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Comparing Analogue and Digital

Since digital systems work with only two levels ( on and off ), they are much more immune to noise and interference than analogue systems. As long as the errors in the digital signal are not so big that the on and off levels can not be distinguished, the digital system will work perfectly. The signal can be cleaned up completely.

Analogue Noise

When noise is added to analogue signals, it sounds like a background hiss (thermal noise) or perhaps cracks and pops (lightning or power switching). Such noise can not be removed so the original clean signal can not be re-created. Techniques such as Dolby noise reduction make the noise less obtrusive but do not remove it. Faint analogue signals can disappear into the background noise completely.

Digital Signal Conditioning and Regeneration

When a noisy digital signal is cleaned up with a Schmitt Trigger, this is called a regenerator. It is an example of signal conditioning. There are other examples of signal conditioning ...

• pulses can be re-timed to remove jitter (the pulse arriving too soon or too late).
• statistical techniques can be used to re-construct data from a very noisy signal. Many samples are taken and averaged to see if the pulse should be a zero or a one.

Repeaters and Regenerators

The repeater amplifies the signal including the noise.

The regenerator removes all but the most extreme the noise so it's a more useful device.

Schmitt Trigger Regenerator

A simple Schmitt trigger NOT gate can be used. This has a speed advantage.

Here are two Op-Amp Schmitt trigger circuits.
These are likely to be slower but there is much more control over the upper and lower switching levels.

In the simpler left diagram use the voltage divider formula to find       V_t = V_out R₁ / (R_f + R₁)

Remember that V_out can be high or low so there are two different values of V_t.

In the right hand diagram, if V_out is high, R_f is in parallel with R₁.
Do the resistors in parallel calculation first.
In the right hand diagram, if V_out is low, R_f is in parallel with R₂.
Do the resistors in parallel calculation first.
 
So there are two different values of V_t.

Now use the voltage divider formula to find V_t.

When noise is added to a digital signal, it is often possible to regenerate the original digital signal perfectly. This means that signals can be transmitted without any noise being added. This is why a phone call from across the world can often sound as clear as one from next door. The digital data crosses the entire network without being damaged.

The diagram above shows random and impulsive ( the spikes ) noise added to a digital signal.

Using a normal comparator to regenerate the digital signal, there would be errors. If the reference voltage is the grey centre line the noise is large enough cross the reference line. This causes the errors.

Using a Schmitt Trigger helps a lot because the signal has to rise above the red line for a ZERO to be registered and go below the blue line for a ONE to be registered. The spikes in the noise are no longer big enough to create errors. Of course, really bad noise would still cause errors but this circuit is much better than a simple comparator.

Digital noise immunity explains why digital phones, radio, TV and CD music all perform so well. On an analogue system this amount of noise would sound very unpleasant. The digital system is able to remove the noise.

If there is too much noise in a digital system, this causes effects like a TV picture breaking up into rectangular blocks. Audio output can be damaged too. Symptoms include gaps, squeaks and gurgles where a sound is repeated because the new data is missing. This last effect is less obtrusive than a gap or a squeak.

Dolby Noise Reduction

Dolby noise reduction is a kind of signal conditioning. When recording to magnetic tape, the high frequencies are exaggerated. When playing back, the high frequencies are reduced back to their correct level. High frequency background noise from the tape is reduced at the same time. This reduces the apparent noise from the tape without unbalancing the sound of the audio.

A similar technique is used for FM VHF broadcast radio. In the transmitter, high frequencies are amplified more than normal. In the receiver the higher frequencies are reduced back to their correct level. At the same time, high frequency noise (hiss) is reduced. The signal from the speaker sounds better as a result.

Digital audio or video recording onto tape does not suffer from these noise problems. Recently it has become common to record to hard disks or sd-cards and tapes are rarely used.