The Advanced Parks-McClellan Specification Editor (APM) is used primarily to design multiple-passband filters, though it also can be used to design single-passband filters. Compared to the Simple Parks-McClellan Specification Editor, the APM allows you to enter any number of passbands and stopbands, allows you to specify certain additional Parks-McClellan parameters, and also allows you to design Differentiators and Hilbert transform responses. The APM looks like:

The Advanced Specification Editor has five elements:

• Sampling Frequency.  Lets you specify the sampling frequency (rate) at which your FIR filter will run.
• Response Type. Lets you select response types of Filter, Differentiator, or Hilbert transformer. In most cases you will use Filter.
• Number of Taps. Lets you specify the number of taps (coefficients) the filter will have. ScopeFIR can also Estimate the number of taps for single-passband lowpass filters, after all other specifications have been entered.
• Grid. This lets you specify the Parks-McClellan Grid Density. Grid Density is a number which the filter design algorithm uses to build a grid of trial response frequencies. The default value of 16 works well in almost all cases, but you might increase this number if the filter design algorithm is not producing results that reflect your parameters. This situation occurs primarily when your filter includes one or more very narrow bands. The time it takes ScopeFIR to design a filter is related to the grid density, so reducing the grid density can speed the filter design algorithm, and increasing it will slow the algorithm. Increasing Grid Density beyond a certain point will not noticeably change the filter design, it will just take longer to do.
• Band Editor. The Band Editor lets you specify the characteristic of each frequency band in your filter.

The Band Editor is where the rubber really hits the road. Each band entered into the Band Editor is either a "passband" or a "stopband". That is, each band either passes or blocks signals in the band's frequency range. For each band you must specify the following:

• Lower and Upper Frequency. These fields specify the frequency range of the band.
• Desired. Generally, you will set this field to "1" or "0" to specify passband or stopband, with "in dB" unchecked. Alternatively, you can enter "p" and "s". For advanced users, this field can also be used to individually set the gain of each passband.
• Attn/Ripple. This sets the attenuation of stopbands or the ripple of passbands. Note that ScopeFIR can automatically set the Desired field to "1" or "0" based on whether the Attn/Ripple exceeds the "Max Passband dB" threshold value, which is set using the "View/Preferences" menu. The default threshold value is 10 dB; values less than or equal to 10 are assumed to be passbands, values greater than 10 are assumed to be stopbands. To use this feature, just tab past the Desired field.
• Actual Attn/Ripple. After the filter has been designed, this column shows the actual values, to help you know if you should increase the number of taps (if specifications were not met) or you can decrease the number of taps (if specifications were substantially exceeded.)

Now that you understand each field, using the Band Editor is straight-forward. Simply fill in the fields for each band and hit the Add button. You can delete bands with--you guessed it--the Delete button. If you make a mistake, click on the band, correct the error, and hit Add again. This corrects errors because whenever you add a band that overlaps an existing band, ScopeFIR replaces the existing band. This can be a bit confusing at first, but since the corrected band will generally overlap the original, you'll find it to be a very quick and easy way to make corrections.

Which brings us to another important point: ScopeFIR not only does not allow bands to overlap (which they can't logically do), but it also doesn't allow bands to touch. When you enter bands that touch, ScopeFIR produces an error message and ignores the new band. This behavior can be confusing to new users who think of bands as being "next to each other". But in fact, bands cannot touch because there must be a transition band, that is, a frequency gap between any two bands. This follows directly from FIR filter theory: the number of taps required to implement a filter increases as the transition bands narrow (as a fraction of the sampling rate). So without transition bands, your filter requires an infinite number of taps! (Let's face it, none of us has the time!) So the moral of the story is: you must provide a transition band, that is, a frequency gap between any two adjacent bands. Sorry, Son, it's not just ScopeFIR: it's the law.