USB 1.1 Interconnection Basics
General Overview
USB is an industry standard interconnection specification for adding peripheral devices to computers. Modern PCs and Macs are fully USB 1.1 compliant. USB support is part of Windows (since 98SE), Apple Mac OS 9 and OS X, and the Linux operating systems. The USB specification is an industry standard and is governed by the USB-IF, a non-profit organization founded by the companies that developed the standard (www.usb.org). USB was developed as a replacement for serial and parallel port connections, and is ubiquitous as robust, medium- to high-speed interconnection protocol for a variety of applications, including digital audio. As a standard, peripheral developers such as Sound Devices can design USB equipped products that work with multiple computer platforms. This allows a single piece of hardware to be useable in many computing environments. Sound Devices USBPre is designed for the USB 1.1 specification.
USB is a quick, easy connection for computers. The USB protocol makes the whole process automatic. There is no need to open the PC (except to add additional USB ports on a desktop machine), and there are no DIP switches or IRQs to set. The host controller automatically allocates resources to USB.
USB is bi-directional, allowing data to flow both ways between computer and peripheral. In the USBPre, audio data can be sent out of the USBPre to the computer for recording at the same time audio data is sent from the computer to the USBPre for listening.
Below are brief notes regarding specific aspects of the USB (1.1) specification. Additional, detailed information about USB is available at www.usb.org.
USB Bandwidth (Speed)
Full speed USB devices signal at 12 Mb/s, while low speed devices use a 1.5 Mb/s subchannel. USBPre is a full speed peripheral. Products like keyboards, mice, and graphics tablets are usually low speed devices. At the full date rate, 18 mono channels of 16-bit/44.1k (0.67 Mb/s) audio can theoretically be simultaneously moved across the bus. Since there is protocol overhead associated with USB, that 18 is brought down to typically 8 mono channels of 16/44.1 audio that can reliably be transferred across the bus. Variations in motherboard designs, motherboard chipsets, and USB host controllers have an effect on the bandwidth maximums of USB. In general, later designs implement USB more reliably than first generation chipsets.
USB Powering
The computer’s USB port provides voltage to power peripherals. This “bus powering” requires no external wall wart or batteries for operation. The computer automatically senses the power requirements from the peripheral and provides it. There are two classifications of USB powered devices, high power and low power. The USBPre is classified as a high power USB device since it draws more than 100 mA from a USB hub (typically around 220 mA). Low power USB devices such as keyboards, mice, and joysticks (or self-powered devices) draw 100mA or less. Computers and active hubs provide 500 mA of power for USB peripherals. Since the USBPre draws power from the host computer, battery runtime with notebook computers is affected. USBPre reduces notebook computer battery runtime from 8 – 15%, depending on the size and number of batteries installed in the notebook.
Hot-Swap
With hot-swapping there is no requirement to shut down and restart the computer to attach or remove a USB peripheral like the USBPre. Plug in the USBPre and it will automatically check in with the operating system and be available for use. (Note: USB devices usually require an installation procedure when first plugged into the computer). In the case with USBPre, audio software queries the operating system upon launch to determine what audio recording and playback hardware (i.e. the USBPre) is available. USBPre should be plugged into the computer before launching audio software for it to property address the hardware. Close out of the audio application before disconnecting the USBPre. If the USBPre is unplugged in mid-session, the application may continue to address the hardware, making the application unstable. The USB hot-swap feature is very convenient, but it is not foolproof.
USB Hubs and Controllers
USB allows for multiple peripherals to be plugged into a single USB port and addressed concurrently. The USB host controller (think bandwidth) enables this simultaneous connection. Hubs (think connection and power) are the physical connection to the USB bus. The USB port on the computer is both a controller and a hub. While many computers have two USB ports, they may be connected to the same controller. Each controller provides 12 Mb/s of bandwidth. When multiple devices are connected to a single USB controller, each shares the controller’s bandwidth. Some desktop motherboards have two USB controllers; peripherals do not share bandwidth if plugged into different controllers. Up to 127 peripherals can (theoretically) be connected to a single controller.
Hubs allow the physical connection of multiple peripherals to the USB controller. Hubs also provide power for peripherals. External hubs are connected to the USB port on a computer to provide additional ports. Active (or self-powered) hubs provide additional USB ports for peripherals and require power – which usually comes from an external power supply. Active hubs provide 500 mA to each USB port. Passive (or bus-powered) hubs provide additional USB ports and are powered by the computer (or an active hub). Passive hubs divide power among the USB ports and are useful only for low power peripherals.
USBPre functions best when it is the only device connected to a host controller, giving it full USB bandwidth. USBPre must be plugged into either an active hub or directly into the PC, since it is a high power USB peripheral.
USB Cabling
The USB specification for maximum cable length between hubs for full speed/power devices (like the USBPre) is 5 meters (roughly 16 feet). For low speed/power devices the limit is 3 meters (roughly 10 feet). To meet CE certification, the USBPre was tested with a single 7.5-meter (25 foot) cable. By using up to 5 active hubs connected with 5 meter cables and a 5 meter cable to the USBPre, 30 meters can be reached and still conform to the USB specification.
Adding more USB Ports (Controllers) to a Computer
Many USB peripherals such as USBPre use the majority of a USB controller’s bandwidth when in operation, so concurrently connected USB CD-RW’s, digital cameras, printers, and scanners means the need to add controllers. Many peripherals, like USBPre, do not reserve USB bandwidth unless they are in use, so check with the manufacturer for specifics on a given peripheral. The need to use multiple USB peripherals along with the USBPre (simultaneously) means that USB host controllers should be added to the computer. Some desktop and notebook computers have multiple USB controllers. USB controllers can be added to desktop machines by installing low-cost USB-to-PCI cards. USB-to-PC card adapters can add USB ports to notebook computers.