Evolving NOS -> MOS
Posted 9th June 2012 at 07:13 AM by abraxalito
Fixing the NOS droop has the undesirable side-effect of making the imaging components worse - HF gain can't be increased up to 20kHz and then suddenly taken away above 22kHz 
In the spirit of taming the near-ultrasonic emissons of a NOS DAC, I'm currently playing with an MOS design - where 'M' stands for 'minimal'. I've been wondering if the attractiveness of the NOS sound will still be preserved if I go to 2X OS in order to fix up the ultrasonics. No practical analog filter can have a sharp enough band edge so a digital one it does have to be...
A DSP implemented digital filter comes at a price - that of throwing away some bits (I'm still using only 16bit DAC chips - TDA1387) so I'm now exploring using a transversal filter (no DSP) to carry out 2X OS. The LTSpice screen grab shows the architecture I'm playing with - a 19 tap delay line feeding 19 separate DACs. The DACs are shown on the right as current sources and their individual weightings are handled by the resistor ladder to their left.
The obtained response agrees with what I saw in MATLAB rather nicely - 19 taps (minimum phase) doesn't give us a great amount of stop-band rejection (almost 23dB!) but its a start at taming the ultrasonics. This also provides an apodizing function as the stop-band does begin at 22.05kHz. The penalty for that is eating into the pass-band somewhat (-1dB @ 17.9kHz).
Further implementation details to follow after I've built the beast...
In the spirit of taming the near-ultrasonic emissons of a NOS DAC, I'm currently playing with an MOS design - where 'M' stands for 'minimal'. I've been wondering if the attractiveness of the NOS sound will still be preserved if I go to 2X OS in order to fix up the ultrasonics. No practical analog filter can have a sharp enough band edge so a digital one it does have to be...
A DSP implemented digital filter comes at a price - that of throwing away some bits (I'm still using only 16bit DAC chips - TDA1387) so I'm now exploring using a transversal filter (no DSP) to carry out 2X OS. The LTSpice screen grab shows the architecture I'm playing with - a 19 tap delay line feeding 19 separate DACs. The DACs are shown on the right as current sources and their individual weightings are handled by the resistor ladder to their left.
The obtained response agrees with what I saw in MATLAB rather nicely - 19 taps (minimum phase) doesn't give us a great amount of stop-band rejection (almost 23dB!) but its a start at taming the ultrasonics. This also provides an apodizing function as the stop-band does begin at 22.05kHz. The penalty for that is eating into the pass-band somewhat (-1dB @ 17.9kHz).
Further implementation details to follow after I've built the beast...
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Comments
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Very interesting, looking forward to hearing about further progress and the build.Posted 9th June 2012 at 10:46 PM by kevinkr
