Specifications

Processing Structure
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ThE dSPXTREmE ANd ITS PROCESSING STRUCTURE
The DSPXtreme broadcast audio processor can be used for processing audio prior to broadcast on FM and
digital radio services. Digital radio encompasses DAB, HD Radio (IBOC) and other radio based broadcasting as
well as internet radio, also known as streaming. The DSPXtreme can also be used effectively for audio post pro-
duction and mastering, ideal for giving CD’s that HOT sound. It is also possible to use the DSPXtreme for other
audio level control/equalization applications such as night clubs and bands. However, this manual will only be
referring to the use of the DSPXtreme for FM and digital radio processing.
Before we discuss the processing structure in full we would like to tell you a little about the final peak limiting
stages of the DSPXtreme. The DSPXtreme employs dual output paths for peak control. Your processing applica-
tion may need you to configure the DSPXtreme in a certain way. Selecting the wrong output path and or not con-
figuring the other settings that affect it may seriously downgrade your audio quality.
The first peak control path is known as 'FM' as it is typically used when processing signals for FM broadcast.
It employs distortion controlled clippers to limit the peaks of the signal. Distortion controlled clipping is the best
method for preserving as much high frequency energy as possible, important when the high frequency loss char-
acteristics of the FM broadcast de-emphasis curve is taking into account. Distortion controlled clipping produces
harmonic distortion which if used moderately can produce a sizzling bright sound but can result in a ripping or
tearing sound if used excessively (overdriven).
The second peak control path is known as 'DR' (digital radio) and is the desired method of peak processing
when the output feeds a codec that employs ‘bit rate reduction compression techniques’. The 'DR' path employs
look-ahead limiting as opposed to clipping. Look-ahead limiting produces less artifacts than conventional clip-
ping, so will reproduce the original audio more accurately with less bits of digital information because it is not
wasting bits encoding non-audible clipping artifacts. Look-ahead limiting produces less harmonic distortion but
produces IM distortion if over driven resulting in a packed, busy sound.
The DSPXtreme can be configured so that each peak control path can be routed to either of the digital or ana-
logue outputs. The stereo encoder is always fed with the 'FM' path. One popular configuration for FM radio sta-
tions is to use the DSPXtreme to process their FM broadcast and to have the 'DR' path feed their digital radio
service or web stream, each service optimally processed for that medium. We suggest that digital radio services
always use the 'DR' path but you are free to experiment with both options.
ThE PROCESSING PATh
Input selection and conditioning
The DSPXtreme offers the user input selection, gain control and a selection from a range of stereo/mono
options. The audio is then routed through defeatable high pass, phase rotating and pre-emphasis filters.
A silence detector provides automatic primary to secondary input failure switching.
AGC xover
The DSPXtreme employs an adjustable gentle-slope digital IIR filtering to split the audio spectrum into 4
bands while maintaining sonic transparency.
Multi-band RMS leveller
For a transparent input levelling function, the DSPXtreme employs an RMS detected multi-band leveller. With
a windowing gating concept, this four band AGC improves consistency and presents a uniform audio signal to
the following stages.
Bass enhancement
The DSPXtreme offers three forms of bass enhancement:
1. A 12dB/Octave shelving filter with up to 12dB of gain.
2. Bass tune control.
3. A peaking filter that can be set to provide up to 6dB of gain on 1 of four frequencies with a choice of 4
Q's. This can be thought of as a simple bass parametric.
Limiter xover
The DSPXtreme employs fixed linear-phase time aligned digital FIR filtering to split the audio spectrum into 6
bands while maintaining sonic transparency.
Multi-band Limiters
Each band has its own dynamic peak limiter. Multiple time constant based detectors with built in adjustable
hold and delay functions significantly reduce distortion.