The figure below illustrates this concept:
- Both the red and blue lines show the frequency response of two channel amplifiers, spaced one channel.
- Both channels interact with each other due to the inherent lack of an abrupt response of any filter in radiofrequency.
- Part of the signal of one channel (e.g. the blue one) bleeds into the another. This interference can be noticed in the TV picture if its level is high enough.
- However, if the response of the blue filter is as good as to keep its level under a minimum value with regards the red channel signal, the interference it may cause will not be of concern.
- The figure shows how much the blue filter response "rejects" the red channel at that particular frequency.
- The more abrupt the response, the higher the rejection of the filter to other interfering signals.
In other words, the filter response is better.
- There is always a compromise between this response (the complexity of the filter) and the cost.
Very good filters can be designed but at an extra cost that has to be evaluated according to the application that they are intended for.