A Microchip PIC based USB sound card
INTRODUCTION
The idea for creating a USB sound card based on a PIC came from discussions of other people creating one on the Microchip USB forum. The hardware of the card is based on all Microchip products. The software uses a modified version of the Microchip USB framework which is interrupt driven instead of the traditional polling. The device is a USB composite device. The first device is an implementation of the
USB Audio 1.0 interface and the other device is a custom interface based on a HID interface. The purpose of the custom interface is for programming the device serial number, upgrading the firmware, and in the future any other configuration that isn’t supported directly by USB Audio 1.0. In a previous version, WinHTTP was oringally used. After testing the card for awhile, I decided it's better to not require drivers for the card especially since the HID/WinUSB interface is rarely used.
The sound card runs at a sample rate of 48KHz, 32KHz or 24KHz selectable by the OS with 12 bits per sample. The quality approaches commercial grade as the sample rate is higher then CDs. 44.1KHz was not implemented due to the difficultly and additional processing overhead. The sample rates that were implemented are all a multiple of the 1ms USB frame meaning each frame sends the same amount of data. 44.1KHz requires the card to handle different amounts of data each frame requiring more advanced buffering and synchronization techniques and more processing power. Microsoft Windows automatically resamples the 44.1KHz audio found on CDs to 48KHz.
