SigmaDSP with PWM or Σ-Δ modulator outputs to drive H-bridge Class-D ?
SigmaDSP is great for digital audio processing, but since the systems are becoming more digital from input to amplification, I'm actually missing PWM or Σ-Δ modulator outputs in order to directly drive an H-bridge for Class-D type amplification.
I've read that SigmaStudio processing clock is limited to its sample rate, so if I'd implement PWM through SigmaStudio it wouldn't have the high frequency needed to keep audio bandwidth and also I wouldn't be able to generate the right timings for the switching dead-time, but could ADI implement a block in SigmaStudio to drive pins (MP or SDATA pins for instance) to do so with a higher frequency?
Or does ADI intend to release a new part of SigmaDSP with such capability?
Regards.
It would be neat to do this, but I don't think the architecture allows it. If all-digital, what clock speed would be required?
I thought the modulation could be implemented in the CLKOUT frequency for instance, but it seems it is not accessible to users. Maybe ADI has the key to do it.
In practice, there is far more to implementing a class-D amplifier of reasonable quality than "just" converting a 24-bit PCM stream into a PWM or PDM stream. If I understand correctly, you are proposing adding an open-loop digital modulator to the SigmaDSP. Although there are successful designs with this architecture, it is a tremendously complex system design challenge. While many digital class-D modulators do use MCLK frequencies in the same range as the SigmaDSP family, that would be a minor factor.
When choosing a class-D architecture, some of the many important design decisions (aside from power) include analog vs. digital modulators, open vs. closed loop, the choice of output filter, feedback around the modulator vs. around the filter, full-bridge vs. half-bridge, power supply control and/or integration, etc. A successful amplifier design requires a much more holistic approach than a peripheral on a DSP could offer, and every application is different.
For exactly this reason, companies, including Analog Devices, have class-D amplifier ICs (or modules) with interfaces compatible with the SigmaDSP serial audio output ports (i.e. either I2S or TDM). While there are plenty of engineers on this forum capable of designing class-D amplifiers from scratch, I would encourage anyone starting a new design to leverage the work of others. At low power levels, ADI's class-D amplifiers are best of breed. At high power levels, you might find a DAC and an analog OEM module is a surprisingly good option.
Ken
HI Samba guy
You might be able to drive the GPIO (MP) pins at higher frequency if you simply use a higher sampling rate. The trade-off will be lower instructions available for other processing.
Custom code is possible through the use of 3rd parties.
Please comment.
Miguel
Hi Ken,
Thank you for such comprehensive answer. I realize now it is not a simple switcher to implement, but it seems that a future implementation with feedback around the modulator could be feasible, right?
I am already wondering if, in such case, it would be possible to estimate the limits of SNR...
Regards
Hi Miguel,
Thank you for your input, this custom code alternative seems interesting.
About the higher sampling rate, it doesn't seem that even 192khz would be enough considering dead-times and width variations, I mean, I think it would be necessary a PWM with a period of this range of frequency or higher to get it to work.
Regards.
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