Automatic Offset Removal Compressor Modes

Using the 20/20 AD	Lexicon

Automatic Offset Removal

Compressor Modes

Dynamic Range

Compression

DC offsets are created by input amplifiers and A/D converters for a number of reasons. In the 20/20 AD, these typically include temperature changes, changes between 2-channel and 4-channel mode, change of sample rate or sync source, or large adjustments of input level controls. The DSP chips in the 20/20 AD automatically remove these DC offsets by passing data from the converters through a digital high pass filter with a corner frequency of less than 0.1 Hz.

This high pass filter will reduce output offsets to less than -80dB within one minute of a change in input offsets. In order to minimize output offsets, wait at least one minute after any of the above changes before recording.

DC offsets created outside the 20/20 AD are removed by blocking capacitors located between the analog input connectors and the first amplifier stage. These capacitors have a corner frequency of less than 1 Hz, which guarantees less than 0.1dB of loss at 10 Hz (referenced to 1 kHz).

It is possible to extend the low frequency response down to less than 1 Hz by shorting these blocking capacitors. This should only be done if the offset of the input signal is relatively constant, and less than 1% of full scale (approximately 150 mV with a 22 dBu input). Transformer-isolated sources are generally safe, but non-isolated sources should be checked with a voltmeter. Offsets greater than 1% may take several minutes for the digital high pass filter to remove.

To short out the blocking capacitors, remove C5, C6, C30, C31, C51, C52, C80 and C81. These are upright tubular capacitors located near the analog input connectors. Solder short lengths of wire in place of each. Alternatively, you can leave the capacitors installed, and solder short lengths of wire across the pins of each on the solder side of the board.

Until you become comfortable with the operation of the rest of the 20/20 AD, you should keep the compressor turned off. Make sure that the DISPLAY, LINK and ON buttons in the compressor block are all off (unlighted) and the compressor gain knob is set to 0dB.

Although the function of the digital compressor is similar to that of a standard analog compressor, its intended uses are somewhat different. Three of the more common uses (Dynamic Range Compression, Non-Linear Transfer Functions and Overload Protection) are described below.

This mode is primarily useful when recording program material with a wide dynamic range onto 16-bit media in 2-channel mode. (Although the compressor will work in 4-channel mode, the dynamic range of 4-channel mode is so close to the 16-bit level that any further compression will only raise the noise floor.) This use of the compressor requires a bit of artistry, because all settings must be optimized to the program material.

With the compressor on, set compressor GAIN to 6dB. This will increase the gain of low level signals (below the THRESHOLD setting) by 6dB, effectively squeezing the dynamic range of the converter in 2-channel mode from 110dB to 104dB. The DITHER switch automatically adds just enough dither to squeeze the remaining dynamic range into the 96dB dynamic range of 16-bit media.