For many audio applications a sampling clock or word clock of 44.1 kHz or 48.0 kHz is available through a distribution network. The audio data converters in such applications are often Delta-Sigma, modulator-based devices that can over-sample the signal by a factor of up to 512, resulting in a system clock of 22.5792 MHz or 24.5760 MHz. This system clock must be synchronized with the low-frequency sampling clock using a phase-locked loop (PLL), but the sampling clock is often too low in frequency for use with many PLL-based clock drivers. Some audio PLLs can accept the low frequency sampling clock, but create so much jitter on the system clock, that performance degrades.
A word clock or wordclock (sometimes sample clock, which can have a broader meaning)[further explanation needed] is a clock signal used to synchronise other devices, such as digital audio tape machines and compact disc players, which interconnect via digital audio. S/PDIF, AES/EBU, ADAT, and TDIF are some of the formats that use a word clock. Various audio over Ethernet systems use broadcast packets to distribute the word clock. The device which generates the word clock - usually with multiple output connectors - is the only master clock source for all the slave audio devices.
Word clock is so named because it clocks each sample. Samples are represented in data words.
Note added at 30 mins (2017-06-28 09:06:31 GMT)
If you have digital connections in your studio (S/PDif, lightpipe, etc), you MUST have a single clock source. If multiple devices are each trying to be the master clock, you will probably experience pops, clicks, or sometimes chirps in your audio. The solution is usually to make sure that you only have one master clock, and that all slave units are getting a clean clocking signal.
But even without obvious problems like pops and clicks, you may still have more subtle word clock problems. Think of it like this...
Ever seen clips from the very early days of film? Back in the days of hand-cranked cameras, it was extremely difficult to get the shutter speed to be perfectly even. The result was a moving image that, when played back at a constant shutter speed, seemed "jittery." the motion was often jerky an unnatural. This was due to the imperfections of the original film speed.
THE NATURE OF SAMPLING
Clock signals are pulses and are represented electrically by
square waves. A perfect sample clock looks something like
Each “On” edge is used to trigger a measurement (a sample) or reproduce a sample. In this simplified drawing, the triggers are perfectly spaced, the transitions from Off to On are instantaneous and there is no noise or distortion in the waveform.
Real life sample clock waveforms are subject to many factors that change their shape so that they only partially resemble the perfect square wave.
Note added at 42 mins (2017-06-28 09:18:42 GMT)
WHAT IS A CLOCK?
Sample clocks, as used in digital audio systems, provide the timing intervals that determine when audio samples are recorded or played back. Unlike clocks that we use to determine the time of day, a sample clock’s primary function is to set a steady pace, much like a very fast metronome. The timing intervals generated by the clock are used for several operations within a digital audio system, such as determining the rate at which audio is sampled, or determining when the samples get transferred to memory buffers as they are moved through the digital system. All of these operations are dependent upon a stable system clock.
Clocks of different but related speeds are used in all digital audio systems. Most commercial analog to digital converters (ADCs) are the delta-sigma type. These converters oversample audio at a very high rate, then use digital signal processing
techniques to produce the final output samples. At 44.1 kHz, for example, a typical delta-sigma converter’s sampling clock runs at 256 times the output sample rate, or about 11.29 MHz. At 48 kHz, the clock runs at about 12.29 MHz. It is this high-speed oversampling sampling clock that determines the actual instant in time when the analog signal is converted, and it is the stability
of this clock that determines the resulting audio fidelity.
It should be obvious that for a sampling system to function properly, an extremely stable clock is required—one that is regular and without variation in speed. Digital audio systems assume that the sampling process is perfect and there is normally no provision for correcting for audio that has been sampled
at inconsistent intervals. If the interval between samples varies during the analog-to-digital conversion, those variations are embedded into the recording in the form of distortion and/or noise. Subsequent playback of the recording will reproduce the recorded distortions. If the playback clock is also not perfect, additional distortion and noise will be added. However, as mentioned earlier, jitter is only problematic at the conversion stages, either analog-to-digital or
digital-to-analog. Jitter in signals transmitted digitally from device to device is not a problem unless it is so great that it causes a transmission error.
To be clear, it is impossible to create a perfect clock or one that is completely free from jitter. Though some systems can claim very low amounts of jitter, all systems have “non-zero” jitter. The challenge in converter design is managing the jitter “budget” and to make the device as stable and sonically
pleasing as possible.
Word Clock (also known as Sample Clock) Digital audio is created by taking a "sample" of an analog signal on a periodic basis, say 48000 times per second (the "Sample Rate"). A dedicated clock, the "sample clock," ticks at that rate, and, every time it does, a new sample is measured. Sample clocks are built into all devices that handle digital audio and video. Thus, your CD player and your DVD player have sample clocks built into them in order to stream the data accurately enough to convert the signal to analog audio or video. Whenever you connect two digital audio or video devices together in order to move data from one to the other, you must ensure they share the same sample clock. Why is this necessary? The oscillating crystals used for sample clocks are generally very stable, but there are always minute differences in the frequency of any two or more clocks. When used individually, this is not a problem, but connect two digital devices together and those minute differences will accumulate over time. Eventually, one of the devices will be trying to read a sample in the middle of the other device's tick, and the result is a small click or pop in the audio stream or noticeable jump in the picture. In the consumer realm, when you connect your CD/DVD player to your home theater processor via a digital interconnect cable, the theater processor will adjust its clock to the incoming data stream, and all works well. In the professional world, many digital devices are often connected together or to a single source (a mixer, for example), and, in order to avoid clocking errors over time, a central clock source is used and fed to all of the equipment. This central clock is known as a Word Clock generator.
I'm afraid I don't know enough about electronics and audio to explain anything to anyone. I hope the information I've found helps you find what you need.
| Helena Chavarria|
Local time: 07:09
Native speaker of: English
PRO pts in category: 4