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Techtalk: Audio clocks
In our articles about I2S and external DACs, we already talked a bit about clocks. In the High End audio community there’s a lot of discussion about this topic and you might think that’s the most important thing to look at. In our opinion it’s important, but the effect should not be overestimated.
What is this clock for?
When recording or playing digital audio, there isn’t a continuous signal, but the signal is sampled more than 40000 times per second. The clock is responsible that this happens at exactly the same intervals. As always, not clock is perfect. Therefore, these intervals will sometimes be slightly shorter or longer. It’s like a mechanical clock that sometimes is a bit faster or slower. This is called jitter.
Source: Wikipedia (https://commons.wikimedia.org/wiki/File:Jitter_timing.svg)
Jitter is bad?
In general: Yes. If the jitter is too large, the sound might not “feel” correct. It’s hard to describe. However, you will also find listening tests that didn’t show differences with a small amount of jitter. Therefore, it’s hard to say how much jitter is really a problem.
Jitter on the Pi
When the Raspberry Pi was designed, it wasn’t designed with audio as the main application. There is one quartz crystal on the Pi that is used to derive lots of different clock frequencies for different subsystems on the Pi. This can work very well, but the one on the Pi isn’t the most flexible. Therefore for some outputs it uses padding which results in a relatively high amount of jitter.
The real jitter
Now comes the big question: How bad is it? Just looking at the jitter measurements: Terrible. However, no modern DAC will simply use this clock. Usually they are using a circuit called phase-locked loop. This basically tries to output a fixed frequency even if there is some jitter on the input (it’s a bit more complex, if you really want to understand this, have a look at the Wikipedia article). Often, this is integrated directly into the DAC chip. This means, you can’t even measure how good this is working. Even with a huge amount of jitter on the outside, the DAC might still perform very well as the jitter is greatly reduced inside the DAC.
Clocks on HiFiBerry boards
All our “Pro” and “HD” boards use their own clocks. But not just that, there’s always an additional PLL used (mostly inside the DAC or SPDIF chip). Some boards use a single clock, others use 2 clocks.
You might think it’s better to have 2 clocks than one. But it’s again more complicated. It depends a lot on the PLL that’s uses. Some PLLs can work very well with a single external clock because they can be configured very fine-grained. Other PLLs don’t have this feature, therefore 2 clocks might just work better.
High-End clocks
You might see “high end” audio clock circuits around that are not just expensive, but should provide a “cleaner” clock and therefore sound better. In our experience the impact of the clock circuit is much smaller than many people think and this isn’t worth the money. As we have shown in “the real jitter”: The jitter on the DAC isn’t the one that you measure on the output of a clock generator. Therefore we don’t use this, but spend the money on other improvements that really improve sound.
Sound
Does a better clock always sound better? “Sound” is always a matter of personal preferences.
Let’s just take an example where we use the same DAC with and without an external clock: The DAC+ Standard and DAC+ Pro – the main difference is the clock design. While the DAC+ Standard uses the Raspberry Pi’s clock, the DAC2 Pro uses it’s own dual clocks. Most people who listened to both prefer the sound of the DAC2 Pro. However, we know of a few users that think the DAC2 Pro sounds better with the clocks on board disabled (which means the board acts as a DAC+ Standard). Do these guys have any problems with their ears? Not at all. While minimal differences are hard to measure, some people might hear them and like or dislike these. It’s not just about better performance, it can be extremely small nuances that are hard to measure at all. What do you prefer? We only can say, that most people that listen to different boards prefer the more expensive boards with on-board clocks. But don’t expect extreme differences. We’re talking about very small nuances here!
March 26, 2021
