Lexicon PCM 80 manual Headroom Display, Unbalanced Balanced, Overload

Product Overview

The PCM 80, with both analog and digital input and output connections, requires Setting Audio Levels some attention to proper setting of signal level.

Analog inputs are first gain-conditioned by the rear panel input gain switch, and then by the front panel INPUT knob. Proper setting of both the switch and knob are important for best performance of the A/D converter. Audio data from the A/ D converter is level adjusted by the Analog Lvl parameter before reaching the effects processors. Digital inputs are also level adjusted before reaching the effects processors via the Digital Lvl parameter.

Analog and digital sources are mixed at the input to the effects processors. For example, setting both Analog Lvl and Digital Lvl to 50% will mix the analog and digital input signals equally and send them to the effects. Creating a mix which exceeds 100% can cause overload.

Proper setting of Input level on the PCM 80 is dependent on:

•Proper signal level into the analog front end to avoid signals causing overload at the DSP input

•Proper adjustment of the signal level into the analog-to-digital converter to optimize noise and avoid overload

•Proper setting of signal level into the digital signal processor to optimize noise.

Headroom Display

The headroom display provides both headroom and overload information from a variety of measurement points. The meters display the sum of both the analog and the digital input data. Examining either the analog or the digital level alone requires that the Level parameter of the subject data stream be set to 100%, while the Level parameter of the other is set to 0%.

The chart below illustrates the adjustment range that will set input levels for both balanced and unbalanced operation.When a choice can be made, it is best to operate at the higher amplitude end of the recommended range to optimize noise performance.

overload:

acceptable:

too low (noisy):

Unbalanced	Balanced
> 0dBu	>+20dBu
0dBu to -22dBu	+20dBu to -2dBu
<-22dBu	<-2dBu

Overload

The 0db (overload) indicators will light under the following conditions:

•A/D overload

•overload at any point in effects processing

For example, internal peaking of high Q filters, or level buildup from certain reverberation modes can result in overload, even when the input A/D or digital receiver data stream is not at full scale. Such conditions are most often caused by a combination of extreme parameter settings. Adjusting parameter/level settings can eliminate these overload conditions.

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