Specifications

Source Quality
Source materIaL QuaLIty
The DSPXmini-HD has the ability to substantially improve the quality of your ON-AIR digital broadcast. However
it can only work with what you provide it. The best performance will be obtained when the processor is fed
with very clean source material. DSPXmini-HD can improve audio somewhat (especially intelligibility) through
dynamic multi-band re-equalisation, but it can't improve audio quality in terms of distortion - poor quality source
material will only sound poorer after processing.
We strongly advise against the use of MP3's and any other loss-less compressed audio formats for audio stor-
age. 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 DSPXmini-HD can violate the frequency
masking characteristics codec relied on, "revealing" distortion that was inaudible prior to the DSPXmini-HD
processing.
Additionally, as digital audio broadcasting systems employ perceptual coding as part of the transmission system,
using compressed sources will further decrease audio quality. If audio is stored in the play-out system in com-
pressed format, this already reduced audio information is subjected to yet another coding cycle for transmission,
often with very low target bitrate. The end result is lower audio quality and increased coding artifacts, than there
would be if an uncompressed audio sources were used and cascading codecs avoided.
fInaL LoWPaSS fILterIng
The DSPXmini-HD has a built-in lowpass filter 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 frequencies, 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 information 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
DSPXmini-HD 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 pre-
sented with the tightly peak-controlled audio such as the one produced by DSPXmini-HD. If the codec does not
provide headroom 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 transmission 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 DSPXmini-HD. Generally, codecs that use spectral band replication (SBR) technology
(such as aacPlus, HDC, Windows Media 11, etc.) have more overshoots than the codecs that don't, especially in
the higher frequencies.
If you're using DSPXmini-HD 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).
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