[Elecraft] Roofing filters

[Doug Person]

Pardon my ignorance...  I'm not completely clear on what a roofing 
filter is.  I understand traditional filters that control the passband 
and DSP that also controls the passband through software means.  But 
what exactly is meant by a roofing filter?  How is it different then say 
the crystal CW filter I have in my old Yaesu FT-900?

thanks

Doug -- K0DXV
(K2 1920 & 5837)