acgd@ihlpf.att.com (Andrew Charles) (06/05/91)
I have some questions which may not make sense (due to a lack of technical background on my part). However, I'll give it a try: 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? Is it possible that circumstances could arise in a CD player which would affect the photocell sufficiently in these ways to either cause a misreading of a bit (wrong voltage value reached) or cause "jitter" (photocell reaction time is too long or too short)? If the answer to either of these is "yes", could these circumstances arise under normal operation if the player is not broken? If "jitter" is possible here, can it be large enough to affect the sound? Another way to approach this is to ask: how can a photocell fail? and if it does (or is degrading over time) what symptoms would one expect to see at the system level? I've just started reading Pohlman's "Principals of Digital Audio" (2nd Ed.) and haven't found any discussion of this. Of course, I may not have seen it yet if it's there. Generally, I'm wondering how much the choice of photocell matters when a CD player is being designed. Email or posted answers would be welcome. Technical references would also be welcome if they aren't beyond my technical grasp (Pohlman's book is in my range). If I'm coming way out of left field I apologize and ask that you bear in mind the disclaimer in the first line of this message. Thanks, Andrew Charles acgd@ihlpf.att.com
basiji@milton.u.washington.edu (David Basiji) (06/06/91)
acgd@ihlpf.att.com (Andrew Charles) writes: >I have some questions which may not make sense (due to a lack of technical >background on my part). However, I'll give it a try: >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? Actually, your questions are very valid... >Is it possible that circumstances could arise in a CD player which would >affect the photocell sufficiently in these ways to either cause a misreading >of a bit (wrong voltage value reached) or cause "jitter" (photocell reaction >time is too long or too short)? If the answer to either of these is "yes", >could these circumstances arise under normal operation if the player is >not broken? If "jitter" is possible here, can it be large enough to affect >the sound? >Another way to approach this is to ask: how can a photocell fail? and if it >does (or is degrading over time) what symptoms would one expect to see at >the system level? A photodiode detector like the ones used in CD players is basically a light EMITTING diode operated in reverse. Of course it is optimized for its use, but the above is essentially true. Like any simple diode, its current/voltage characteristics are exponential, that is, as one shines light of the appropriate wavelength on it and increases the intensity of that light the output voltage rises from a negative value very quickly to a positive value with very little current output. As the intensity of the light increases, the output current rises dramatically with very little change in output voltage above a nominal 0.6 volts or so. What the CD's laser does then is to turn on and off a small current source (the photodiode). The current is passed through a resistor to ground and the voltage is measured at the top (diode side) of the resistor to convert the current to a voltage (V=IR). That voltage is then passed into a device called a Schmidt trigger which outputs 0volts if the input is low (<1.5V)and outputs about 5Volts if the input is higher than 1.5volts. Now the signal is in a form useable by the digital circuitry downstream. The photodiodes commonly have response times of less than 1uS which corresponds to a frequency limit of 1MHz. Since the data is sampled at about 50KHz and there are 16bits per sample, one needs a minimum response time of about 1.3uS. Obviously, things seem to be cut pretty close, but it's not so simple. There'sa truism in engineering that the more you pay for a part the better the documentation. Since I only have a sketchy tech sheet (I'm cheap), I don't know any more than a single number to characterize the diode's response rate. In fact, response time is meaningless. How is it responding? Does it have a maximum and minimum current? Does the response time tell how long it takes to swing between them? What I want is a slew rate. This is the time it takes the diode to change its output current by a fixed amount. Ideally, I'd get a graph of slew rate, since it may swing faster between two higher or lower current values even though the difference is the same. That way, I could operate the diode in its fastest region. Or, given a slow diode, I could run it very close to the threshold voltage for the Schmidt trigger so that the diode wouldn't have to change its output very much to swing the Schmidt trigger. The problem is that you increase the noise level since it only takes a small perturbation to turn a 1 to a 0 or vice versa. All told, in my experience, the diodes are faster than they need to be and if there was any problem, it would probably come from the IC's downstream. The best thing you can do is to keep the diode clean so the output current is sufficient to swing the Schmidt trigger. Sorry for the long winded response. David Basiji
png@netcom.com (Peter Glaskowsky) (06/06/91)
> 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
miker@polari.UUCP (Mike Ranta) (06/07/91)
In article <12783@uwm.edu> acgd@ihlpf.att.com (Andrew Charles) writes: >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? The photodiodes used to "receive" the reflected laser beam are very fast devices. I know the devices used in the fiberoptics communication area must respond at several Megahertz which is at least an order of magnitude above CD frequencies. >Is it possible that circumstances could arise in a CD player which would >affect the photocell sufficiently in these ways to either cause a misreading >of a bit (wrong voltage value reached) or cause "jitter" (photocell reaction >time is too long or too short)? I doubt the photodiode could cause any problems. Again, other factors, such as the optics in the transport are far more likely to be marginal. Speed would not be the problem, but signal level from say dirty lenses might be. >Another way to approach this is to ask: how can a photocell fail? and if it >does (or is degrading over time) what symptoms would one expect to see at >the system level? As with most semiconductors, I would expect the photodiode to either work, or not work. The sensitivity of some optoelectronic devices can be effected if they are overloaded. In the case of the photodiode, this would mean being hit with a much higher intensity of light than is probably possible. >Generally, I'm wondering how much the choice of photocell matters when >a CD player is being designed. I would say it's one of the easier things to specify. The widespread use and technology of communication laser/fiberoptic equipment has made this sort of thing fairly "cookbook". The more critical aspects are the servo design for tracking and focus, optics design, and the laser diode itself. miker@polari.uucp (Mike Ranta)