25 ~31 MARCH 2013 (WEEK 10)

HIGH PASS FILTER VS LOW PASS FILTER

- Filtering describes the act of processing data in a way that applies different levels of attenuation to different frequencies within the data.

A high pass filter will apply minimal attenuation (. leave levels unchanged) for high frequencies, but applies maximum attenuation to low frequencies.

A low pass filter is the reverse - it will apply no attenuation to low frequencies by applies attenuation to high frequencies.

There are a number of different filtering algorithms that are used. The two simplest are probably the Finite Impulse Response filter (. FIR filter) and the Infinite Impulse Response filter (. IIR filter).

The FIR filter works by keeping a series of samples and multiplying each of those samples by a fixed coefficient (which is based on the position in the series). The results of each of these multiplications is accumulated and is the output for that sample. This is referred to as a Multiply-Accumulate - and in dedicated DSP hardware there is a specific MAC instruction for doing just this.

When the next sample is taken it's added to the start of the series, and the oldest sample in the series is removed, and the process repeated.

The behavior of the filter is fixed by the selection of the filter coefficients.

One of the simplest filters that is often provided by image processing software is the averaging filter. This can be implemented by an FIR filter by setting all of the filter coefficients to the same value.

-Phototansistor 

-Are designed specifically to take advantages of this fact .the most -common variant is an NPN bipolar transistor with an exposed base region.Here,Light striking the base replaces what would ordinarily be voltage applied to the base so, a phototransistor amplifies variation in the light strking it.Note that phototransistor may or might not have a base lead (if they do, the base lead allows you to bias the phototransistor light response .