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Bits are bits
- Thread starter alfe
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What am I looking at?
Are we looking at measurements of the raw output of the optical doodah, kind of thing? If so, I see no contradiction with the idea that 'bits are bits'. Of course, if an optical storage medium is damaged or corrupted (due to wear in the stamper or whatever they use), noise margins will be reduced and, in extreme cases, errors introduced. If the system has extra redundancy for error correction then the errors may never get through, however.
As long as there are no errors passed through and the system never lets its output buffer empty, then bits are indeed bits.
As we discussed before, the optical system and its motor is slaved to the system clock and therefore the output sample rate - not the other way round. What is seen in between is of interest to technicians only.
The tests are nine weeks apart.
What is the "Tester Number" ?
What is the "Machine Number"?
What is the "Sys ID"?
What is the "Tester Number" ?
What is the "Machine Number"?
What is the "Sys ID"?
Kees de Visser
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Luckily the voltages have margins. As long as the bits arrive unchanged (and in time, if speed is an issue), all is fine.
I just made these QRs to illustrate the case:
Just my 0.02 bits
I just made these QRs to illustrate the case:
Just my 0.02 bits
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I have heard them talk about voltages, currents, power, rpm, torque, force... all kinds of things. I'm sure they mention bits from time to time, too.
But you make my point for me: technicians are indeed interested in the analogue voltages that come out of the optical doodah. The end user isn't. And the DAC isn't.
I don't really follow (understand) the charts. There are many, many reasons noise floors vary among measurements; we usually do averages and such to minimize variance, but it is noise after all so there is always some. As for the block error rate, if it is higher after 2000 plays for whatever reason, then the same bits are not being pulled off the disc, right? Different bits, different results...
The techs I work with understand bits. And voltage. And many other things.
The techs I work with understand bits. And voltage. And many other things.
When we send digital signals on a wire, the bit get distorted all the time. Even the slightest change on the cable can show variations. As noted, digital systems don't look at the analog value of those waveforms but rather gross detection of one or zero (land or pit in optical media). That gives them huge margin of error. As Don says though, you can get errors if things get bad and for that moment, you will have an audible distortion and usually pretty noticeable too.
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Worst CD I have gotten was a Beatles reissue. Straight out of the jewel case and into the PC to rip. I was using software that reads multiple times to compare and see if results match. It took several attempts per track to get matches. I redid the entire rip a week later, and the resulting files were all bit for bit matches. It also took multiple attempts the second time. Later compared to a burned version of my rip, or the CD there was no difference during listening.
So they replicated 2000 times, and the 2000th disc had higher block errors? That is an issue for the duplicating machines, disc handlers, or something... To use an analogy, one of the misconceptions people have about CNC machines is that they produce perfect replicas every time. They are programmed and the program does not vary, but there is mechanical play and such that must be compensated, wear and tear in cutting heads, variations in the handlers and feeders that place the material to be machined, and so forth. Make a million parts with a CNC machine and there will be variance; the goal is to reduce it below design specs. Just as the goal with a CD and DAC is to reduce the impact of errors below audibility.
With voltage, I think I am not understanding your terminology. When people talk about "voltage vs. bits" I assume they are implying voltage is a continuous signal whilst bits output by a DAC (also a voltage by the time it is applied to a speaker) represent sampled data that is quantized in time and amplitude. I am not sure your goal here; is it to show that the digital code pulled from a CD can vary and not be bit-perfect? That is certainly true for a variety of reasons (bad media, encoding errors, playing (decoding) errors, noise in the receiver that converts the received optical signal to a bit stream, etc.)
There are also non-deterministic things like noise, design sensitivity to temperature or power supply variance, and so forth that can cause the output from a DAC to vary even when the input is (bits are) identical. The changes are (or should be) inaudible for a decent design, however.
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Yes I did.
What's the typical lifetime of a CD stamper?
I caution you against feeding the folklore that bits are not bits: Yes, a system like CD can fall prey to errors if you smear the disc with peanut butter, but they are not inevitable - and quite clearly CDs and DVDs are bit perfect when used for distributing software. Other, more modern, systems that store and distribute data are bit perfect by default; the modern world could not operate if this were otherwise.
I was talking to DonH and his comment which, although obviously correct, emphasises the errors over the much more common situation of 100% bit perfection
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Why, what's not perfect about your bits or the numbers they constitute?Sorry I didn't get it, I'm storing bits since 25 years and believe me if there is someone who believe that bits are bits it's me.
But" bit perfect"is a Philips marketing BS.
Yes, lucky and some hard work.
I wonder how "quantum" data will be checked and corrected, since you won't know what it is supposed to be since you haven't looked at it yet...
You smart guys always put me to shame:
https://en.wikipedia.org/wiki/Quantum_error_correction
"Classical error correcting codes use a syndrome measurement to diagnose which error corrupts an encoded state. We then reverse an error by applying a corrective operation based on the syndrome. Quantum error correction also employs syndrome measurements.
We perform a multi-qubit measurement that does not disturb the quantum information in the encoded state but retrieves information about the error. A syndrome measurement can determine whether a qubit has been corrupted, and if so, which one.
What is more, the outcome of this operation (the syndrome) tells us not only which physical qubit was affected, but also, in which of several possible ways it was affected.
The latter is counter-intuitive at first sight: Since noise is arbitrary, how can the effect of noise be one of only few distinct possibilities?
In most codes, the effect is either a bit flip, or a sign (of the phase) flip, or both (corresponding to the Pauli matrices X, Z, and Y). The reason is that the measurement of the syndrome has the projective effect of a quantum measurement.
So even if the error due to the noise was arbitrary, it can be expressed as a superposition of basis operations—the error basis (which is here given by the Pauli matrices and the identity).
The syndrome measurement "forces" the qubit to "decide" for a certain specific "Pauli error" to "have happened", and the syndrome tells us which, so that we can let the same Pauli operator act again on the corrupted qubit to revert the effect of the error.
The syndrome measurement tells us as much as possible about the error that has happened, but nothing at all about the value that is stored in the logical qubit—as otherwise the measurement would destroy any quantum superposition of this logical qubit with other qubits in the quantum computer."
Huh?
You can tell if a "bit" is wrong even if you aren't allowed to know what it is (yet).
Nice trick!
I want one!
I wonder how "quantum" data will be checked and corrected, since you won't know what it is supposed to be since you haven't looked at it yet...
You smart guys always put me to shame:
https://en.wikipedia.org/wiki/Quantum_error_correction
"Classical error correcting codes use a syndrome measurement to diagnose which error corrupts an encoded state. We then reverse an error by applying a corrective operation based on the syndrome. Quantum error correction also employs syndrome measurements.
We perform a multi-qubit measurement that does not disturb the quantum information in the encoded state but retrieves information about the error. A syndrome measurement can determine whether a qubit has been corrupted, and if so, which one.
What is more, the outcome of this operation (the syndrome) tells us not only which physical qubit was affected, but also, in which of several possible ways it was affected.
The latter is counter-intuitive at first sight: Since noise is arbitrary, how can the effect of noise be one of only few distinct possibilities?
In most codes, the effect is either a bit flip, or a sign (of the phase) flip, or both (corresponding to the Pauli matrices X, Z, and Y). The reason is that the measurement of the syndrome has the projective effect of a quantum measurement.
So even if the error due to the noise was arbitrary, it can be expressed as a superposition of basis operations—the error basis (which is here given by the Pauli matrices and the identity).
The syndrome measurement "forces" the qubit to "decide" for a certain specific "Pauli error" to "have happened", and the syndrome tells us which, so that we can let the same Pauli operator act again on the corrupted qubit to revert the effect of the error.
The syndrome measurement tells us as much as possible about the error that has happened, but nothing at all about the value that is stored in the logical qubit—as otherwise the measurement would destroy any quantum superposition of this logical qubit with other qubits in the quantum computer."
Huh?
You can tell if a "bit" is wrong even if you aren't allowed to know what it is (yet).
Nice trick!
I want one!
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Is there a "Dummies" reference that can explain how the CD bits or a WAV file bits gets translated into a digital signal?
The classical presentation will be just fine, I will tackle the quantum data problem next week......
The classical presentation will be just fine, I will tackle the quantum data problem next week......
But thats the point - error correction does exist for this very reason. The correct data is recovered regardless of the media imperfections.Lucky error correction exist
https://en.m.wikipedia.org/wiki/Reed–Solomon_error_correction
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