BW Broadcast DSPXtra-FM, Operational Manual

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Setting Up The Processing
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the control or the secondary ones have most of the control, or a balance, which works best. To lessen the effect
of the secondary time constants set the attack to 10 and the decay to 1. This minimalises energy distributed into
the secondary time constant circuit. The peak time constant circuit will dominate and control the audio. If we
were to do this with each of the bands of the look-ahead limiter we would significantly increase the loudness but
more business and IM distortion would be introduced to the audio especially under higher levels of drive. If we
were to flip the settings of the secondary time constants around to distribute most of the energy into the second-
ary time constant circuit we would see an improvement of audio quality at the expense of reduced volume due
to the longer release time constants being used. As previously pointed out a balance is best and that balance
will vary with each band of the look-ahead limiter.
One of the most important features of the look-ahead limiter is the shelf filter control. This is effectively an
adjustable low pass filter that is used to tame high frequencies. The shelf is needed when the DSPXtra-FM
has its pre-emphasis control engaged, for several reasons. The first being that the DR processing path usu-
ally serves a broadcast medium that does not have de-emphasis so outputting pre-emphasised audio will not
sound very nice. Secondly, the pre-emphasised audio will dominate the peak control of the look-ahead limiter
and create spectral modulation pumping. The shelf allows us to compensate for the effect of this increased high
frequency energy restoring a more natural tonal balance to the DR processing path. The shelf controls range is
0 to -17dB with negative numbers equating to more cut. The numbers are the gain reduction being applied at
15 KHz. As an example a processor setup for DUAL use that had its pre-emphasis set to 75uS would require
a shelf cut of between -17dB and -14dB to restore a more natural tonal balance and 50uS pre-emphasis would
probably require -15dB to -12dB. Even if the processor does not have pre-emphasis enabled you may find that
you may want to experiment with small cut figures of between -4dB to -1dB to tame the high frequency energy
that may have built up in the multi-band processing.
We need to draw your attention to one other control that can have an effect on the look-ahead limiter and which
may not be immediately obvious. The HF clipping control (located in the clipper menu) redistributes control of
high frequency energy from the multi-band limiters into the following processing stage. For the FM path this
HF energy gets handled by the HF clipper but in the case of the DR path and its look-ahead limiter this high
frequency energy can modulate the rest of the audio waveform and introduce a pumping type sound. If you are
using the processor to process for DUAL services (FM and digital) then you can use the shelf control to compen-
sate for pre-emphasis. If you are not using pre-emphasis (non FM use) then do check the HF clipping control if
you are experiencing excessive HF energy and/or pumping from the look-ahead limiter.
ITU BS.412 limiting
The ITU BS.412 standard recommends that the power of baseband composite signal integrated over any 60
seconds interval, does not exceed the power in a sine wave that modulates the carrier to +/- 19 kHz. This
requires significant reduction in loudness compared to the usual requirement of m/- 75 kHz peak
deviation only. The ITU limiter in the DSPXtra , when activated, will reduce the loudness of your broadcast signal
and maintain its power at the level required by the BS.412 standard. It will do so with the slow limiter to avoid
noticeable pumping which is the consequence of the standard requiring measurement of the pre-emphasized
composite signal and not employing any frequency weightning to make it more natural sounding.
When activated, the ITU limiter will reset the Multiband Clipper Drive, Main Clipper Drive as well as Composite
Clipper Drive to their default values. Keep in mind that even if you then turn the ITU limiter back off, those val-
ues will not return to the values they were before you tuned on the ITU limiter, but will remain at their factory
defaults. This means that if you have a custom preset that changes the values of those parameters, turn the ITU
limiter on and then back off, it will not sound the same as the mentioned parameters will be reset to their factory
values.
The ITU limiter in DSPXtra will preserve the sound texture of your preset (excluding the distortion and effects of
the main clipper). However, as the ITU limiter reduces the power of your signal anyway, you may wish to relax
the processing and actually make use of the increased rms-to-peak ratio. We would therefore advise raising the
MID and HF clipper thresholds to -1 dB and BASS clipper threshold to -3 dB. You may also want to increase the
limiter peak attacks to let more of the transients through.
Composite clipper
The composite clipper in the DSPXtra-FM's stereo encoder is highly over-sampled and allows you to gain an
extra dB or two of modulation loudness when using the multiplex output to drive your FM transmitter. The range
of the composite clipper is -0.5 to 2dB.
Here at BW Broadcast we prefer to use the multi-band and final clipper to generate the required amount of loud-
ness because composite clipping introduces stereo crosstalk. When we do use the composite clipper we prefer
to use small amounts of no more than 1dB. A pilot protection filter is provided which will protect the pilot region
from contamination by composite clipping products.