggest difference between compact discs and earlier systems such as magnetic tapes and vinyl records is the density of information stored, and the fact that this is digital. The Philips CD930 compact disc player contains a low cost 16-bit digital to analogue converter circuit.
Two digital bit streams are recovered from the disc by the optical reading head and these are converted into the Left and Right analogue signals ready for amplification. An important part of the D/A conversion process is the filtering necessary. In the CD930, the D/A is contained in the SAA7350 integrated circuit and it creates an analogue output by allowing accurately timed pulses to be integrated by a capacitor. In this case the filtering circuit based on the LM833 op-amp eliminates the noise associated with the modulated pulses.
The aim of your investigation should be to understand how the D/A conversion works and to be able to describe the important features of the filter circuit.
The main sources of useful information are likely to be:
* Available to you when you start doing the activity.
The filter circuit is reproduced on breadboard and replaces the internal circuit of the CD930. The response can be obtained by conducting tests on the filter as a stand alone circuit. The components may be modified and the resulting affect on the playback quality can be heard when operated as part of the CD930.
3. SUGGESTED APPROACH
Read one of the general books on CD players to gain a general feel for the data readout spec.
Read the article on the SAA7350, aiming to understand how it works. The article included with it may help, although you are unlikely to be able to understand the whole article in the time available.
Investigate the filter circuit: with the CD Player on PAUSE, examine the output on an oscilloscope. Watch what happens to the noise as you change capacitor and inductor values.
Conduct further tests on the breadboard circuit and analyse the circuit in the appropriate way.
What are the most important things you have discovered ?
What is you audience likely to be interested in and able to understand in a short time?
You will not have time to describe all you have found, so you must select what you regard as the most important items. It will also be worth spending some time deciding what the best method of presenting you data will be.
Dr P.R. Palmer, October 2002