prk@charm.UUCP (Paul Kolodner) (06/02/85)
A recent posting asked if it's not possible for bits to be lost when digital recordings are copied. This question brought several thoughts to my mind: 1. Good digital systems have noise levels which are much smaller than the least significant bit. That's why you can, in principle, copy over and over without ANY degradation. My bet is that the signals in digital recording satisfy this condition by a large margin. 2. Computer memories reserve a significant fraction of their space for error-correction bits. They suffer from having bits shot out at random by ionizing radiation, chiefly due to cosmic rays and radiactive impurities in IC packages. With the right error-correcting code, however, the mean time between uncorrectable error can be made much longer than the obsolescence time of the machine. Scientific American published a nice little article about this within the last two years. 3. About digital recordings: Even though CD-player blurbs talk about "error-correction", I'll bet there is none - there's no time for it. The real issue is, if there is an incorrect bit now and then, could you hear it ? NO! A single glitch in a digital record leads to white noise of very low amplitude, as I have verified by toying with artificial data sets simulating my experimental data. You can't hear it. Think of this another way. Suppose Beethoven is playing along, and then one bit gets out of place. This causes a click of duration 20 microsec. How sensitive would you be to that? Not very sensitive. These glitches would have to be quite frequent to be annoying. They're not - we're talking about rare events anyway, or else digital recording would never get off the ground to begin with (see point #1 above). Stop worrying and get some sleep.
karn@petrus.UUCP (06/03/85)
> 3. About digital recordings: Even though CD-player blurbs talk about > "error-correction", I'll bet there is none - there's no time > for it. The real issue is, if there is an incorrect bit now and then, > could you hear it ? NO! A single glitch in a digital record > leads to white noise of very low amplitude, as I have verified by > toying with artificial data sets simulating my experimental > data. You can't hear it. Think of this another way. Suppose > Beethoven is playing along, and then one bit gets out of place. > This causes a click of duration 20 microsec. How sensitive > would you be to that? Not very sensitive. These glitches > would have to be quite frequent to be annoying. They're not - > we're talking about rare events anyway, or else digital recording > would never get off the ground to begin with (see point #1 above). > Stop worrying and get some sleep. I believe this is incorrect. Error correction IS provided in CD players, and it IS essential. The "raw" error rate on CDs is something like 1 bit in 10,000, although they generally occur in bursts. At the net data rate of 1.4 megabits/sec, this would be about 140 bad bits/second. In PCM, the audibility of a bad bit depends on its significance within the sample. If the low order bit were bad, you would not likely hear it. But it is entirely possible that the high order bit is corrupted, and this you would DEFINTELY hear. Try listening to one of the test CDs with an impulse-response test signal to see what this would sound like. I suspect that listening to Beethoven at the proper volume would not only be intolerable without error correction, it would probably also fry your tweeters. On a related topic (digital vs analog generation degradation), the advantage of digital is not that bits cannot EVER be read in error; as somebody pointed out, this can happen as long as there's noise in the raw analog signal coming off the media. The real advantage of existing systems is that through error correction, occasional errors in raw data bits can be fully corrected before they're re-written to the copy. The over all effect is that the bit-error-rate vs signal-to-noise ratio on the tape or disk can be made even more strongly nonlinear. At moderate-to-high S/N ratios, the probability of getting a bad bit after the error correction circuitry is virtually nil, despite occasional errors in the "raw" data. The raw data errors simply don't occur close enough to destroy enough redundancy to keep the error correction circuit from doing its job. However, at some lower value of S/N (the threshhold) the output error rate skyrockets as the correction circuit is suddenly overwhelmed by the number of input errors. As long as you operate with enough margin (just a few dB is often enough), however, the copying process is virtually error-free. Phil
pchow@Shasta.ARPA (06/05/85)
> 3. About digital recordings: Even though CD-player blurbs talk about > "error-correction", I'll bet there is none - there's no time > for it. The real issue is, if there is an incorrect bit now and then, > could you hear it ? NO! A single glitch in a digital record > leads to white noise of very low amplitude, as I have verified by > toying with artificial data sets simulating my experimental > data. You can't hear it. Have you ever put your favourite CD up to a light source? Choose one that doesn't have an opaque label. There are definitely defects there that are more than 1 bit! Thank goodness for error correction. Paul Chow Stanford University decwrl!shasta!pchow pchow@shasta
mraspuzzi@kl2116.DEC (Michael Raspuzzi) (06/07/85)
>3. About digital recordings: Even though CD-player blurbs talk about >"error-correction", I'll bet there is none - there's no time >for it. The real issue is, if there is an incorrect bit now and then, >could you hear it ? NO! A single glitch in a digital record >leads to white noise of very low amplitude, as I have verified by >toying with artificial data sets simulating my experimental >data. You can't hear it. Think of this another way. Suppose >Beethoven is playing along, and then one bit gets out of place. >This causes a click of duration 20 microsec. How sensitive >would you be to that? Not very sensitive. These glitches >would have to be quite frequent to be annoying. They're not - >we're talking about rare events anyway, or else digital recording >would never get off the ground to begin with (see point #1 above). >Stop worrying and get some sleep. Of course there is error correction! A popular misconception about the CD players is that they work like LPs. That is the "data" (music) is read and sent to the amp (of course after it has been converted from digital to analog). Most CD players do NOT work like that. The data is read off disc but is then stored in memory. From there they are CLOCKED out using a vibrating quartz crystal (usually). With all that time between read and clock out, this gives the circuitry plenty of time for any error correction. Mike Raspuzzi DEC Software Specialist ARPA: mraspuzzi%kl2116.dec@decwrl ...decwrl!dec-rhea!dec-kl2116!mraspuzzi --------