Specifications

Source Quality And Pre-emphasis
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SOURCE mATERIAl QUAlITY
The DSPXtreme has the ability to substantially improve the quality of your ON-AIR broadcast. However the
DSPXtreme can only work with what you provide it. The best performance will be obtained when the DSPXtreme
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 DSPXtreme.
We strongly advise against the use of MP3's and other compressed audio formats for audio storage. If you must
use compressed audio we advise rates of 256 Kbps and higher. Linear formats are always to be preferred.
Compressed audio formats employ frequency masking data reduction techniques to reduce the bit-rate. Through
re-equalisation the DSPXtreme can violate the frequency masking characteristics of the bit reduction process,
creating distortion that was inaudible prior to the DSPXtreme processing.
PRE-EmPhASIS
If you are using the DSPXtreme to process for FM broadcast you will need to enable the pre-emphasis filter in
the DSPXtreme. Even though your STL or transmitter may contain pre-emphasis we recommend disabling it, let-
ting the DSPXtreme handle the pre-emphasis for the transmission system. The DSPXtreme uses sophisticated
processing methods to limit the high frequency energy of the pre-emphasis curve while maintaining a 'bright'
sound. Using de-emphasis and then pre-emphasising again will only degrade performance and possibly casue
overshoots, resulting in lower average deviation.
The exception to the rule is when the DSPXtreme is feeding discrete left and right outs to a compressed audio
STL. Bit rate reduction codec's do not cope with pre-emphasis very gracefully and artifacts will be generated.
The best option in this case is to de-emphasise the output of the DSPXtreme prior to the STL system. At the
transmitter site the pre-emphasis can be enabled in the transmitter to restore the processed signal back to nor-
mal prior to transmission.
The best solution is always to locate the processor at the transmission site. This way overshoots are minimised
and quality is maintained.
FINAl lOwPASS FIlTERING
The DSPXtreme has a built-in lowpass filter in the HD path, adjustable from 4kHz to 20kHz that limits the
amount of high-end spectrum and energy delivered to the codec. Codecs are especially sensitive at higher fre-
quencies, where most of the annoying artifacts like "metallic high-end", "phasing" or "squishing" occur. Reducing
the amount of high-end puts less stress on the codec and can lower the amount of codec-induced artifacts. The
actual setting will depend on the type of codec and the bitrate used. Lower bitrates require more audio informa-
tion to be thrown away by the codec, so lower filter settings would have to be employed to reduce artifacts. If
you're using higher bitrates, you can set lowpass filter more towards 20kHz without causing excessive artifacts.
As a rule, low pass filter frequency should be set at or below codec's half sample rate frequency (i.e. 15 kHz for
codec running at 32 kHz sample rate).
OUTPUT lEvEl SETTING
DSPXtreme has an adjustable output level that can be set to drive subsequent equipment with full scale digital
audio level. However, caution should be made here if the following equipment using perceptual coding (codec)
does not take into account internal codec overshoots. Overshoots can occur in the codec when presented with
the tightly peak-controlled audio such as the one produced by DSPXtreme. If the codec does not provide head-
room internally, these overshoots would be clipped leading to increased distortion and artifacts. Therefore we
would advise setting the output level of the processor lower than 0.0 dBfs to leave the headroom in the trans-
mission system for codec overshoots. The actual output level setting would depend on the amount of codec
overshoot occurring in the system, which depends on the type of codec used and how aggressively you're
processing audio with DSPXtreme. Generally, codecs that use spectral band replication (SBR) technology (such
as HE-AAC (aacPlus), iBiquity HDC, etc.) have more overshoots than the codecs that don't.
If you're using DSPXtreme to process for HD Radio (IBOC), DAB+, DRM+ or web-streaming, a good starting
point to set the output level would be -2.0 dBfs. This would ensure maximum audio quality at the expense of
slightly reduced level (which the users can always compensate with volume control on their radios).