Austin, There are at least two of us esteemed engineers who disagree with you on this list...
At 03:06 12/06/02, Peter wrote: >Julian, > >I am in total agreement with you. > >Peter, Nr Clonakilty, Co Cork, Ireland I point this out not to score a point, and I would never say or believe that the majority must necessarily be correct - BUT, I would ask that you at least draw the conclusion that it might be worthwhile looking seriously at what we (or I) am saying. I spent 2 hours yesterday constructing the following post which was carefully thought out to try and make the points clearly, you appear to have somehow misconstrued who wrote what and given a very partial and non-contextual dismissal of a couple of points which of course I don't agree with. Could you do me a favour and read and respond to the whole post below, especially the parts headed DYNAMIC and SCANNERS? To be very explicit, so to avoid confusion, every word is mine except those quotes that are explicitly quoted with > symbols or inside "quotes". Thanks, Julian R ----yesterday's post follows, please respond---------------------------- This post has 3 sections, headings are: RANGE - discusses the confusion about range DYNAMIC - discusses the confusion about what dynamic means SCANNERS - applies dynamic range terminology and discusses the relationship between density and dynamic range. 1) RANGE *********** Someone else wrote: > > However my point is that if you can reduce the noise level then you > > can > > increase the number of steps (by halving the step size) with real > > benefit, but ******without altering the range******. Austin responded: > Correct, but that INCREASES the dynamic range. The asterisks are mine to draw attention to the problem. Here goes! Big breath... Put some numbers to the above. Let's say max signal is 1000mV and noise (or min signal in this case) is 10mV at first. Then you reduce the noise level by half. So min is now 5mV. OK, so now use the PLAIN ENGLISH language definition of a range. In the first case, the RANGE of the usable / measurable / instrumentable signal is "10mV to 1000mV", or 100 to 1, or 40dB (volts remember, so 20log, not 10log). In the second case the RANGE of the signal is "5mV to 1000mV", or 200 to 1, or 46dB. Do you see? The plain english RANGE of the signal you are dealing with has changed from 40dB to 46dB. You HAVE altered the range. Why do you say in the quote "without altering the range"? The range is NOT zero to 1000mV, it never was, and never will be. It is 10mV to 1000mv, then 5mV to 1000mV. The range changes when you change the noise level, that is why you would change the noise level, to increase the range. And look! The calculation for plain english RANGE is the same as the calculation of DYNAMIC RANGE! That is because they are the same thing here. The DR is, as I have said many times, a RANGE. It is not something else, it is a RANGE. Look at the definitions that you quote, it is a range. It is usually measured as a ratio, and usually quoted in dB. It is not a number of levels, or anything else, it is a range. It IMPLIES a number of levels, but it is NOT a number of levels, it is a range. The thinking that leads you to state that by changing the noise level you don't change the range is at the heart of this problem. It does change the range, and it must. And the true definition of DR is no different in maths from the definition of plain english range. You seem to have laid an unnecessary layer of additional complexity over all this, and the result is total confusion. I can see WHY you might like to do this, but I don't see how it is useful, and it is at variance with other usage. Here is what you (Austin) said in another response to me: >Surely, you can understand that you can have two exact same ranges, with >different noise? That can't be hard to understand? No I don't understand that because I most explicitly don't agree. By DEFINITION of the most basic kind, the range we are discussing here is from the smallest (noise in this case) to the largest. The RANGE we are discussing is noise to max signal. Change the noise and you change the range, so long as the smallest signal is noise which is usually the case. Arbitrary ranges are pointless, in particular zero does not exist, because it is not measurable or includable and we are not discussing it. The whole point of discussing the range is to specify the smallest signal and largest signal, it is NOT to choose two arbitrary points inside or outside those figures. The RANGE is the distance between the smallest signal and the largest signal. It is NOT measured from somewhere "smaller than smallest signal", or zero or anywhere else. I repeat - Zero by definition is not in the range of (AC) signals we are taking about - because of noise. Noise not only always exists, it is at the heart of what we are discussing. I repeat - zero does not exist. That is usually the whole point of discussing signal ranges, to see how close we can get our noise to zero. Look Austin - we are discussing a situation in which we are trying to do something intelligent with a smallest measurable signal, and a largest measurable signal. So we use the concept of the RANGE. The range is the difference (or ratio if you want, it doesn't matter) between these two. It is not complicated, it is very very simple, and it is NOT ambiguous. It is NOT something else, and it is NOT up to you to specify arbitrary end points to some range and still expect that so-defined range to have any meaning. If you DO specify arbitrary end points, then you have thrown away the basic premise of your SIGNAL RANGE and you need to invent something else to get it back again. I think you do this and you call your newly defined range the Dynamic Range. But you DON'T need to do this. The actual range is inherent and unambiguous. Why do you have to invent this other "range" that goes from 0 to X volts? It means nothing, it is not used anywhere, it is not relevant, it doesn't get into the maths, its existence is impossible and it only means you spend your life explaining something that doesn't need explaining and so have to invent another term to explain what we all originally meant anyway. What's worse, to define your "new" range you steal a term that has another meaning entirely, and so we all end up here discussing "dynamic range" -- and hours at the keyboard defending to the hilt truth and our chosen way of life! 2) DYNAMIC *****(******** Dynamic. What is the point of the term dynamic? OK, another big breath. Imagine please a receiver, quite sophisticated, sitting over there in the corner. It happens to be a radar receiver, but that doesn't matter, except that this is what I know for a FACT. This receiver has an input, and an output that can go to some A/D converter or a radar display. It also has a plate on the front with two specifications from the manual inscribed on it in big letters. They read: "Receiver dynamic range: 60 dB" and "Receiver input range: 90dB" What's this? How can it handle 90 dB with a dynamic range of only 60dB? Well that depends on what Dynamic Range is defined to be, and here is the purpose of having the term Dynamic Range. Dynamic range is the range from noise to max signal that the receiver can handle (output) at one particular time - that is, without changing receiver parameters. Yep, there it is, just "max / noise" or "max / min", as we have discussed a million times. (it is actually max/min, but when the min is determined by noise, then we can rewrite it as "max/noise"). The absolute range or max range or non-dynamic range or input range or total range or whatever you want to call it is "max signal that can be handled under any settings" / "min signal that can be handled under any settings". The difference is that when the total range is greater than the dynamic range, you cannot achieve that whole total range without changing some parameter in the system - usually the gain. There is a very useful distinction between the dynamic range and the total range. To demonstrate: Take your trusty signal generator over to this receiver and plug it in to the input, and then plug your equally trusty scope onto the output. You adjust the sig-gen level and observe the receiver output on the scope, and as you twiddle you soon see that the output limits or saturates when the input is (say) +0dB, and gets lost in the noise at -60dB. You have confirmed the range - the dynamic range - of this receiver is 60dB. And you see that the way it is set up, the smallest signal you can see is -60dB. BUT, the first stage of this receiver consists of an expensive preamplifier with two important characteristics - this pre-amp has a huge dynamic range of 90dB, and it has variable gain. Suddenly, a light comes on the front panel. It says "Pre-amp gain now 30dB, it was 0dB before". You twiddle your sig-gen knobs and you see that now you can see a usable output on your scope with an input of -90dB. And now the signal saturates at -30dB. The Dynamic Range is still 60dB. BUT you have managed to measure a smallest signal on this receiver of -90dB and, previously, a largest signal of 0dB. This is the total range that this receiver can handle, 90dB. But not all at once, at one time it can only handle 60dB - the Dynamic Range. You might like to call the 90dB figure the TOTAL RANGE, INSTRUMENTED SIGNAL RANGE, CALIBRATED RANGE, NON-DYNAMIC RANGE, SIGNAL RANGE, INPUT RANGE or some such term. It is clearly different from the Dynamic Range, and THAT is why we dignify the Dynamic Range with its own name. 3) SCANNERS **************** This is not entirely irrelevant in the scanning world, and is specifically involved in the relationship between density range and scanner dynamic range. A scanner - a bad one - might have a dynamic range of 20dB i.e 100:1 (or a D of 2.0 in the usual loose scanning jargon). BUT it may be able to satisfactorily get detail in slides whose density varies from nearly transparent to 1000 times darker than this i.e cover a range of 1000:1 or D of 3.0. How? Same as the receiver discussed above, by changing the gain. With such a scanner you could NOT properly scan a single slide which contained the 1000:1 range in a single pass, but you COULD set the gain and scan the darkest parts getting all the detail in those dark parts, then re-set the gain and scan the light parts and get all the detail in the light areas. So your total recoverable density range might be 1000:1 although the scanner only has a dynamic range of 100:1. This fact can lead to some of the stupid specmanship that confuses everybody. For the above scanner, you would say the Dmax = 3.0 (that is the log version of 1000:1). But if you specified the dynamic range you would have to say 2.0. Trouble is many people confuse the two terms (and Dmax is possibly not being used correctly anyway) and assume that Dmax and Dynamic Range are synonyms which they most definitely are not. Despite the (accurately specified) Dmax of 3.0, you could NOT scan a slide with a range of 3.0, but you COULD scan a slide with max density 3.0 and range of 2.0. I really hope this helps someone to understand what is a logical, consistent approach to all this, which can be used to understand what is going on and to therefore make informed scanner decisions and comparisons. Now I stand to one side hoping to avoid the blast! Julian R *****PS to Austin****************************************************** AUSTIN - here's a related response to some of your post: Julian: "(definition of Dynamic Range has) Nothing to do with resolution at all," Austin: "It absolutely does have to do with resolution, and for some reason, you're not understanding that. The ratio is overall range to noise, fine. The noise IS the resolution, you can only resolve to the level of noise, that's plain and simple and indisputable. If you have a range of 1 and noise of 1/4, you can only get four different steps within that range. You can NOT get 8 discernable values, because your tolerance is simply 1/4...which is the finest you can resolve to. ...the noise is ALWAYS the resolution, whether it's the same as the low signal level or not." I agree with everything you say here. BUT, noise is NOT involved in the calculation of dynamic range in the general case. It is min signal that is in the definition. In most cases the min signal is the same as the noise level, but noise is NOT part of the definition. Dynamic Range does have a lot to do with resolution, in a deterministic sense under the circumstance when min=noise, but NOT in a definitional sense. You can have noise OR min signal in your calculation but NOT both at once, because noise ONLY gets into the equation when it is the same as the min signal. This is the problem, because here you start putting in this extra parameter, either noise or another level depending on the time and place. In relation to dynamic range there is only one bottom-end parameter, and that is the lowest detectable value. This might be noise, or it might be something else. The dynamic range is defined as highest value divided by the lowest detectable value. That's it. No divided by noise, no nothing, it is the ratio of max to min. So - dynamic range is max/min (a range). When min is the noise as is usually the case, it becomes max/noise. Finished. It is not (max-min)/noise or anything else. Example: This system for some reason has a noise of 1V, a minimum detectable signal of 2V and a max signal of 10V. Dynamic Range = max/min = 10/2 = 5. Not (max-min)/noise =(10-2)/1 = 8 It is 5. In this case, the DR is 5 and the number of levels to best quantify your signal is 8. DR is NOT related to noise. PLEASE read this carefully, the dynamic range in this case has NOTHING to do with noise. It doesn't in the general case either, except when min signal = noise, and even then only by a derived relationship, not by definition. The fact that min signal is the same as noise in the usual case is not a good reason for mixing the definitions up. What exactly do you think Dynamic Range is if it is not a range? ---------------------------------------------------------------------------------------- Unsubscribe by mail to [EMAIL PROTECTED], with 'unsubscribe filmscanners' or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or body