How to select the parameters and order of a low pass filter?

I want to design a low pass filter with passband frequency of 4khz.. but I am not clear about which order filter should I use...and I am not clear about what all these means too..
Active or Passive?
Order of filter?
Filter type: Butterworth,Chebuchev, etc?
Gain: ?
Q quality: ?
Passband attenuation: ?
Stop band attentuation: ?
How should I choose these parameters...I have a filter designing software which asks all these specifications...what should be given to get a good response...I am all new to this... please help me out...

Re: filter designing

>> active or passive?
passive filters use only passive components (resistors, capacitors, ...)
active filters also use transistors or op-amps

>> order of filter
the higher the filter order, the steeper the transition from passband to stopband will be. also, the higher the filter order, the more components you need to use to realize the design.

>> filter design
different filter types optimize certain design criteria such as ripple in the frequency response.

>> gain
do you want the filter to amplify or attenuate the signal in the passband? if not, you would want a gain of 1.

Take a look at the wikipedia entry for analog filters, it gives a decent overview of most of these parameters.

Re: filter designing

ananya.M,
Choice of active or passive is influenced by the frequency of interest. In general, low frequency filters are not practical, since inductors must be used (except for very simple filters). Inductors are physically quite large for low frequency filters. For very high frequency filters, op amps are not available that have sufficient bandwidth. In this case passive filters are the only choice. Of course, passive filters have the advantage that no power supply is required.
.
The order of the filter and filter type depend on the required performance. The performance is defined by the required passband attenuation, and stopband attenuation. For example, if you require a maximum passband attenuation of 1 db at a specified passband frequency, and a required attenuation of 20 db in the stopband at a specified frequency, these two parameters partially specify the performance. The selection of filter type depends on several additional requirements, such as:
. Is ripple allowed in the passband?
. Is ripple allowed in the stopband?
. Is phase linearity important?
"Electrical Networks and Filters" by G.H Tomlinson gives a good introduction to both passive and active filters. For a good beginners book on active filters, see Don Lancaster's "Active Filter Cookbook".
Regards,
Kral

Re: filter designing

thanks for the ideas... I want to filter the audio input in to four bands... so I decided of using one lowpass,two bandpass and one high pass filter( for the range between 20 hz to 20 khz)..how can I decice the frequency range for each band and design the filters accordingly?should I divide it in to four equal bands?

Re: filter designing

ananya.M,
Deciding how to divide the audio spectrum really depends on your application. However, If you want to divive the spectrum so that there are an equal number of octaves in each band, then you would select 20Hz, 200Hz, 2000Hz, 20000Hz. The High pass filter would have a cutoff frequency of 20 Hz. The lower frequency band pass would have cutoff frequencies of 200 and 2000. The upper frequency bandpass would have cutoff frequencies of 2000 and 20000. The lowpass would have a cutoff frequency of 20000.
Regards,
Kral