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

RE: Some kind questions about reclocking

Hi,

> 1) the one embedded with the input digital signal

Yes. This one MUST be used if the DAC cannot control the information flow rate from the source. This clock can however be filtered to have a similar level of phase-noise as a decent crystal clock. All you need is the right kind of tech.

An example is the TC Audio DICE/DICE2 chip which is found in some Firewire DAC's where it imparts in effect the jitter performance of something close to an asynchronous USB DAC, while using adaptive Clock Firewire or SPDIF.

> 2) one internal

In this case the transport (CD-Driver, Computer etc.) must be slaved to the clock from the DAC. In USB this is taken care of by using the asynchronous protocol, for SPDIF a clock link of some description back to the transport is needed.

> 3) one taken from an external clock generator

This is mostly limited to pro-audio, generally as word clock or AES3id Sync. Usually external clocks offer little advantage over the internal clock generators (actually, they often make things worse). Much depends on the detailed design of the DAC.

> If i have a dac with a internal quartz and no way to select
> the clock source i guess the dac always reclock the incoming
> digital signal, if not what would be the reason of the presence
> of a quartz inside ?

The DAC may use a microprocessor to control a number of functions in the DAC without being at all related to the audio clock. Some receiver chips can use a crystal clock to detect the incoming sample rate.

Other DAC's use Sample Rate converters that convert from all external sample rates to an internal one. Generally using sample rate converters will embed at least some of the source jitter in the output signal. Some recent DAC Chip's have sample rate conversion build in and hardwired into the audio path.

> So in the end what should really matter is the quality of the
> dac's internal clock and not the quality of the incoming spdif
> signal.

No, this is generally not the case. There are many ways to design things. But as in principle you cannot directly replace the SPDIF source clock with a fixed rate crystal clock.

If the local crystal clock is for arguments sake 50ppm different from the source, you will get either 50 dropped samples for every million source samples or 50 repeated samples. This repeat/drop is generally audible as a more or less faint click or tick.

> It is very important for me to understand this because it will
> free me to chose even a very cheap source and focus all the
> attention and allocate most of the budget to get a good dac.

A few DAC's indeed incorporate such internal clocking, but they are extremely rare. I can show you what such a system implemented well is capable of though.



All traces show a 24-Bit/48kHz J-test signal from an AP2 testset via coaxial SPDIF, with the AP2 set up to add 50,000pS (50nS) of jitter as 400Hz squarewave.

For the red/green traces the "reclocking" system (in effect a digital Frequency Locked Loop - FFL - wrapped around a memory buffer) was switched off and the DAC was was allowed to behave like most current DAC's using generic receiver IC's (AKM, CS, BB/TI,Toshiba etc). As one can see, the level of distortion is gross.

For the yellow/cyan traces everything is the same, except the FFL system was enabled. As we can see, 50,000pS of jitter vanish as if they had never existed.

So,the proof is in the pudding. It can be done, it just is not done very often.

Thor

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?


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