The Importance Of Digital Clock In Modern Digital Audio System

- Jun 19, 2018-

In the analog era, we use time codes to synchronize and record or play back from the same time code. During the entire process, all devices and tracks display the same time code. This is our standard for recording and mixing. All The equipment is controlled by this time code as a standard. This synchronization method has been used in today's digital systems in addition to the analog era.

The time code of a digital system is called a word clock. It is considered as a kind of pulse signal used to coordinate digital audio signals of different sampling frequencies and communicate in the same system. The word clock signal has an accurate and stable sampling frequency, which ensures that the transmitting and receiving ends of the signal operate at the same sampling frequency in all aspects of the digital audio system, and the bits in the transmitting and receiving signals start simultaneously. Under the action of the word clock signal, each device in the system not only achieves the synchronization in frequency, but also achieves phase synchronization, and can continue to work normally under stable conditions.

The word clock synchronization method plays an important role in stabilizing the transmission quality of digital signals in the system. However, when the system's word clock synchronization problem causes serious degradation of the signal quality such as an error in the transmission of a word clock signal causing sudden noise in the system, it is often overlooked from the direction of the word clock to solve the problem.

To implement the word clock synchronization of a digital audio system, one (and only one) word clock source must be set in the system. In fact, digital devices such as your sound card, audio interface, ADDA, and digital effects can generate word clocks on your own, but if you have more than one digital device in your system, you must pay attention to your The system can only set up a word clock source, because the word clock signal in each device determines the sampling frequency in this device. If a different word clock signal appears in a system, it means that the sampling frequency in this system is not uniform, and it cannot achieve the synchronous working state, and various unexpected problems will arise.

So in your actual work, the first step should be to select one of the main digital devices and set it as the source of the word clock signal in this system, the master clock, and set the synchronization mode of the device selected as the host to "Internal". The other digital device word clock signals are slave clocks, set to "external" and it is necessary to ensure that each slave device can stably receive the word clock signal from the master device. When the entire system reaches the synchronization state, all slave devices in the system will work according to the sampling frequency determined by the master device's clock.

The impact of the clock, passing standard clock signals for each device in the system is an important step to ensure digital audio quality. Because of the difference of clock oscillators, each device's clock signal will have a certain amount of deviation. This deviation will cause the digital signal to be inaccurate. , thus affecting the quality of the audio.

Clock degradation usually originates from jitter. In digital transmission systems, jitter is defined as the momentary change in the important moment of the digital signal from its ideal position in time. The important moment is the best time to sample the audio stream in a time period. Ideally, the sampling is performed at a set precise time each time, but in practice, the actual signal will move periodically along the edge of the ideal signal. This change is considered to be a kind of jitter also called the phase noise of the digital signal.

If the jitter is too large, errors will occur. The impact of the error on digital audio is enormous. Therefore, the larger the jitter is, the larger the error and the greater the loss of digital audio is. The size of the jitter is closely related to the integrated circuit system. A large number of circuit designs, processes, component quality, and system-level factors all affect jitter. In addition, if the unbalanced system has ground potential drift and there is a voltage offset between the differential inputs, Changes in the rise and fall times of the signal, etc., may also cause this digital distortion in jitter.

The use of an independent clock signal device is more professional than the transmission of a clock signal into a digital signal such as AES. Clock signals and digital audio data are included in composite digital signals such as AES, and delays in wiring between the clock paths are different. The difference in wiring delays, the different load conditions between the data and clock paths, the difference in packet lengths, etc., may affect the accuracy of the clock signal.

In addition, on the digital device link, each device is locked to the previous device, and the jitter on the last stage of the link has the contribution of the previous stages. Each device adds its own inherent jitter, and each connecting cable contributes to the jitter introduced by the wire. There is also some jitter gain or attenuation at each level. This process is called jitter accumulation. Accumulation results vary with individual device jitter characteristics and data patterns at each level. But at some point, all the jitter will not be properly combined when there is a "pathological" signal. During long-distance transmission, jitter accumulation occurs at many points in the master device and the slave device, and such an accumulation effect is greater.

As the jitter level increases, the decoding of the signal from the device begins to fail, which then leads to decoding failures—occasionally silent, popping or sometimes out of lock. To solve the digital signal distortion caused by clock jitter, there are two important points. One is to use a star topology to transfer the clock signal as much as possible, and the other is to use an independent clock to solve the impact of the clock signal on audio quality.

 

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