Hi, I'm new to the forum, as well as DSP. I am currently doing a project which involves filtering a square wave signal with a heavily skewed duty cycle which is going to have all sorts of noise imparted onto it. The basics of the project are such that this square wave is to be propogated down a railway line. When it hits a S/C or O/C it will be reflected back towards the source. The time delay between sent pulse and reflected pulse can therefore be worked out. In order to work out this time delay, some filtering will have to be done on the reflected signal, as a lot of noise (traction return current, harmonics from rectifying circuits for DC motors on trains) will be imparted onto it and therefore the pulse may not be initially visible. This project is not intended to result in a working design but rather have several different thought out designs (with simple simulations where possible) which could be implemented and then choosing the best one. The square wave will probably have a duty cycle of around 2/10, with complete pulse duration being less than 20us. The frequency of pulses is quite slow, roughly 1-2 seconds between pulses. Currently I am trying to find out the characteristics of the above noise signals. Can anyone suggest filtering ideas which could work. Is it possible possible to do some simulation/modeling of digital filters using Matlab? I have only had very brief encounter with Matlab and therefore any help regarding getting started with filter design/modeling in Matlab would be very appreciated. If I've missed out any crucial information then please let me know. Thanks to anyone who can add anything to this topic.