Specifications
The DSPX And It's Processing Structure
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THE DSPX AND ITS PROCESSING STRUCTURE
The DSPX broadcast audio processor can be used for processing audio prior to broadcast on FM and digital
radio services. Digital radio encompasses DAB,HD,IBOC and other radio based broadcasting as well as internet
radio, known also as streaming. The DSPX can also be used effectively for audio post production and mastering,
ideal for giving CD’s that HOT sound. It is also possible to use the DSPX for other audio level control/equaliza-
tion applications such as night clubs and bands. However, this manual will only be referring to the use of the
DSPX 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 DSPX. The DSPX employs dual output paths for peak control. Your processing application may
need you to configure the DSPX in a certain way. Selecting the wrong output path and or not configuring 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 artefacts.
Look-ahead limiting produces less harmonic distortion but produces IM distortion if over driven resulting in a
packed, busy sound.
The DSPX can be configured so that each peak control path can be routed to either of the digital or analogue
outputs. The stereo encoder is always fed with the 'FM' path. One popular configuration for FM radio stations is
to use the DSPX 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 DSPX 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.
Bass enhancement
The DSPX offers two 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.
Wideband AGC.
For a transparent input levelling function The DSPX employs an RMS detected AGC stage that has various
hidden (automatic) intelligent controls. Return to platform level and speeds interface to the intelligent silence
gating preventing hunting and suck-up during quiet passages. A sophisticated window gating system prevents
small unnecessary level changes.
Xover
The DSPX employs linear-phase time aligned digital FIR filtering to split the audio spectrum into 4 bands
while maintaining sonic transparency. Two Xover frequencies are selectable for each Xover point.
Multi-band AGC
Like the wideband AGC the DSPX processes each band with RMS based levellers. Each bands gain control
processing function can be configured in different manners to provide different effects. Adjustable ratios, band