The Dolby System is extensively used for reduction of noise in audio video systems. There are three Dolby methods.
Dolby A, Dolby B, Dolby C.
Dolby A: Used for professional recording, which is a complex method.
Dolby B: Used for domestic recording and is comparatively simpler method.
Dolby C: This is the recent developed method and is very effective in the noise reduction.
The basic dolby system (Dolby B) – used for domestic recording:
In this system, the signals below a pre-determined level called ‘dolby level’ (say below 40 db and above 1 KHz) are emphasized (boosted) while recording (coding) and are de-emphasized (reduced) back during replay (decoding) in the same amount.
As the high frequencies have low intensity, therefore they are passed through a high pass filter circuit. They are then amplified and sent to a limiter circuit, which limits them to a constant amplitude. Now the limited signal and the splitted signals are added by an added circuit.
The reverse is done in the replay process. The Figure (a) shows circuit for recording (coding) and Figure (b) for replay (decoding).
1.The noise of the tape recorder is basically a hissing noise during recording a low level audio signal.
2.Dolby system reduces the noise by 50 percent, or so.
3.The other noise reduction systems (not so popular)are: Dolby spectral recording, Telcon (developed by Telefunken) and dBx.