Building and Wiring the Compression Suite

After the hardware is installed, we can wire the components. We will look at the professional connectors and cable used to connect inputs and outputs, and provide an overview of the wiring tasks.

Audio Cables and Connectors

The standards used for analog audio inputs and outputs are different for professional and consumer equipment. Professional audio equipment often uses balanced connections; consumer equipment uses unbalanced connections. Standard signal levels and other audio characteristics also vary for each type.

We won’t go into all the details of audio circuits here. Suffice it to say, when connecting devices, a balanced circuit has three connections (high, low, and ground), and an unbalanced circuit has two connections (high and ground.) If a connection is made between a balanced input and a balanced output, or unbalanced input and output, the signal levels and audio characteristics will usually match, and the audio signal will be optimized. If a connection must be made between an unbalanced and balanced input or output, a device that matches the level and audio characteristics, such as a matching transformer, can be inserted.

In the case of the compression suite Fabrikam is building, all the equipment in the chain provides professional balanced connections, except for the consumer S-VHS VCR. To solve that problem, they will route the audio through the mixer, which accepts both types of inputs.

Balanced audio is routed by using an audio cable with two wires surrounded by a wire sleeve called a shield. The high and low signals are connected to the wires, and the ground is connected to the shield. Balanced connections typically use an XLR connector. The XLR connector and other connectors are pictured in the port and connector sidebar in chapter 2.

Unbalanced audio is routed using an audio cable with one wire and a shield, and typically use RCA, quarter-inch, or eighth-inch connectors. There are quarter-inch and eighth-inch connectors that can support three internal connections, called tip-ring-sleeve (TRS) connectors, that can support three internal connections. One connection is made to the tip of the connector, another to a ring just below the tip, and a third to the sleeve below the ring. TRS connectors can be used for a balanced line, but in computers they are more often used to carry a stereo signal, where the tip is the left signal, the ring is the right, and the sleeve is common.

The shield on an audio cable is connected to ground in an audio device, which is usually the metal chassis or shield surrounding the electronic components. The chassis is then connected to the ground connector on the AC line, which ultimately connects to a metal pipe or long metal stake driven into the ground. Grounding is very important in audio circuits for preventing external electrical fields from interfering with the low-voltage audio signal.

Video Cables and Connectors

Shielding is even more necessary with video signals. Video signals are routed through a coaxial cable that can contain several layers of shielding. The video coaxial cable for carrying standard composite video has one wire surrounded by shielding. Cables used for carrying other types of video signals, such as S-video and IEEE 1394 data, can contain multiple wires with shielding. Video production facilities typically standardize on a particular type of video signal to make video routing easier.

The Media department has chosen the NTSC standard for composite video. Composite video consists of one analog signal that is a composite of the red, green, and blue components of a video image, and the synchronization pulses that a video monitor or television uses to display the image. NTSC is the standard system used in North America and some other countries. Other standards used in the world are PAL and SECAM. (For more information about video standards, see chapter 2.)

Even though the department has standardized on composite video for its routing system, connections can still be made directly using other types of video. The video capture card the Media department uses will be able to input various types of video. For example, a technician can connect a digital camcorder directly to the card using the IEEE 1394 digital connector.

The BNC connector is the professional connector for composite analog. It is also used for SDI video, which is the standard in broadcast digital video. A Digital Betacam deck can be connected directly to the capture card with the SDI connector, providing the most direct—and therefore the highest-quality—transfer of video. RCA connectors are often used instead of BNC connectors for composite video on consumer-grade equipment. However, many analog devices, including the S-VHS VCR, have S-video ports that provide a higher-quality analog video signal than composite video. You can, for example, connect the S-VHS VCR directly to the capture card with an S-video cable.

Video Wiring

As you can see in figure 15.6, one end of most of the video lines connects to the video patch bay; the other end connects to an input or an output of a device. The exceptions are the two connections between the VCRs and the waveform and video monitors. Both VCRs provide two composite video outputs, so one video output can be routed to the monitors and the other to the patch bay.

Figure 15.6: Wiring for video equipment in the compression suite.

The waveform monitor, like most professional devices, has two BNC input connectors for each input, even though only one source at a time can be connected to an input. The reason for the extra connector is so signals can be looped through to other devices. One cable runs from the Digital Betacam to one BNC connector. Then another cable is run from the loop-through connector to the video monitor. The signal from the Betacam is displayed on the waveform and the video monitor. Keep in mind, if a device has a termination switch next to the loop-through connectors, termination must be turned off if loop-through is used; otherwise termination must be on.

For higher-quality captures, the SDI outputs of the Digital Betacams are connected directly to the capture cards. When capturing digitally, the waveform monitor is not used because the video parameters do not need to be adjusted. For more information about capturing digitally, see chapter 10.

Audio Wiring

The concept behind the audio wiring is the same as for video. One end of every line connects to a patch bay input or output, and the other end to a device (see figure 15.7). Because all of the devices are stereo, two lines must be run for every connection.

Figure 15.7: Wiring for audio equipment in the compression suite.

The main output and the inputs of two channels of the mixer are routed to the patch bay. This provides the technician with a way to mix two signals, adjust levels, or add mixer equalization to a source before it is fed to the encoding computers. For example, suppose audio levels fluctuate significantly on an S-VHS tape. The S-VHS VCR can be patched into the mixer, and the mixer output can be patched into the encoding computer. Then the levels can be adjusted during capture.

The outputs from the two encoding computers are fed into two channels of the mixer, and the powered monitor speakers are fed from the mixer’s monitor outputs. “Solo” buttons on the mixer channels are used to monitor either or both encoding computer outputs. The channel faders are normally turned all the way down. This is to avoid a feedback loop when the mixer output is connected to an encoding computer input. A feedback loop is created when the output of a device is fed back into its input. Feedback can caused damage to electronics and speakers, not to mention your ears.

As with digital video, the AES/EBU connections on the Digital Betacams and capture cards are used to capture audio digitally. (Again, see chapter 10 for more information on digital capture.)