If your word clock generator has several outputs, you can connect each device directly to the clock master as shown in Figure 5.27.
Figure 5.26 – This is a dedicated synchronization tool from AvidĮxternal word clock synchronization is typically accomplished using low impedance coaxial cable with BNC connectors. An example of a dedicated synchronization tool is shown in Figure 5.26.
WORD CLOCK RECORDING CODE
It may also have the ability to generate and sync to other synchronization signals such as MIDI Time Code (MTC), Linear Time Code (LTC), and video black burst (the word clock equivalent for video equipment). A dedicated word clock generator has a very stable word clock signal and several output connectors to send that word clock signal to all the devices in your system. If the box only has to do one thing, it will probably be able to do it well. Any time you can go with a dedicated hardware solution, chances are good that the hardware is going to be pretty reliable. The first word clock synchronization strategy is to slave all your digital devices to a dedicated word clock generator. The strategy you choose depends on the capability of the equipment you are using. There are three basic strategies for word clock synchronization. To avoid the errors that result from word clock drift, you need to synchronize the word clocks of all your digital devices. Even if two word clocks start at precisely the same time, they are likely to drift at different rates and thus will eventually be out of sync. At a sampling rate of 44.1 kHz, this equates to a potential drift of $$44100\ast \frac\approx 2.2$$ samples per second. This imperfection is measured in parts per million (ppm), and can be up to 50 ppm even in good quality equipment. If the word clock of the receiving devices falls out of sync, it starts chopping up the blocks in the wrong place and the audio data will be invalid.Įven the most expensive word clock circuit is imperfect. If the device receiving the digital audio stream has a word clock that is running in sync with the word clock of the device sending the digital audio stream, each block that is transmitted is received and interpreted correctly. Every time the clock ticks, the digital device grabs a new block of data – sometimes called an audio word – from the audio stream.
To solve this problem, each digital device has a clock that runs at the speed of the sampling rate defined for the audio stream. If the stream is split up in the wrong place, the data block is invalid. The stream of bits is only meaningful when it gets split back up into the blocks containing the sample data in the same way it was sent out. These data blocks are a constant stream of bits moving down a cable. The signal is transmitted as a stream of small blocks or frames of data containing the audio sample along with timing, channel information, and error correction bits. There are several different digital audio transmission protocols, but all involve essentially the same basic process in handling data streams. Interconnecting these devices digitally can remove the latency and potential signal loss of analog interconnects, but digital transmission introduces a new set of problems, such as timing. (See Chapter 8 for more on setting gain structure.) If you practice good gain structure (essentially, controlling amplitude changes from one device to the next) and keep all your sampling rates consistent, the loss of quality is minimal, but it is still something to consider when using analog interconnects. You may also pay for this in a loss of quality in your audio signal as a result of multiple quantization errors and a loss of frequency range if each digital device is using a different sampling rate. As described in Section 5.2.1, you pay for this strategy with increased latency in your audio system. Transmitting in analog involves performing analog-to-digital conversions coming into the devices and digital-to-analog conversions coming out of the devices. When transmitting audio signals between digital audio hardware devices, you need to decide whether to transmit in digital or analog format.