Specifications

Setting Up The Processing
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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 adjust-
able low pass filter that is used to tame high frequencies. The shelf is needed when the DSPXtreme has its
pre-emphasis control engaged, for several reasons. The first being that the DR processing path usually 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 frequen-
cy 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 compensate
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 sec-
onds 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 maximum +/- 75 kHz peak deviation only.
The ITU limiter in the DSPXtreme , when activated, will reduce the loudness of your broadcast signal and main-
tain 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 values
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 DSPXtreme 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.
Stereo generator
The DSPXterme-FM digital stereo generator creates the multiplexed output with superb stereo separation,
excellent pilot stability, low distortion and low crosstalk between main and sub-channel. MPX level is adjustable
via front panel and has a wide range to produce proper modulation level with any exciter. Pilot injection level is
adjustable as well, with the option to turn it off for mono operation. If you are already broadcasting mono audio,
turning pilot off is a good idea as it will increase your signal to noise ratio by about 23 dB. There is also a switch-
able 19 kHz pilot reference output for RDS encoders.
Composite clipper
The composite clipper in the DSPXtreme'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.
If you are using high composite clipping drive (lots of composite clipping), an optional pilot protection notch filter
will protect the pilot area from being contaminated by clipping harmonics. Be aware that the filter will remove