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In Reply to: RE: Why does my 20 year old NON-NOS ladder DAC sound so good? posted by ahendler on June 02, 2014 at 09:33:49
Alan,
The great majority of current production Delta Sigma DACs are not single bit, but rather, utilize relatively low resolution multibit PCM resistor-ladder quantizers. Examples include, Burr-Brown's PCM179x series, Analog Devices' AD1853 and AD1955, Cirrus' CS4398, AKM's AK439x series, Wolfson's WM874x series, and ESS' Sabre series. True single bit DACs featuring noise-shaping, such as utilized for DSD, are the less common devices.
Multibit sigma-delta DACs are a sort of hybrid, combining low resolution, but still PCM, resistor-ladder quantizers with noise-shaping.
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Ken Newton
Follow Ups:
The ESS SABRE DACs running in stereo mode convert digital data to 8 bits at a very high sampling rate, typically 44 MHz or higher. (When run in mono mode they operate at 9 bits.) Allowing for the 1024x sample rate increase and without even considering noise shaping, this gives an effective resolution of these DACs of 18 bits. With noise shaping, the effective resolution is considerably greater measured in the band below 20 kHz, around 23 bits if the chip is used in the best possible circuitry.
The SABRE chips use MOS current switches, all identical, not resistor ladders. There are 256 one bit switches used per channel to achieve the 8 bit resolution in stereo mode, a technique called "thermometer code". Because these are not all exactly identical there is a technique used to randomly rotate these bits and this evens out small variations in individual components, so that one hears random noise (e.g. at -138 dB) rather than music related distortion when the individual switches differ due to manufacturing tolerances. The details of the SABRE chips are described in the ESS technical white paper and the ESS patents.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
I was under the impression that multi bit delta sigma dacs take the pcm signal and convert it to single bit and then do the noise shaping from there and only a true ladder dac keeps the signal as a pcm signal. Is this true or not or is it more complicated then that
Alan
Multibit sigma-delta-modulation (SDM) DACs take a higher resolution PCM signal and re-quantize it to a lower resolution PCM signal. This lower quantizer resolution (producing higher quantization noise) of multibit SDM DACs is more than compensated for by noise-shaping processing. Such noise-shaping is essentially the same as for 1-bit SDM, except that the quantizer resolution is now as low as possible. Which therefore requires 1-bit SDM DACs to utilize the strongest degree of noise-shaping to compensate. Strong noise-shaping processing is the key innovation of SDM. My recollection is, that the first SDM DACs were single-bit resolution, and that the multibit versions were later developed in recognition of the performance compromises of single-bit SDM.What both multibit and single-bit SDM DACs share is the spectral relocation of their quantization noise to outside the desired audio band. Full resolution PCM DACs don't typically need to do this. The relocated energy is often termed, out-of-band-noise. Since full resolution PCM DACs natively feature lower quantization noise, they don't require noise-shaping. So, you may be wondering, if the first thing SDM DACs do is to reduce the available resolution of an PCM audio signal, why are they so common today? I think the primary reason is because it's very difficult (meaning, expensive) to make a low distortion, yet full native resolution, PCM quantizer. SDM processing enables the utilization of an inexpensive, low distortion quantizer while delivering the low quantization noise of an expensive, full resolution quantizer.
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Ken Newton
Edits: 06/02/14 06/02/14 06/02/14 06/03/14 06/03/14 06/03/14 06/03/14 06/03/14
From what I have heard, the sonic consequences of doing all of this complicated math to move the noise "out of band" has some audible consequences that people seem to like to sweep under the rug. For me, it is often audible when the DAC is not a ladder type...regardless of the analog output stage.
Could be that your amplifier is non-linear in the presence of high frequency noise. This can convert inaudible artifacts into audible artifacts.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
unlikely as I have heard this when listening through many different types of amps...I seriously doubt that they all had issues...of course I could also win the lottery...if I actually played that is.
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