Unfortunately, even if sourcing equipment provides a reference clock with low jitter, cabling always adds unacceptable amounts, especially poor quality or high-capacitance cable, which results directly in sampling jitter in the analogue interface if jitter-filtering is inadequate. But this is expensive and, in any case, the 'pull-range' of an ordinary quartz crystal oscillator is still generally insufficient to meet the tolerance demands of the digital audio interfacing standards.Īs well as a very stable clock oscillator, a good sounding converter must have a PLL (phase-locked loop) with a loop-filter which steeply attenuates incoming reference jitter towards higher frequencies. On the other hand, if we limit the range of rates at which the oscillator needs to operate to small ‘islands’ around the standard sample rates we could use a bank of oscillators, selecting the appropriate oscillator according to our desired sample rate. But such an oscillator would inevitably have poor stability – at least in terms of the stringent requirements for high-quality audio conversion.
Word clock frequency plus#
The externally-clocked design challenge has traditionally been a trade-off since the more stable a clock oscillator is, the less is its range of frequency adjustment: but we would ideally like an oscillator which can operate over a wide range of sample rates, perhaps from 48kHz, plus multiples thereof. The real problem is that in many installations the analogue interfaces can almost never operate from their own internal clocks since they must be slaved to an external reference sync, or maybe to a clock from a host computer. Most analogue interfaces can provide workmanlike performance when internally clocked, since this is only a matter of providing a stable clock oscillator (or range of oscillators) at a fixed frequency (or frequencies) – although even this is not always well-executed. But this is a very tall order for circuit designers, especially if they are on a budget. would exhibit no jitter (frequency variations) at the point of conversion, whether operating from an internal clock or from an external synchronization reference of any format and at any sample rate. The ideal clock system in an A/D or D/A converter would be ultimately stable, i.e.
![word clock frequency word clock frequency](https://www.thanksbuyer.com/image/cache/data/202008/64192/1597828837-5-750x750.jpg)
Why is good clock stability so rare? Probably because most conversion equipment has to compromise between clock stability, operational requirements and cost. With the linearity of modern A/D and D/A converter chips beginning to rival and exceed the performance of the best analogue circuits, digital recordings would already be ‘beyond reproach’ if clock stability did not so often degrade their potential quality.
![word clock frequency word clock frequency](https://www.thanksbuyer.com/image/cache/data/202002/63875/1582431151-2-750x750.jpg)
![word clock frequency word clock frequency](https://images.twinkl.co.uk/tw1n/image/private/t_630_eco/image_repo/11/0b/t-l-4541-100-high-frequency-words-word-ma_ver_4.jpg)
Good clock stability is probably the single most important issue separating good-quality analogue interfaces from the rest.