Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!spool.mu.edu!uwm.edu!csd4.csd.uwm.edu!info-high-audio-request
From: png@netcom.com (Peter Glaskowsky)
Newsgroups: rec.audio.high-end
Subject: Re: Photo-detectors in CD players
Message-ID: <12827@uwm.edu>
Date: 6 Jun 91 12:53:47 GMT
Sender: news@uwm.edu
Lines: 43
Approved: tjk@csd4.csd.uwm.edu
Originator: tjk@csd4.csd.uwm.edu

> What factors affect the output voltage of a photocell such as those used
> in CD players? What factors affect the time it takes a photocell to reach
> its "expected" output?

CD players typically use photodiodes, not photocells. So-called "PIN"
photodiodes are extremely fast, sensitive, and accurate.

I'll stick a disclaimer in the middle here: my experience with photodiodes
comes from high-speed fiber optic receivers (in the GHz range), which I
worked on about a year ago, and from low-speed photometers for color video
monitor calibration (which I'm working on now, at SuperMac Technolog). I've
never worked on CD players, but I've seen several application notes on the
subject.

With that out of the way, the brief summary:

CD players bounce a laser off the disc, and a photodiode measures the
reflected beam (much detail omitted). The photodiode may be connected
so that there is no voltage across the terminals (which is common in
low-speed applications), but is typically connected in reverse-bias
mode, to maximize frequency response. The output of the photodiode is a
current, proportional to the incident light. This current is converted
to a voltage, and then a comparator produces a logic-level output which
goes on to the digital side of the circuit.

While it would be possible for the photodiode receiver circuit to have
problems with amplitude or timing accuracy, it's extremely easy to
design the circuit so that these problems don't arise.

Error rates in digital communications systems, which is basically what
we have here, are usually measured in terms of "Bit Error Rate", or BER,
expressed as a unitless ratio (error bits/total bits). At the bit rates
found in CD players (about 1.4 Mbps of data, plus overhead for format
codes, redundancy, etc.), it's almost trivial to achieve BER's of one
error per billion bits with just a couple dollars worth of parts. (That's
just for the receiver section, folks, assuming no errors in the source
material, no dust or scratches on the disc, etc. Your mileage WILL vary.)

The short-form answer is, I wouldn't expect to see a significant
contribution to the overall error rate because of problems in the
photodiode receiver circuit.

.                      png