Looking at the Clocks to be generated

The clock generator details

Several clocks are needed for the correct function of the stereo generator, both to drive the DSP and synchronise the input and output data streams. The clock choosen has to be precise since the DSP derives the pilot signal (19KHz) and sub-carrier (38KHz) from the main DSP core clock. Also the frequency has to be a multiple of the sample rate choosen for the ADC, this in turn decides the DAC sample rate. The MPX spectrum extends to a minimum of 53KHz, ie: a mininimum sample rate of 106KHz however we need to minimise the DAC sinX/x droop so we choose the highest possible sample rate for the DAC (192KHz). We need an integral interpolation figure for simplicity so for 1:4 interpolation the input sample rate has to 48KHz. The 48KHz sample rate is over sampled by the ADC (x256) to reduce the requirements of the anti aliasing filter. This gives our bit clock rate of 12.288MHz for both the ADC and the DAC. The sampling is synchronised with the Left/Right frame clock. The ADC stream consists of a multiplexed 32bit frames for each of the left and right samples so the L/R frame clock is 192KHz. Likewise the DAC frame is 768KHz because there are 4 x MPX sample per input sample.

The ADC only needs the bit clock to generate the L/R digital stream. The DSP SPORT input triggers on the frame clock flank to sample the audio stream, so this frequency is very inportant for accuracy of the pilot signal. Thus a syncronous 4 bit divider chain is used to derive all the clocks from the 24.576MHz master. This master MUST be a high stabilty (in absolute term for frequency, temperature stabilty and aging) crystal oscillator. The computation time of each 4 x MPX sample set is less than the frame time so in order to ensure correct output sample streaming both the bit clock and MPX frame clock are used both by the DSP sport port and the DAC. Hence the MPX output is decoupled from the audio input.

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