|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: DEMODULATION AND ERROR CONTROL CODING
In the CD player, the signal is read from the disk by an electro-optical front end, and then after amplification and filtering by front-end electronics, the resulting electronic signal is fed into the SAA7310 decoder IC. This powerful IC demodulates the signal (and in the process extracts bit-timing information which is used to control the disc rotation speed), and then it performs the major task of decoding the signal and applying error correction.
The main aim of this exercise is for you investigate the performance of the demodulation and error control coding functions, and explain why they have been designed the way they have.
To do this, you will need to draw on a number of sources of information. You may also want to use some custom electronics which we have designed to let you "get at" some relevant signals inside the CD player.
The main sources of useful information are likely to be:
* Available to you when you start doing the activity.
To make efficient use of your time, you need to plan your investigation. The following is a suggested outline plan, but you are free to vary it:
Find out about the physical characteristics of the data on the CD disc: the bit density (bits /mm), track spacing, rotation/linear speed (NB the characteristics of the CD reading process are very different from that for vinyl LPs). What is the EFM system and why is it used?
What is the bit rate out to the Digital-to-Analogue converters (DACs), and in what format (sample rate, number of bits)?
Investigate the CD930 technical manual, locate the relevant signals from the front-end amplifiers to the SAA7310, and find out how it converts the analogue signal into a 1/0 stream. (Concentrate on the signals HF, HFI and FB).
Find out about the digital audio output and control lines from the SAA7310 - concentrating on EFAB, WSAB, DAAB, CLAB. See how these signals are processed by the ICs following the SAA7310.
Consider the likely physical causes of errors (including manufacturing blemishes, dirt, and scratches) and work out how many bits they are likely to corrupt and in what patterns (e.g. how many bits in a burst of errors, and how many bits between bursts of errors).
See what you can find out about the error correction scheme (NB it is quite complex, and you don't need to understand it fully, but it could be useful to find out about its general error correcting abilities). Extracts of relevant books will be available to you.
Read the descriptions of the custom electronics, and find out what they are designed to let you do.
Plan initial investigations and then a scheme of measurements to allow you to characterise the error correction process and explain it to others. Carry out the experiments, and analyse the results.
Assess the design decisions that led to the CD system being specified as it is:
Put together your presentation, aiming to explain briefly what the modulation and error correction systems were designed to acheive (in terms, for example, of the needs of the manufacturer and the user), what technical solutions have been chosen, what their overall performance characteristics are, and how successfully they meet the requirements.
Two circuit boards are provided. The first is mounted inside the CD player; a logic-level input to it operates an analogue switch IC which can short-circuit the HFI and FB inputs to the SAA7310 together. This can be used to simulate loss of data (e.g. due to a scratch on the CD).
The second board is external to the player; it (a) supplies a number of tapping points which allow you to see signals from player on a 'scope (specifically, HF, HFI, FB, EFAB, and WSAB), and (b) includes a useful circuit which processes the EFAB and WSAB signals to give a logic-level output.
Full details will be available to you when you start the activity.
Dr P.R. Palmer, March 2004