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DNA Dictator Peak Program Limiter (part 2)

One of the problems with limiting, and compression for that matter, is that you set it up perfectly for the signal you are dealing with, and then something unexpected comes along...


One of the problems with limiting, and compression for that matter, is that you set it up perfectly for the signal you are dealing with, and then something unexpected comes along. Often this is someone kicking a mic stand, or a simple case of popping. Either way the resulting burst of low frequency energy causes momentary severe limiting of the entire signal. I’m sure you know how terrible this sounds. DNA have solved this by providing their Interactive Low Processing, or ILP, feature. When ILP is switched on, signals below 150Hz are channelled through separate VCA and detection circuits. When a sudden low frequency surge comes along it will be squashed independently from the main signal and the result will be a much more natural sound. For further naturalness there is a soft knee switch with two positions so that the knee can have a range of 3dB or 6dB.

In applications such as broadcasting and digital recording, over modulation is an absolute no-no. Over modulate your FM transmitter and the aerial will start radiating frequencies that are not rightfully yours to radiate. You can only do this for so long before the FM police will get you (and thousands of listeners to other stations who are having their enjoyment impaired). Go into the red on your digital recorder and a nasty splat will probably be the result. To prevent either of these occurrences, DNA have included a switchable clipper circuit which ensures that the signal does not exceed the threshold during the attack phase. As I said earlier, the attack time is fixed at 100µs, so there is time for fast transients to get through. Activating the clipper reduces the limiter threshold by 2dB to 3dB (dependent on the release time), so that there is a margin between limiting and clipping so that only significant peaks get clipped. DNA advise that for FM broadcast the clipping threshold should be set to 1dB below the maximum allowable level since the addition of the 19kHz pilot tone can theoretically increase the signal level by this amount.

Although the standard version of the Dictator could be used for broadcast, there is also a specific broadcast version which additionally features pre-emphasis and de-emphasis. In the UK, FM broadcasts and TV sound are pre-emphasised with a time constant of 50µs so that high frequencies are boosted. Of course this has implications for limiting since if a signal is already at the maximum allowable level and then high frequencies are boosted for transmission, there is an obvious risk of over modulation. The answer is to provide the pre-emphasis in the limiter so that this can be taken care of. The broadcast version of the Dictator also includes de-emphasis in case pre-emphasis cannot be switched off in the transmitter. Also, since the 50µs time constant is not a universal standard, the alternative 75µs curve is internally settable.

In use, the DNA Dictator is efficient and offers totally adequate sound quality. The ILP and clipping features are a bonus not normally found on compressor/limiters, and the calibration controls offer set and forget operation. Recommended for your inspection.

Controls and Specifications

  • On: Switches between hardwire bypass and operating mode
  • Input L/R Cal: Switches to internally calibrated level
  • Input level: Adjustable ±12dB
  • Meter in/out: Switches between input and output signal
  • ILP On: Activates Interactive Low Processing circuit
  • Threshold Coarse: Adjustable ±12dB
  • Threshold Fine: Adjustable ±1dB
  • Threshold Cal: Switches to internally calibrated level
  • Clipper On: Activates clipper circuit
  • Soft Knee On: Activates soft knee
  • Soft Knee 3/6dB: Sets range of soft knee
  • Release Time: Adjustable 50ms to 500ms
  • Release Auto: Sets automatic release time
  • Gain Reduction meter: Measures mount of gain reduction 0dB to 10dB
  • Broadcast version only
  • HF Limit: LED indicating attenuation of pre-emphasis signal
  • Pre-emphasis: Activates pre-emphasis circuit (50µs or 75µs)
  • De-emphasis: Activates de-emphasis circuit (50µs or 75µs)
By David Mellor Thursday January 1, 2004