Filter or source frequency measurement does not match that of SigmaStudio

In order to properly process signals, the system master clock, SigmaDSP PLL settings, and SigmaStudio parameters (filter coefficients) need to align. If measured frequencies differ from expected frequencies, it is possible that there is some mismatch in the design.

For example, consider the following situation:

The MCLK input to a SigmaDSP is 12.288 MHz.

The PLL is set to accept an input MCLK of 256 * fs. Therefore, fs = 12.288 MHz / 256 = 48 kHz.

In SigmaStudio, the system sample rate for the schematic was mistakenly set to 44.1 kHz.

A sine tone generator is placed in SigmaStudio and routed to the outputs. The frequency of the generated tone is set to 100 Hz.

The measured frequency of the output is approximately 109 Hz.

The reason for the frequency shift is that the DSP's stored parameters are configured for a 44.1 kHz sample rate, but the hardware is being clocked at 48 kHz.

In order to solve the problem, two methods are available:

1. Change the sample rate in SigmaStudio and propogate this value to all cells. Recompile and download the new parameters to the hardware. The system is now aligned at a 48 kHz sample rate.

2. Change the system MCLK frequency to 12.288 MHz * (44.1 / 48) = 11.2896 MHz. The system is now aligned at a 44.1 kHz sample rate.

Additional comment:

Some SigmaDSPs, notably the ADAU1701, ADAU1702, and ADAU1401, require hardware changes (resistor values) when the system sample rate is changed. Read the datasheet carefully and implement the changes in hardware.

Message was edited by: BrettG (corrected spelling)

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