It is important not confuse sources of noise. Even though the modern world is training each one of us to think "digitally" the transport medium for the voltage levels representing the 0 and 1 states is, without a doubt analogue, where, as a simple example, the 0 state is represented by 0volts and the 1 state is represented by 5volts. Although the bit stream is said to be "digital" it is essentially an analogue electrical signal with two set levels, rather than the infinite number of levels in the analogue output from an amplifier.
The time required to change from one state to the other is referred to as the rise time and may be influenced by the electrical environment, such as the cables used. A good digital design engineer must also be a good analogue design engineer to understand how the various parts of a digital system can influence the shape and timing of a "digital" pulse, such as the rise time and ringing. Ringing will be one component of "jitter". Expensive external reclockers and master clocks are recommended to minimise jitter, as well as linear power supplies; SMPS devices are electrically "noisy" and can inject noise into the system, whether on the power line or by EM interference. Carefully designed filters are included in the better DAC circuits in an attempt to remove all forms of unwanted noise (for example
https://www.project-audio.com/en/product/pre-box-s2-digital/).
A good illustration of a "real world" digital pulse is provided by mitchco in the posting at
https://www.audiosciencereview.com/...link-by-stack-audio-and-signal-detoxing.7578/. Signal detoxing is really another way of talking about removing superfluous noise from the "digital" signal. The USB output port of any computer will effectively inject noise and timing uncertainty (aka jitter) into the output stream to a USB DAC. Timing. or clocking, and pulse shape is what makes or breaks a digital system, loose the precision synchronisation between the clock signal and the digital data stream has a real time impact on the quality of output from the DAC and the sound quality of the music system. Jitter can be thought of as a measure of uncertainty of where the digital signal is in the time domain and the actual voltage level of the data signal when the system clock signal is active, compared to where it should be (as in the original source). In simple terms any pick-up that will affect an analogue signal will also affect a digital signal, but in a different way.
All of the devices listed previously are used to control the music files streamed to a USB DAC and some, through good audio / analogue design and other strategies, are able to "detox" the signal and improve the synchronisation of the system clock with the digital data stream, thereby reducing jitter.
The SMSL SU-8 and various Topping DAC designs have balanced outputs which should remove much of the noise between the DAC and the amplifier, but still leaves the digital stream to the DAC input to be considered. It also requires an amplifier with balanced inputs.
All of my digital music files are held on external USB hard drives and the current DAC has a USB input, which explains why I would like to add a control device that at the same time will detox the data stream. Perhaps part of this explains why contributors have posted something along the lines of "yeah, but its not all about the DAC chip" ! This is not about the noise measured within a DAC under test but about reducing the noise and signal timing inaccuracy in the data stream input to the DAC.
