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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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In Reply to: RE: "More DACs"..... posted by 13th Duke of Wymbourne on July 14, 2015 at 21:43:24:
I mentioned point one, because some people have argued that the achievable signal to noise ratio of a DAC is limited by Johnson noise. This is not true. If the thermal noise can be made 20 dB below the other sources of noise then it won't have a significant effect on the overall noise floor.
The second method comes from averaging, and if simple methods are used you get a 3 dB gain. You also get a 3 dB gain within a given noise bandwidth by doubling the sampling rate for similar reasons. However, if the sampling rate is increased a much greater gain can be had within a given noise bandwidth by using noise shaping. In addition, if the separate chips are fed different signals it is possible to get more than a 3 dB gain without using noise shaping. (The SABRE chip takes this approach when the number of audio channels is reduced. Halving the number of channels doubles the number of switches available for each channel and this results in a 6 dB gain, not the 3 dB gain one might expect.
One hard limit comes from jitter noise on the master clock. This noise modulates the audio signal and it also modulates the idle audio signal if noise shaping has been used. It may be possible to average out this noise by using separate clocks and get the 3 dB effect, but this will require the clocks to be unsynchronized (random jitter) while still being synchronized (otherwise high frequency response will deteriorate through aperture effect). In the end, the only hard limit is that fixed mathematically by the format. The bit rate of the digital channel provides an upper bound on the possible signal to noise ratio of the analog output of the DAC within a given bandwidth, in accordance with information theory.
Distortion is hard to analyze because it is usually not random. Therefore statistical methods (which gave the 3 dB noise values discussed above) are not properly justified. This is particularly true where the distortion comes from design errors or manufacturing defects (e.g. two DAC chips coming from the same lot may be barely within specification, but their distortion may be highly corellated). Analysis is complex and depends on internal design of the DAC chip and its algorithms and this is likely to be proprietary.
The SABRE chips have two modes of operation, voltage and current. The voltage mode consists of a built in I/V converter and op-amp. This is geared to mid-fi applications of the chip. All high quality applications use the current mode output which requires an external I/V converter. Some DACs such as the PS Audio direct stream DAC takes a single bitstream and uses it to drive high power switches, making it possible to use a transformer to serve as an I/V converter, low pass filter without requiring any output amplifiers. The presumption is that passive components have less distortion than active components. There still will be distortion in this approach. It will come from non-linearities in the sigma-delta modulator. There will also be noise caused by master clock jitter.
If you parallel separate sigma-delta DAC chips that are running off separate modulators, if they are fed with the same digital input it is likely the will generate the same digital noise provided they run a deterministic digital algorithm. This is going to create problems, because performance will be better when the multiple DACs are running with unsynchronized modulators. This may or may not happen. The solution is to add randomness into the input of the modulators from time to time. Whether anyone actually does this or not remains to be seen. This os a variation of Murphy's law of clocks, "If a system requires clocks to be synchronized for good performance, then clocks will not remain synchronized. If a system requires clocks to be unsynchronized for good performance, then clocks will remain synchronized.". We found this out at Digital Equipment in the early 80's while designing Ethernet controllers and chips.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
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Follow Ups
- RE: "More DACs"..... - Tony Lauck 07/15/1508:31:50 07/15/15 (2)
- RE: "More DACs"..... - 13th Duke of Wymbourne 15:43:18 07/15/15 (1)
- RE: "More DACs"..... - Tony Lauck 16:09:17 07/15/15 (0)
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