Philips PSC805 Appendix B, USB Hub Considerations, Introduction to USB Hub, Power Limitation

Philips Sound Solutions

8. Appendix B, USB Hub Considerations

8.1. Introduction to USB Hub

A hub is a USB device that uses one USB port for connection to a PC and typically provides four additional ports for connection to USB devices. The function is similar to an electrical plug expander or an extension cord with multiple receptacles.

Some of the earlier computers have only two USB ports that are usually located on the back- side of the computer. In this case if we have more than two USB devices for connection to the PC we need to use a USB hub. Using a hub not only provides more USB ports, but also provides easy access to the ports that are located in the back of the computer.

Most newer computers have two USB ports in the back and two additional ports in the front. The front ports are usually concealed under a flip up or flip down cover. Refer to your PC document to locate these ports.

A USB host controller card may be added to a computer if it does not have any USB ports at all. These cards need to be installed inside the computer in a PCI slot.

8.1.1. Power Limitation

Hubs, like any USB device, may be bus-powered or self-powered. A self-powered hub has an external power supply, which plugs into an electrical outlet, and can provide maximum specified power to all its USB ports. A bus-powered hub draws power from the root USB port, and thus, it has limited power available to the connecting USB devices.

8.1.2. Bandwidth Limitation Issues

Each USB port on the PC has a maximum bandwidth of 12 Mega-bits per second (Mbps). Different USB devices require different amount of bandwidth. Connecting more than two high bandwidth-demanding devices to a hub may cause degradation of functionality for some or all of the devices on that hub. For example, connecting a web camera and an audio device to the same hub may cause gaps in the displayed video, or gaps in the audio stream. It may even cause the devices not to function at all. This usually happens to the device that is used last when other USB devices are active.

PSC805 Aurilium™ Sound Processor Users Guide 36