Lexicon
18
Using the 20/20 AD
Automatic Offset
Removal
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.
Compressor Modes
Dynamic Range
Compression
