Dynamic Noise Reduction (DNR) is a method of removing noise from an audio signal. Media, which stores a signal in analogue format, is prone to such issues. An audio magnetic tape, that stores data on grains of magnetic particles, may exhibit noise after a few years. This may be due to the tape wearing out, or perhaps damage due to heat, which demagnetises the particles.
Noise issues also apply with audio signals stored on records, which may exhibit noise as the groove wears out. The audio signal may have noise components in the higher and lower frequency bands of the audio spectrum.
The Ideal Filter
An ideal filter is one which will remove the noise component and leave the rest of the signal unaffected, however many filters are far from being ideal.
There are also software based filters such as Audacity, which will filter noise, however this involves digitising the signal first.
I was looking for a hardware solution that could filter the noise components on the fly and decided to try this circuit.
LM1894 DNR System
This circuit works very well because it uses psychoacoustic techniques to reduce the noise level. These are some very clever and advanced tricks with much science behind them.
Psychoacoustic techniques are used widely in MP3 compression, where parts of the sound spectrum that are outside of the human hearing capability are removed, thereby reducing the size of the data footprint.
This chip uses white noise to mask sound, which is an interesting method not commonly found. In addition, since the human hearing cannot detect sound lasting less than a millisecond, this IC uses compression techniques to minimise the noise level. Finally, it also reduces the bandwidth of the signal to the precise audible range; hence, eliminating any noise falling outside the range. All of these techniques together provide a generous 10 dB noise reduction, whilst leaving the original signal relatively unscathed.
Hence, this type of filter is very different from active and passive filters that are readily available. Those types of filters including graphic equalisers generally do not perform as well and end up changing the characteristics of the original signal.
Although this circuit has wide applications, it is very popular with people who still listen to tape, and reel-to-reel, and especially where high audio quality is important. This type of circuit is ideal if you are a reel-to-reel user and you want to listen to your archive with an on-the-fly filtering system.
Tuning Out the Noise
This DNR is a low pass filter with a variable bandwidth from 1 kHz to 30 kHz. A variable bandwidth is useful as it allows one to “tune” out the noise precisely depending upon the recording media.
For example, if you have a tape with a constant noise at a particular frequency, then adjusting the 1 kΩ potentiometer such that pin 10 just begins to register a voltage will tune out that noise. To provide a tuning support, a bar graph meter circuit can help.
This type if tuning works where you may have a reel tape where the lead-in exhibits the noise component alone, hence, it is possible to use this reference noise to tune the filter into removing it.