One of the criticisms often made of digital audio versus analog is that digital sound quality is often perceived as -too hard'. Other ways to express this quality in words include -brittle', -glassy' or -edgy'.
No-one ever says that about analog - analog electronics, analog tape or vinyl records.
So it is clear that softness in a sound is often a desirable quality. Even a distorted electric guitar, where plainly the sound is meant to be distorted, can often sound too harsh. Backing off the gain simply takes away the distortion where what is required is to keep the distortion but just make it less harsh.
But how is softness achieved? Just like -upfrontiness', there is no -softness' control.
The most obvious means of softening a sound is to back off the high frequencies. This is a tricky balancing act because if you back off too far, the sound becomes dull rather than soft. Also, it is commonly also necessary to back off the low frequencies too so the sound retains its overall frequency balance.
Clearly though, this is not a complete answer to the problem.
Let's look at what happens in analog tape and vinyl disc to relieve excessive harshness...
To achieve a good signal-to-noise ratio in analog tape recording, the level has to be pushed high enough to keep the average signal level well above the tape hiss. But this causes peaks to go well into the distortion region. However, short peaks of less than around 4 milliseconds duration are not particularly noticeable when they are distorted. But the side effect is that their level is reduced.
The same happens in vinyl. The acceleration of the cutting stylus is limited so that the playback stylus is not given an impossible groove to attempt to follow. Once again this has the effect of lowering peaks.
So it could be then that clipping short peaks is a good way of achieving softness. So would a limiter be the answer?
Well, it's well worth trying. A fast acting limiter set to capture just the short peaks can help to take away excess harshness. The trouble is it just might not be fast enough - although some digital limiters that can -look ahead' in time can be.
So the answer is to borrow a trick from analog tape and apply soft clipping. -Soft' clipping is where peaks are brought down in level, but they are not allowed to -square off' as in everyday clipping.
Instead, peaks have rounded tops that do not draw attention to themselves through excessive high frequency energy. You can do this using a plug-in that offers a soft clipping feature, or by putting the signal through a tube stage and pushing the gain.
It does have to be said however that soft clipping, whether you do it using a tube stage or plug-in, can make a sound tiring to listen to after a while. So taste and caution are necessary.
If you really wanted to go the whole way, you could lay off your track to analog tape, then copy it back. If you really wanted to achieve a certain kind of sound, then nothing would be too much trouble.
Or you could have an analog tape simulator?
No. They don't work. Why... will have to wait for another day...
By David Mellor, Course Director of Audio Masterclass
Friday November 25, 2005