Data and signal: are we progressing?

Apparently, an old “bits is bits” issue is repeating from time to time, so sometimes it looks like we did not move at all. So let me try to rephrase this problem, supposedly from the year 2013 point of view.

The term “data” denotes ones and zeroes i.e. bits. Normally, we want a reliable system, one that can preserve data as such, so it remains unaltered when it is used or moved. Fortunately, as long as we talk about data rates required for audio, today’s technique easily preserves data integrity, and in fact, even the early CD equipment had quite reliable data coding / reading / correction process, so even early CD system was already data perfect (“bit perfect”).

So usually bits IS bits – all zeroes are equal, and all ones are equal too. However, it is like that only until data remains in the digital domain. The harder part comes from our inability to hear this digital data directly – once we have to do something to convert it into the analog domain. And this is where the basic data integrity requirements become insufficient.

Namely, once you start operating with digital data inside the hardware, data will be represented by some waveform: zeroes will be “low” state (0 Volts), and ones will be “high” state (usually equal to the digital supply voltage). And waveforms usually suffer from different kinds of inaccuracies and artifacts, such as noise, overshoot, ringing, finite slope, ground bounce, crosstalk, jitter, and alike, all being caused by hardware imperfections. It is normally considered that all these imperfections can not violate the data integrity so long as the error does not violate “low” and “high” state threshold requirements in the voltage domain, or exceed one half of the signal interval in the time domain.

But as said, that is the easy, data integrity part. The harder part is that, since our converters operate in real time, these inaccuracies in the digital domain can, and will break through to the converter’s analog output.

For this purpose, you can think about the converter as the black box with two sorts of inputs:
(1) digital data (zeroes and ones) input, and
(2) triggering signals (clocks) inputs, which are necessary because, as said, the conversion is real time event.

In other words, once the D/A conversion takes its part, you don’t anymore have 16 or 24-bit data written in the disc, and protocol that “knows” which 16 or 24 bits are one word, but you have a real time stream of bits, with other time signals (clocks) that tell converter the moments the single bits (zeroes and ones) arrive, the moments the word starts or finishes, and consequently the moment the output audio analog signal has to change its level. That is why this last clock signal, the one that triggers the converter’s output, is essentially fully analog – its properties directly translate into the resulting audio output.

Among these “properties”, the jitter or phase noise became recently the most known. Ideally, the triggering clock signal would come in perfectly equal time intervals, otherwise its time uncertainty produces frequency modulation of the audio signal, a.k.a. jitter. As said, in a purely digital system any time uncertainty that stays below one half of the time interval is acceptable, but in the D/A conversion, even the picoseconds of such uncertainty can be audible. Normally, the generation of this clock is a part of the data reading process, and the way this clock is managed in (through) the system hugely determines its sonic qualities. But problems are not at all limited to jitter, because the waveform of the data input signal can also bring a classic noise into the converter, and translate itself into its analog output.

In fact, the whole science lies behind such issues. Many phenomena related to so called mixed signal circuits (digital + analog) became recently well understood and documented, and many problems are seriously taken into account in order to solve them, instead of denying their existence, or their audibility. Thus, today we can differentiate between the good and bad clocking schemes, and we know why and how we should design for low jitter clocks, or low noise or low RFI emission inside the units.

For us, the sonic qualities are always what ultimately matters, but this also means that these issues are not only audible but also measurable. Even more, some recent findings in this domain opened a whole new path to understanding the relationships between subjective perception and particular forms of distortion and noise.

Finally, a notable part of the improvements achieved in digital audio in the last two decades happened right here, in the area of designing for higher signal integrity criteria. So not that much for instance in the area of converter chips, which, during this time, were developed mostly for lower production costs. As a result, even if Philips D/A chips from the ’80s are in many areas still unsurpassed, today it would be unimaginable for any serious design to repeat the layout or clocking schemes as they were done in Philips CD players from the ’80s (or in the majority of players from that time, for that matter).

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