Digital Drive

RE: Truncation of LSB's

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This Post Has Been Edited by the Author

In Reply to: RE: Truncation of LSB's posted by sideliner on June 03, 2017 at 10:24:52

Your seemingly simple questions have some complex answers. There are two well known ways to feed a data stream featuring a given bit-depth to a D/A unit featuring a lesser native bit-depth. The first way is to dither the imcoming data to the desired bit-depth rather than simply truncate it. This will remove the distortion of truncation at the price of increasing the channel noise floor.

The other way is via sigma-delta-modulation(SDM), which can deliver the full dynamic range of the original data, but only within the audio band. This is what most commercial D/A chips do, even when fed 16-bits. These units typically feature only a 5-bit or 6-bit quantizer core, and utilize SDM to deliver the full resolution (SNR) of the original 24-bit data, within practical circuit limits of course.

As far as various hybrid converter topologies, most of these have been tried. The currently in production PCM1794A DAC chip is based on a hybrid converter core, while also utilizing SDM. The out of production PCM1704 R-2R DAC chip was actually a hybrid of two internal 23-bit R-2R cores operated together to function as a single 24-bit core. An architecture Burr-Brown refered to as 'co-linear' in the data sheet. As a rule of thumb with the complex merging of multiple converters, each extra bit of resolution requires a doubling of converter circuitry. Simply paralleling converter outputs does not increase bit resolution. It only reduces random noise by 3dB for every doubling of the number of converters.

High resolution discrete R-2R converters that reliably retain high accuracy are non-trivial to design and consistently manufacture. Such converters are becoming in vogue among DAC vendors seeking to differentiate themselves in a crowded market. However, none which I'm aware objectively match the consistent accuracy of today's top multi-bit SDM converter chips. Subjective assessments may and do differ, of course.
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Ken Newton

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