DIGITS.HTM --- Part of Manual for Driver Parameter Calculator --- by
Claus Futtrup.

Created 2. February 1997, last revised 15. June 2003.
Ported to XHTML 1.0 on 2. October 2004. Last modified 26. October 2004.

With DPC it is possible, through the DPC.INI configuration file or alternatively through Change setup (the main menu, item 4), to specify desired number of digits to be displayed for each variable given in DPC by specifying a .DIG file. You may of course enter more precise data, and DPC will use full precision, but this will not be displayed on the screen.

For each set of .CU file (custom units), there is a corresponding .DIG in the distribution, which does this. These files are just suggestions from my side and may be modified as desired. Besides this I have made one digits file called MAX.DIG, which gives you as many digits as possible within the limits of space on the screen.

The format of the DIG file simply is: each line specifies the variable in question, eg. the resonance frequency fres, and then two numbers are specified, say x:y, where x specifies the (maximum) number of digits you would expect ahead of the decimal separator and y specifies the number of digits you would want after the decimal separator.

For the example fres, you would expect fres to be below 10 kHz = 10000 Hz (5 digits), so x = 4 is maximum, also there should not be any decimals for fres, so y = 0. The line for fres would in other words look like this:

fres=4:0

It is important to note, that if fres for once is 10000 or higher, it will of course be displayed, but with the decimals pushed backward and perhaps into some text on the screen. Therefore x + y should be below about 12 or so, to make up a certain margin for outrageous numbers. While x is always displayed, y is always rounded to the specified number, so we need to ensure that enough digits are displayed for any possible driver.

A special situation appears if you specify eg. fres=0:0. In this case DPC interprets this as a desire for removing the parameter from the list, and the number will never be displayed (though it will be calculated). This could be positive to users, who feel they do not want to see one of the more special parameters calculated by DPC, perhaps because it is disturbing with unnecessary parameters in the display. This technique is only supposed to work for normal parameters (displayed in the paramter editor and similar places), not for special stuff like rho and c.

Please note that DPC has plenty of room on the screen for large numbers, like 15-17 spaces or so (eg. 14 digits and a decimal separator). DPC uses REAL or DOUBLE precision, which should give you no higher than approx. 15-16 digits.

There is no degradation of the precision within DPC, but if you want to specify your own .DIG file, you should consider the relative accuracy of all the numerical values you're going to specify to DPC. How precise do you actually know fres? it is probably within the range 1 Hz, if we're talking about woofers with low resonance frequencies. For tweeters you should not expect higher precision than approx. 10 Hz.

When measuring loudspeakers, the precision should (hopefully) stay within 5 %, but that is actually also quite a large figure. For this reason there is no "real" value in specifying your Q-values with an extremely high precision, it just makes the screen harder to read and the values/figures harder to understand.

When reading a datasheet (from manufacturer) you will rarely see any number specified with (very) high precision. Manufacturers know that variation for a particular kind of driver can be significant, eg. I have a Vifa C17WH-09-08 driver, which I measured Qm = 2.29, but the fact being that Vifa had specified Qm = 4.3 on their datasheet! The most precise figure I have seen was a Peerless driver, where Cms was specified with 4 digits, which I find to be a bit much but perhaps fair enough because it is not influenced by environmental parameters (eg. like Vas, which is influenced by the actual air pressure, and therefore usually only specified with up to two digits precision). Cms is usually specified relatively precise, but actually Cms often varies with load/power input because spiders are not linear as well as common manufacturing tolerances of up to 10%.

Talking significant digits there is a big difference between fres = 50 Hz and fres = 50.0 Hz or perhaps even 50.000 Hz -- I am sure no manufacturer would specify fres that precise. On the other hand, if one uses the MKSA.CU (also called SI-units), then Vas may be something in the area like 0.0403 cubic meters (40.3 liters), many digits yes, but only three significant digits, the zeros ahead are ignored. In this case I have chosen the following setup in MKSA.DIG: Vas=1:4 -- which I think will suffice for any driver that supposedly goes into a box (otherwise Vas is not relevant). Please note that you should not specify Vas=0:4 because DPC expects that you reserve at least a zero ahead of the decimal separator, so there must always be a digit ahead of the decimal separator.

For calculated numbers (ie. numbers you have not entered into the Driver Parameter Calculator directly), we need to round off these numbers, so that they usually will be displayed with a meaningfull number of significant digits. There is no reason to believe the number is more precise than the figures used to calculate them. MANUAL.HTM contains a list of so-called data connections checked by DPC (with in-depth equations in FORMULAS.HTM). For example Rms is involved in one equation:

- Qm, fres, Cms, Rms

If Qm is specified with three (significant) digits, Cms with (up to) four digits and fres with three digits, there is no reason to specify Rms with more than three digits. It would simply be erroneous to believe that you know Rms with four (significant) digits precision.

In this case Rms should be specified with 3 significant digits, one ahead of the decimal separator and two behind. It will look like this:

Rms=1:2

With DPC you will have to think through the extreme cases---how many digits ahead and behind the decimal separator will you need, always to get a full representation of the number on the screen.

You can always exit the data parameter and go back to the main menu. Here you may enter setup and load MAX.DIG and this way you will be provided with further information.