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Browse Prior Art Database

Frame Adjust Counter

IP.com Disclosure Number: IPCOM000007075D
Original Publication Date: 2002-Feb-22
Included in the Prior Art Database: 2002-Feb-22
Document File: 3 page(s) / 2M

Publishing Venue

Motorola

Related People

Edward J. Yurchik: AUTHOR [+4]

Abstract

In a system that is routing audio between two devices that have different clock masters, timing differences need to be collected and processed so that adjustments can be made to minimize audio distortion. In the system described herein, a Frame Adjust Counter collects timing differences between a Serial Audio Port (SAP) and a universal serial bus (USB) device interface. A software algorithm processes the differences and adjusts the Start Of Frame on the USB interface responsive to the detected difference.

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Frame Adjust Counter

Edward J. Yurchik, Carl Wojewoda, Eric Overtoom, and Jay Cantrell

Abstract

In a system that is routing audio between two devices that have different clock masters, timing differences need to be collected and processed so that adjustments can be made to minimize audio distortion.

In the system described herein, a Frame Adjust Counter collects timing differences between a Serial Audio Port (SAP) and a universal serial bus (USB) device interface.  A software algorithm processes the differences and adjusts the Start Of Frame on the USB interface responsive to the detected difference. 

Body

In an embedded system that requires timing differences to be monitored and processed between a SAP interface and USB interface, a mechanism is provided so that the system software has access to timing information.  Counters are employed such that the system software can use the information after it has been collected over a period of time instead of. burdening the software on a frame basis.

The Frame Adjust Counter compares the frame rates of a SAP interface and a USB device.  In effect, this is a simple frequency counter using the SAP frame synch as the frequency reference and the USB slave frame rate as the unknown frequency.  Referring to the drawing below, a fixed value is programmed into Count_targ.  This value is the number of SAP frame synchs that will be counted(divided by 8).  Equivalently, since SAP frames are 125 micro-seconds, which is prescaled by 8, it is the time that the unknown frequency is measured, in steps of 1 milli-second.

When Counter 1, which counts the pre-scaled SAP frames, matches the target value, the following sequence starts:

  1. Both counters stop counting
  2. The contents of Counter 2 is transferred to the FrameRegister
  3. An interrupt is generated (if enabled)
  4. The Stale bit is set[1]
  5. Both counters are reset
  6. Both counters resume...