This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
Audio data is stored on CD as 16-bit words. It is the job of the
digital to analogue converter (DAC) to convert these numbers to a
varying voltage. Many DAC chips do this by storing electric charge in
capacitors (like water in buckets) and selectively emptying these
buckets to the analogue ouput, thereby adding their contents. Others
sum the outputs of current or voltage sources, but the operating
principles are otherwise similar.
A multi-bit converter has sixteen buckets corresponding to the sixteen
bits of the input word, and sized 1, 2, 4, 8 ... 32768 charge units.
Each word (ie sample) decoded from the disc is passed directly to the
DAC, and those buckets corresponding to 1's in the input word are
emptied to the output.
To perform well the bucket sizes have to be accurate to within +/-
half a charge unit; for the larger buckets this represents a tolerance
tighter than 0.01%, which is difficult. Furthermore the image
spectrum from 24kHz to 64kHz must be filtered out, requiring a
complicated, expensive filter.
Alternatively, by using some digital signal processing, the stream of
16-bit words at 44.1kHz can be transformed to a stream of shorter
words at a higher rate. The two data streams represent the same signal
in the audio band, but the new data stream has a lot of extra noise in
it resulting from the wordlength reduction. This extra noise is made
to appear mostly above 20kHz through the use of noise-shaping, and the
oversampling ensures that the first image spectrum occurs at a much
higher frequency than in the multi-bit case.
This new data stream is now converted to an analogue voltage by a DAC
of short word length; subsequently, most of the noise above 20kHz can
be filtered out by a simple analogue filter without affecting the
Typical configurations use 1-bit words at 11.3MHz (256 times over-
sampled), and 4-bit words at 2.8MHz (64 times oversampled). The
former requires one bucket of arbitrary size (very simple); it is the
basis of the Philips Bitstream range of converters. The latter
requires four buckets of sizes 1, 2, 4 and 8 charge units, but the
tolerance on these is relaxed to about 5%.
MASH and other PWM systems are similar to Bitstream, but they vary the
pulse width at the output of the digital signal processor. This can be
likened to using a single bucket but with the provision to part fill
it. For example, MASH allows the bucket to be filled to eleven
different depths (this is where they get 3.5 bits from, as 2^(3.5) is
Lastly it is important to note that these are all simply different
ways of performing the same function. It is easy to make a lousy CD
player based around any of these technologies; it is rather more
difficult to make an excellent one, regardless of the DAC technology
employed. Each of the conversion methods has its advantages and
disadvantages, and as ever it is the job of the engineer to balance a
multitude of parameters to design a product that represents value for
money to the consumer. [Chris]