Apogee Duet audio interface features soft limiting to prevent clipping

OXSA Labs | February 18, 2013

Should a pro engineer prevent clipping through correct gain setting? Or can soft limiting be relied on to save the day?

"This superior analog design prevents the digital clipping that causes distortion by instantaneously rounding off transient peaks before they hit the analog-to-digital converter. Soft Limit allows several more decibels of apparent level to be recorded while subtly providing an analog-like warmth to the sound." - Apogee

Apogee features 'Soft Limit' on several of their audio interface products, including the new Duet. The idea is that if you set the gain too high, Soft Limit will prevent digital clipping. Not only that, it adds warmth to the sound too. But is this a good idea for the budding engineer or producer?

What is clipping?

Clipping occurs in a recording when the preamplifier gain is set too high. The signal attempts to go over the maximum level that can be stored in a digital audio file (0 dBFS) and as a result the tips of the waveform are sheared off flat. The sound is distorted and harsh. Note 'harsh', not 'warm'.

How can clipping be prevented?

The usual way is to do a level test before recording. So the singer sings a few bars, the drummer goes round each drum, etc. - The level of each performer is tested and the gain of each channel set so that clipping does not occur.
But...
It is commonplace for players and singers to perform louder in an actual take than they do during the level test. The engineer will anticipate this and set the gain lower than the level test might have indicated. The engineer may choose to set the gain 6 decibels lower than the setting that was just below clipping. This is known as leaving 6 dB of 'headroom'. The more unpredictable the signal level, the more headroom should be allowed.
Headroom is an almost foolproof method of preventing clipping, in the hands of an experienced engineer.

So why Soft Limit?

Although the wise engineer will allow plenty of headroom, there is always the possibility that he or she might have underestimated. If so, the recording will be clipped. If the recording is being made under studio conditions, then the answer is to lower the gain and go for another take. But if it's a live recording, then the clipping has done its damage and is permanently fixed in the recording.
But if a limiter is used between the preamp and audio interface, or an Apogee interface with Soft Limit used, then an instance of unexpectedly high level doesn't have to mean disaster. The recording won't be entirely clean - it will be limited or soft clipped and will sound as such. But that's a lot better than it being clipped.

Should you rely on Soft Limit?

In professional terms, no. An engineer who can't set the gain correctly should find another profession that doesn't require such a high degree of care, attention and precision. Keeping Soft Limit as a last fallback position is OK, but relying on it is completely the wrong thing to do.

What about the 'warmth' Apogee claims?

If Soft Limit provides a kind of sound texture that you like, then there is no reason why you shouldn't use it. Bear in mind however that any processing that is used at this stage is locked into the recording and cannot be undone. You might consider that it is more flexible to make clean recordings that you can process any way you like during the mix.

More decibels?

"Soft Limit allows several more decibels of apparent level to be recorded, as Apogee says."
Let's say that you push the level 3 dB over the clipping point. Soft Limit will round off the top 3 dB of your recording, leaving the peaks just below 0 dBFS, so there is no actual extra level. The lower levels that would not have clipped will however be 3 dB higher, at the expense of the overall accuracy of the recording.

Takeaway...

Soft Limit is a useful tool as a last backstop of protection, but a professional engineer would hardly ever need to use it. If you like the sound of Soft Limit, then it's fine to use it, as long as you realize that it can't be undone later in the mixing process.