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Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it.

RE: Looking for serious reading material-Hardware

These chips output both analog (sine and cosine) waveforms and digital clock pulses. The analog waveforms are used for software controlled radios, while the digital output is the one that we are concerned with for digital audio applications.

Looking at the spikes, I would think the true harmonics wouldn't matter after conversion to digital since they will be synchronized with the fundamental frequency and hence would affect the transition times of all transitions equally. This statement does not apply to the bogus 4th harmonic in the plot. It looks like some kind of aliasing. This could be bad because this can cause periodic jitter and the pattern of this could vary as a function of the synthesized jitter. This could cause people to hear different sound as a function of transport clock frequency.

It seems like digital synthesizers can be made to work very well if the synthesizer DAC uses analog circuitry (including its own DAC) that works sufficiently better than the audio DAC that is actually clocking the audio. This seems like a serious fail to me, not only in cost, but if one can build a better DAC one should just use it for audio in the first place. If the two DACs are equal in performance then it appears like there will be more jitter noise in the audio output compared to just using a fixed reference clock and ditching all the DDS machinery. Here the problem is the noise floor in the analog signal that results in the digital clock. Any noise here appears as jitter in the output audio when the audio DAC uses the jittery clock.

Having built some software generated audio clocks (square waves) I am aware of the various artifacts these devices can produce. The ones that I built suffered from different tone quality due to varying aliases with different synthesized frequencies. This was a hard fail for me, as I was interested in human musical pitch perception at the time, i.e. how different tuning systems for keyboard instruments sound. I was generating square waves, as this was all I could do at the time (early 80's) using software in an Apple II. Eventually, I coded up a generator that produced synthesized square waves at a sampling rate of 1 MHz. At this point the aliases were sufficiently low level that performance was adequate for my purposes. (If you are wondering how one can generate samples and play them at the computer's clock rate when a single instruction uses several clock cycles, that's a different story.)

Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar



Edits: 03/19/14

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