brbrock
Electrical engineers, forgive me if I misspeak anywhere below
I cannot comment on the DACs in mitch2's comparison, but I can comment on the benefits of a DDC with most (not all) streamers or CD transports.....and specifically the Denafrips Pontus II/Iris combination, but that applies to most other DACs. Sorry for the detailed information, but I appreciated the education when I first got it.
(1) Crystal Oscillator - The basic clock in the Pontus II is a standard (though very high quality) FEMTO clock. Crystal oscillators (clocks) and their performance are affected by temperature variation, thus the need to keep the clocks at as stable a temperature as possible.....but that, and the implementation of, can get expensive. The better the clock, the better the stability and the sound quality, the higher the price.
Most low to mid quality DACs have a FEMTO clock that has no compensation for temperature, thus minor fluctuations in it's performance.
The next step up is a TCXO (temperature compensated crystal oscillator), where the temperature is controlled by the use of thermistors and insulation of the crystal oscillator.
The next step up from there is an OCXO (oven controlled crystal oscillator), which uses an internally adjustable "mini oven" to keep the crystal at optimum operating temperature.
As I mentioned, the clock in the Pontus II is a high quality FEMTO clock with no temperature compensation, The crystal oscillator in the Iris is the higher quality TCXO temperature compensated crystal oscillator. The next step up Hermes and the top of the line Gaia use an OCXO oven controlled crystal oscillator.
(2) Galvanic isolation - This isolates signals by using magnetic components. All Denafrips DDCs use a galvanic opto coupler, optically and galvanically isolating the signal
(3) Optical isolation - This isolates the signal by converting to an optical transmission using lasers, thus eliminating electrical contamination......usually with an opto-coupler. All Denafrips DDCs use a galvanic opto-coupler, optically and galvanically isolating the signal
(4) USB - USB transmission does not contain a clock signal, it is assigned by the receiving device. USB directly into a DAC has the DAC assigning the clock, USB into a DDC has the DDC assigning the clock which would then be passed to a DAC. High end streamers do a very good job of transferring USB data, so less phase shift and jitter; and some with very high end streamers have reported little/no benefit from a DDC.
(5) AES/EBU and SPDIF - These signals do include a clock signal that is generated by the sending device, however the clock is imbedded with the data signal and must be separated by the DAC/DDC. Higher end DACs/DDCs will correct or even regenerate the clock signal, and some have the ability to turn that feature off if the incoming signal is superior
(6) I2S - This is the protocol that DACs (and other digital devices) use internally to process data, so any USB, AES/EBU, SPDIF signal must be converted to I2S in the DAC/DDC. The great thing about the DDC is that it handles this conversion, and then can (if the connection is available) transmit that same I2S data to a DAC. The I2S signal has a separate pinout to transmit the clock, so it does not have to be parsed by the DAC; the DAC just converts the digital signal to analog with the already converted I2S signal using the separate signal for the clock.
I suspect this is partially why so many people report that the I2S connection sounds best versus other inputs. Some external devices (streamers or CD transports) can natively output I2S, so a DDC might not be needed. Jay's Audio and PSAudio CD transports have I2S output, and PSAudio/HiFi Rose/Silent Angel/Matrix/Pi2/Holo Audio all have I2S outputs.
