USB Speech Handset Development

NHT Design was given the task of updating a Handset for a Wired Communications System. Limitations in the existing system meant that it was unsuitable for increased functionality (to allow full-duplex speech), and did not allow for future improvements.

The development of the new handset was done in tandem with the development of a new node device, containing a significantly faster processor (than its predecessor) with greater resources such as an Ethernet interface and USB Host capability, under a Linux operating system. As the Linux environment promotes the use of re-useable and standardised protocols, it was decided that the new handset would have USB 2.0 Device support to avoid the need for proprietary software on the Host. The high bandwidth capabilities and USB protocol also meant the interface could be used for any number of capabilities without affecting performance. Use of a standard shielded USB cable would also improve on the customer's existing cable.

The speech requirements were:-

16-bit resolution
8kbps minimum sample rate
Full-duplex

A 7-Segment LED display was also needed on the handset to indicate volume level and channel number, along with switch inputs and other LED outputs, which all had to fit into the existing handset enclosure design.

The updated handset design contained the following hardware features:-

NXP LPC2141 ARM7 processor implementing USB hardware and software stack
Three USB classes; Audio class to transfer the full-duplex speech, HID (Human Interface Device) class for switch and LED report data, and DFU (Device Firmware Upgrade) class for in-field software upgrades
An external codec, giving 16-bit resolution ADC and DAC components, with associated configurable gain amplifiers, and reducing the total part count and PCB space requirements
High efficiency LEDs to give the same brightness as previous design, but with much lower current draw
Low-profile, removable, custom 7-segment display, with surface mount connection to reduce component height and PCB area requirements, and an SPI communications interface to allow easy upgrade of the display
6-layer PCB construction.

The new handset now provides clear full-duplex audio with the ability to adjust volume using the USB Audio class and associated mixer interface. Switch inputs can be read and LED outputs set using the HID class, and software can be upgraded using the DFU class. Future enhancements can also be added without the need to re-engineer the communications protocol, as in the previous design.

Northern Hi-Tec
Wasco House
Willow Lane
Lancaster
LA1 5NA
Tel: 01524 67833
Fax: 01524 67544
Email: sales@nht.co.uk