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9.5.6 BASS CLIPPING
The DSPXtra-AM Encore, like most competent broadcast audio processors, has a bass
clipper prior to the final clipper.
The purpose of the bass clipper is to keep low frequency energy to a pre-determined
level to allow for the summation of the other bands. Without the bass clipper the bass
signal can push the mid and HF audio waveforms into the final clipper, creating audible
IMD, the worst type of distortion. By restricting the bass to a certain level the mid and HF
energy has its own reserved space in the summated waveform and we reduce the
likelihood of bass generated IMD.
The downside to bass clipping is you are restricting the bass to a lesser level than what it
would be without it. The upside is that moderate levels of bass clipping won't cause a
large loss of bass loudness and should have minimal audible artefacts.
When bass-clipper is being driven more aggressively you will start to notice generated
distortion. This distortion can be used to actually give the illusion of more bass, especially
on smaller radios that are incapable of producing the lower frequency fundamental
bass waveform.
This can be viewed as an upside of bass clipping. You need to decide what level of bass
clipping is acceptable to your format, both in creating room for summation from the
other bands and making the punch/distortion trade-off.
9.5.7 FINAL CLIPPER
The final clipper is a sophisticated highly over-sampled peak limiter that incorporates
distortion-controlling techniques, and has an embedded 1kHz low-pass filter.
This section of processing is the last line of defence in the processing and is also the most
critical part in the loudness/quality trade-off. While each of the preceding processing
stages play a part in reducing the peak to average ratio of the audio waveform none
has the same effect on the peak to average ratio as the final clipper.
Great care is needed in setting the final clipper drive control. This control needs to be
adjusted carefully and only you can make the decision on the balance between
loudness and quality.
As you increase the drive you will obviously obtain more loudness but at the expense of
distortion. There is a fine line between artistic distortion and distortion that your listeners
will find uncomfortable to listen to, especially for extended periods of time.
The final clipper has an additional control to help reduce IMD distortion. The ‘clipper
hardness’ control is an additional program dependent mechanism that helps to reduce
distortion by analysing the level of IMD, and dynamically adjusting time constants.
The control is subtle and its range has been limited to restrict the amount of control,
preventing pumping and a loss of loudness which would undo what we want to use the
clipper for - which is gaining loudness.
