BW Broadcast DSPXtra Encore, Руководство пользователя, Страница: 30 / 56

9 - 5
9.2.3 CLIPPING
Img. 4
Clipping the audio waveform will not produce any audible side effects if performed in
moderation. Excessive clipping however, will produce a form of distortion that is
unpleasant to hear. Clipping can also be used as an effective method of high
frequency peak control when used in conjunction with distortion controlling filtering. See
image 4.
9.2.4 LOOK-AHEAD LIMITING
Often used instead of a clipper in systems that feed bit rate reducing audio codecs,
look-ahead limiting examines the audio waveform and prepares a gain control signal in
advance of the delayed audio waveform arriving. This prevents overshoots while
minimising distortion. A look-ahead limiter behaves in the same way as a soft clipper.
Competent look-ahead limiters will usually be of the multi-band variety.
9.2.5 SOURCE MATERIAL QUALITY
The DSPXtra-AM Encore has the ability to substantially improve the quality of your
broadcast. However the DSPXtra-AM Encore can only work with what you provide it. The
best performance will be obtained when the DSPXtra-AM Encore is fed with very clean
source material. After dynamic multi-band re-equalisation is performed, poor quality
source material will sound poorer when processed with the DSPXtra-AM Encore.
We strongly advise against the use of MP3s and other compressed audio formats for
audio storage. If you must use compressed audio, we advise bitrates of at least 256
Kbps. Linear formats are always to be preferred. Compressed audio formats employ
frequency masking data reduction techniques to reduce the bitrate. Through re-
equalisation the DSPXtra-AM Encore can violate the frequency masking characteristics
of the bit reduction process, creating distortion that was inaudible prior to the DSPXtra-
AM Encore processing.
9.2.6 HF BOOST (PRE-EMPHASIS)
Today’s typical AM receivers have narrow bandwidth (typically 3 kHz) and it is usually
beneficial to compensate for the high-end roll-off by boosting the high frequencies in
the processor. This can help restore some of the high-end response and brightness. The
DSPXtra-AM Encore has an HF equalizer that can provide as much as 20 dB of high end
boost. The graph below illustrates pre-emphasis curves in 2 dB steps. The blue curves
show the response with the shape control set to 1, and the red ones show the response
with the shape control set to 10.