Johnny B Good wrote:


Philips realised that they could use a 4 times oversampling technique
using cheaper yet higher precision 14 bit DACs to achieve exactly the
same dynamic range performance of a conventional 16 bit DAC using a times
one sampling rate.

** Philips/Marantz used a pair of 14bit, TDA1540 DACs in a few early CD players - see:

http://www.lampizator.eu/lampizator/...tda%201540.pdf

To make one of the above convert all the bits on a CD, each 16bit sample was replaced with 4 sequential 14 bit samples with values modulated by a digital filter algorithm. Averaging the result recreated any of the otherwise 4 missing values that a 16bit DCA would normally output.

The same algorithm also included a steep LP filter which had the interesting effect of producing pre and post ringing on each step transition.

It worked well enough and some of the noise was pushed out of the audio band - but the specified THD was rather higher than with good 16 bit DACs as used in the Sony machines of the same era.


Not only where they able to solve the 'monotonicity' issue at a stroke,
the oversampling technique also introduced two additional benefits. The
first being that the inevitable digital hash and aliasing products were
all pushed into the 88KHz part of the spectrum, well clear of the
problematic 22KHz region that had mandated the use of 'brickwall'
analogue filtering required of the primitive methods using expensive 16
bit DACs. The second benefit being that cheaper, less ripply analogue
filtering could be employed to protect the following analogue stages (and
the listener) from both digital artefacts and unwanted aliasing products.

** In fact, CD players using TDA1540 DACs usually had high levels of supersonic hash at the output - viewed on a scope, this amounted to 20 or 30mV rms whenever a CD was playing. This hash defeated most reviewers and others attempts to verify the s/n ratio and THD figures claimed by Philips.

Many early players like the Sony CDP101 included ceramic filter assemblies with 100dB/oct roll off slopes that removed all signs of such hash.



The remaining issue with DACs was the analogue output stage clipping
that afflicted some of the earlier products due to inadequate voltage
rail provisioning derived from the "Join the dots" peak amplitude
calculations by some rather naive designers who didn't fully understand
the process of handling a bandwidth limited analogue signal encoded into
the digital domain.

** Not real sure what you are on about here, but the maximum signal level possible from a CD player is 2Vrms or 2.83V peak. Given that most players have dual 12 or 15 volt supplies for the op-amps, there is no such issue.



.... Phil