do DACs have LPF? do they affect the phase? - diyAudio
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Old 11th January 2013, 10:33 PM   #1
tsiros is offline tsiros  Greece
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Default do DACs have LPF? do they affect the phase?

the title says it all, really...

if my question doesn't make sense, please tell me.

yes, i know, there is not much sense having an LPF at >20K since the human ear can't hear it, nor is there significant musical content that high up, but this questions is niggling me :/
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Old 12th January 2013, 04:34 AM   #2
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Typically the transfer function is specified in the datasheet along with the group delay, from which it's easy to infer whether the antialiasing filter was synthesized as minimum, intermediate, or linear phase. Some vendors also present typical impulse responses, often in the corresponding eval board datasheet, which provide more insight into the phase.

One or two research papers have found statistically significant subjective preference for playback with infrasonic. It's been a while since I looked at them but, as I recall, they didn't control for the narrowing of the DAC's own impulse response so it was unclear if the benefit was actually ultrasonic. Don't have the links but you should find them with a search of Lynn Olsen's posts here on DIYA or of his website. You may also find Wolfson's paper and ESS's whitepaper on the topic of interest, as well as some of the discussion in the rePhase thread over in the multi-way forum. And possibly some of abraxalito's discussions of NOS DAC sync compensation in this forum too.

The primary purpose of on DAC antialiasing is to provide a high order digital filter such that the stopband between the baseband and first alias is wide enough that it's easy to implement an analog filter which rejects the first (and subsequent aliases). The value of alias rejection depends on the rest of the signal chain and I'm not aware of any rigorous subjective studies on the topic, but the motivation for antialiasing is to offload the down chain amplifiers' GBP and slew rate (op amps, power amps, whatever) so their excess loop gain is more fully brought to bear on the audible bandwidth. In modern DACs it's common for the slow roll filters to emit around half of the first alias, which is pretty well fine given parts with decent GBP and reasonable choices of analog lowpass filters. Bruno Putzey's paper on feedback is a good starting point for GBP requirements and DAC eval boards a good starting point for the filters.

OT: Which eight string is that in your avatar?
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Old 12th January 2013, 02:36 PM   #3
tsiros is offline tsiros  Greece
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agile interceptor pro 828

28" scale (aka baritone) neck through with EMG actives

my dad has a lenco turntable and i think of improving it and getting a preamp and a respectable needle for it.

i want to see if there is any improvement of stereo image between cd and vinyl.

another hurdle is how to order a calibrating vinyl disc :/ would you trust it to be shipped to you?
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Old 12th January 2013, 05:42 PM   #4
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Nice. Tuned B to B or something else?

Can't help with the vinyl; I've given up on spinny things and my sources are now USB stick or SD card.
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Old 12th January 2013, 06:04 PM   #5
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tsiros, it is typical, almost manditory, for vinyl to be cut with bass frequencies (typically below 300-200hz) in mono rather than stereo. Dance music will likely be all mono below 300hz while classical might hold out to 200hz, but if there is any significant bass content it is almost always (99.99999% of the time) cut in mono. The cutting head and cartridge just can't handle the required excursions were it in stereo.

So your vinyl will always be limited stereo, a good set-up might achieve 25-30db separation at very best.
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Old 12th January 2013, 06:06 PM   #6
tsiros is offline tsiros  Greece
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high to low e b g d a e b g#, the g# might be anything... e, f, f#, g, g#...
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