Inretia, even though you've just said you won't post anymore in this topic, i want to add a few comments, some specifically for you, most for anyone interested in this topic
1. i missed your previous post http://www.cdrlabs.com/phpBB/viewtopic. ... 2056#72056
i was in between my 2 posts and didn't see that you posted in the meantime.
Inertia wrote:I guess you just can't trust those science teachers. No wonder our schools are in trouble.
seeing as i'm a science teacher (math), should i be offended ?
Inertia wrote:See Understanding of the Speed and Acceleration Formulas.
Also, see Acceleration Formulas.
believe you me, i've no need for those references, i know all these formulae by heart! i guess you do get something
from a B.Sc. in physics!
Inertia wrote:"As we know with CAV, when the acceleration is constant the velocity increases linearly."
Far from being God-given, I felt that this statement would be so obvious to the technical know-how of this group that I did not expect it to be challenged.
i quite agree, this requires no derivation, this is obviously true.
to my eyes it is so negligible as to be unremarkable and not an issue. Have you considered that the graph may be distorted? As cfitz has explained, either we are dealing with CAV or not. I don't think anyone is claiming to have evidence that the burner manufacturers are lying to us about using CAV because someone imagines they see something different in a CD Speed graph. As I have stated, the fixed RPM of CAV provides constant acceleration in a linear fashion, i.e., a straight line, from the inner hub to the outer edge of the disc. This is a fact, and not a matter of opinion.
cfitz wrote:True CAV (constant angular velocity) by definition requires that RPM be constant. Otherwise the angular velocity would not be constant. So if you are developing a complex model of true CAV that requires a non-constant RPM, then you are going down the wrong path. Regroup.
well, come now. because a manufacturer says so and so, that makes it true??? i can't belive i'm hearing this...
since when have we questioned the accuracy of the CDSpeed graphs? this is the first i see of it in these forums after more than a year browsing through them.
why are all
the CLV and ZCLV graphs "really straight" lines, but the RPM not? if the drive manufacturers can make a drive that does maintain a fixed linear velocity, why can't the RPM be fixed too?
if the CDSpeed graph of a CLV and ZCLV is indeed a straight horizontal line, it seems to me that CDSpeed is accurate enough. so if the RPM is not, why are you questioning CDSpeed now?
if you go this way, why trust any
of the data from CDSpeed? why trust the starting and ending speeds, the total time etc. etc. ?
i take these graphs at face value. and they all
show that the RPM isn't really constant. this is in reference to KCK's comments and mine about non-fixed RPMs. it should
be true CAV, but apparently, according to the empirical data supplied by CDSpeed, it's not.
and this could be another possible explanation for the speed graph not being linear.
Inertia wrote:As cfitz has alluded, sometimes those that are used to complicated issues may have difficulty with an uncomplicated approach and a simple solution. To my knowledge, there is only one way of calculating an average speed for a CAV data transfer. In the example given, the test disc was exactly 74 minutes. It took 2:04 minutes to transfer the data. 74 / 2:04 is equivalent to an average transfer rate of 35.835x. This average transfer rate was the same rate that was calculated without benefit of minutes in my earlier post, so this is a proof of the average rate. An average transfer rate should be equivalent to a CLV rate that would produce a 74 minute data transfer in a given amount of time. At at (theoretical) CLV rate of 35.835x, a 74 minute disc would be completed in 2:04.
The fact that the transfer was completed in 2:04 minutes is a virtual proof that there can be no "concave" speed curve or RPM change in the CAV transfer. If the provided starting and ending speeds are used, linear acceleration is the only model that fits the completion time of 2:04 minutes. Any variation from constant acceleration would produce a different time.
i still disagree about some of the average speed issues.
granted, dividing the total number of bytes transferred by the time to transfer, will give the speed of an equivalent CLV transfer of the same data at the same time. and this is the "formula" for average speed that cfitz has advocated.
however, this most definitely does not proove
your calculation that the average speed is the algebraic average of the starting and ending speeds. not does it proove, as you claim, that there is no concavity. and to claim that "linear acceleration is the only model that fits the completion time of 2:04 minutes. Any variation from constant acceleration would produce a different time." is patently a false statement.
i'm looking and the empirical CDSpeed data. the plot is
concave. you can't
ignore it, i'ts visible to the eye. if i can see it with my eyes, it's significant enough that i can't just ignore it.
5. as to the usefulness of this topic, well, like i said before, i too don't clearly see what's the point.
it's a nice model and so forth. but from a practical perspective, solving it (for the total time,say) and comparing it with the empirical results from CDSpeed, well the only useful thing it does is check the validity/consistency of the model. it does not give us, as far as i can see, any new info that cannot be taken directly from the CDSpeed empirical info.
6. however, it does prod us to think a little more carefully about exactly what's going on inside of CDSpeed, what is it measuring, what is it reporting etc.
so far the most interesting question to me were KCK's questions about the empirical CDSpeed graph:
KCK wrote:2. The final point on the horizontal axis is always at least 2 minutes short of the displayed Disc Length; e.g., 74 min vs 77:45:70 in the fourth CDFreaks picture, where I would expect the final point to be 76 minutes. BTW, for one of my discs, CD Speed displays 79:01:35 in Disc Length and the graph stops at 76 min, whereas KProbe reports 79:03:34 and the final tested MSF = 79:03:23.
3. It's not clear whether the Elapsed Time and the final speed correspond to the final displayed length or the larger Disc Length. I don't think the Elapsed Time includes the spin-up time.
so far nobody has responded to or commented about this.
i can add another one of my own:
question: from my (little) experience with CDSpeed, when you do the test the progression along the x axis (location on the disc) is constant with respect to time. i mean the speed at which the test progresses along the x axis is fixed.
however if the reading (or writing) is really true CAV, it souldn't be! because the radius (proportional to the velocity in true CAV) is not linear with the location on the disc. this is what i noted in one of my previous posts.
i guess it all boils down to what you want to get out of the CDSpeed output.
i believe for most of us (all of us except KCK???), we are (were?) all happy to just accept the output as it is, no questions asked and no delving any deeper in...
a small aside, Inertia:
i was sorry to see that your discussion with KCK has "heated up" a bit (not wanting to use a stronger word). i hope you gather from the smilies i splashed throughout that i'm taking all of this rather light-heartedly. hope you do too