In typical stereo encoders, overshoots in the program material result in over-modulation in the stereo generation process. With the continued proliferation of audio delivered digitally using satellites or the internet, overshoots are common place, especially when bit reduction codecs are used – which typically can change the waveform amplitude by up to 3dB or more. Tightly controlled audio levels are no longer peak controlled by the time they eminate out at the FM transmitter end of the digital link – which means unless the transmitter or stereo generator has some way of handling the overshoots, over-modulation will occur, resulting in the need to reduce peak levels with the resulting loss of loudness and signal to noise.
Advanced stereo generation for digital delivered audio
Some stereo generators have brute force clippers on the inputs of the generator to deal with this, often audibly, generating clipping distortion, or audible ducking of the audio. Other generators may use composite clippers, with the resulting ‘mush’ that infects the spectral purity of the broadcast.
6 Digital Signal Procesing chips inside the DSPmpX provide unprecedented peak control and reference grade stereo generation. With typically over 70dB of stereo separation and very tight linear phase 15Khz brickwall filtering, your radio stations composite signal will be exceptionally clean, both in terms of audio quality, and in spectral content, ensuring you do not cause interference, or increase levels of multi-path.
Inaudible amplitude and bandwidth control
The DSPmpX borrows techniques from the $10,000 flagship broadcast audio processors and integrates the following into an extremely simple to setup product.
- Reference grade stereo generator
- Composite clipper
- Pilot protection filter
- SCA / RDS filter
- Linear phase overshoot compensated brickwall 15 Khz filters.
- Distortion controlled audio clipping and limiting system
In front of the stereo generation system is a highly advanced DSP audio subsystem with the sole job of preventing the stereo encoder from being over-driven (unless you turn up the composite clipper control). This system contains an overshoot compensated distortion control clipping system with dynamic limiting feedback mechanism. In layman’s terms, this translates to an extremely low distortion gain control mechanism that is free from pumping and ducking. The carefully tuned system kicks a fast limiter into play when there is excessive amounts of audible distortion being generated by over-driving the clippers. The result is a system that is audibly transparent for typical overshoot levels generated by digital links. When the system is over-driven the system will do it’s best to reduce audible distortion, while switching off all dynamic limiting and clipping otherwise.
The combination of program dependent limiting and clipping, overshoot control, stereo generation, composite clipping and filtering provides an extremely effective tool to radio stations who may want to feed a network of transmitters over satellite, or maybe just a community radio broadcaster who wants to process audio at the studio and use audio over IP to send their audio to a remote hill top transmitter site.
Oh….. One last thing – the DSPmpX has a built in silence detector that can switch between its analogue or digital (AES / EBU) audio inputs or vice versa. Perfect if your internet or satellite feed fails where you can play audio from back up device - keeping you on the air.
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