Just a note on how upsampling and oversampling differ:
Upsampling involves increasing the sample rate of a digital signal from its base rate (eg 44.1KHz for Red Book CDs) by any selected factor which need not be an integer value.
Oversampling is a limited subset of upsampling where the factor values are limited to integer values.
So, upsampling can be done using non-integer factors such as 1.3333 or 2.5 etc. while oversampling uses factors that are integers such as 4x or 8x.
Using the Red Book standard for CDs, an upsampling at 1.3333x will result in a sample rate of 44.1 x 1.3333 or 58.8 KHz, and oversampling at 4x will result in a sample rate of 44.1 x 4 or 176.4 KHz.
In a Non-Oversampling (NOS) DAC, the digital signal's sample rate is not raised and is fed to the D/A converter which converts to an analogue signal, but the D/A conversion process introduces "aliases" (at multiples of sample rate) which, for optimal SQ, need to be filtered out via the use of anti-aliasing filters which, have to be implemented in the analogue stage (as the signal is already an analogue signal after D/A conversion. These analogue anti-aliasing filters are complex circuits and are difficult to implement.
This difficulty is introduced by the need to provide three filter types that all need to work together:
a) A pass filter for signals with frequencies below 20 KHz (the audio signal)
b) A stop filter for signals with frequencies above 24 KHz (any aliases)
c) A transition filter covering the 20KHz to 24KHz "gap" that transitions from for the 20KHz requirement to "pass" to the 24KHz requirement to "stop" (or in gap terms needs to attenuate at a steep slope of "pass" (zero attenuation) at 20 KHz to "stop" (infinite attenuation) at 24KHz - a small frequency band of just 4KHz
This transition filter is where the complexity arises and the steep slope has resulted in the use of the term "brick wall filter".
The concepts of upsampling (non-integer sample rate multiple) and oversampling (integer sample rate multiple) were introduced to widen the gap between the pass and stop filters in order to make the slope of the transition filter much less steep (and, thereby, less complex to design and build).
The two basic options (if we exclude upsampling) are, therefore, oversampling (or OS) DACs and non-oversampling (NOS) DACs.
Decent NOS DACs with effective analogue anti-aliasing filters tend to be costly due to the complexity and component costs associated with such filters.
OS DACs can be built at a lower cost as less-steep filters can be built for less.
NOTE: I've attempted to simplify the above and, as a result, may have strayed into over-simplification (as opposed to up-simplification - sorry, couldn't resist...
) - so if anyone needs to clarify or expand on this, please feel free.
Dave