Radiospurs

Looking at the DSP features

DSP

The heart of the stereo generator is the DSP (digital signal processor). A specialised beast orientated to real time processing of digital signals. An ordinary RISC/CISC CPU would not be up to this job.

The DSP details

Here we are talking about the ADSP21161 digital signal processor by Analog Devices. This is a 32bit floating point 100MHz SIMD processor. The chip has multiple serial digital ports (SPORT) with seperate IO DMA engines. When this design was first originated (circa 2004) this DSP was a state of art product but now seems to have converted into a cash cow line. This DSP model no longer appears in A.D's recommended design line. However there is no problem in upgrading to the ADSP214xx series. These are cheaper, more processing power and offer more internal peripherals.

DSP Flow Diagram ADC DAC initialisation Pilot Sub-carrier initialisation L/R synchronisation Trigger sampling SIMD split Low Pass Filter Split into three bands Audio processing Processed audio summing Poliphase Interpolation Filter Construct MPX Ring Buffer

Click on the flow box for more detailed information. Maybe even some code.

The older ADSP21161 has two major sticking points: 1) BGA packaging which complicates the manufacturing and 2) lack of an AES3 receiver. The newer ADSP214xx series is an advantage here. It is available in the more normal pin packages which is much more production friendly for small batch runs. Also there are multiple asynchronous receivers. Thus for any new design the ADSP214xx series is to be highly recomended. The software is almost identical. The availablity of SPI ports with DMA means any RDS generator incorporated can be dynamically configured, likewise the audio processing engine can be much more sophisticated and the MPX parameters (eg: pilot level). Use of SPI DMA means in memory parameters can be change on the fly without interrupting the main CPU calculation loop.

The use of the ASDP214xx is discussed in another part of the web where the combined stereo generator and FM digital exciter is presented.

In order to complete the design several extras need to be includes. A VU meter simulator to measure the incoming signal levels and the outgoing MPX. A mechanism for entry of the configuration parameters, pre-emphasis choice, compression/expansion/limiter coefficients.

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