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We have known Alex from APL Hi-Fi for quite a few years now. When he first came to us with a consultancy inquiry, his idea was to take his DA converter DSD-S with DSD conversion using a monolithic chip a little bit further and add a PCM playback functionality through the DSD path. This required PCM up-sampling and a 1-bit modulator with input decoding and synchronization. DSD over PCM was another feature perfectly suited for this kind of product. Later on, an additional requirement was to establish a proprietary encrypted raw DSD transfer between the DAC and the disc transport, called DTR-M. The transport was also supposed to run the up-sampling and the modulator.
The trickiest part of the development was definitely the modulator. While the up-sampling filters were relatively straight forward to fine-tune to achieve the desired transparency, the sonic effects of different modulator architectures and modulator orders were tremendous. Just to clarify, there was nothing wrong mathematically. The raw technical performance of any of the modulator variants was excellent, with huge dynamic range and decent signal to noise figures. The differences were most probably caused by the interaction of a spectrally very rich DSD signal with the downstream circuits. It is quite difficult to model and simulate such a modulator on its own, let alone predicting its sonic performance within a complex reproduction system. And this can be seen as a general rule – it is always about the whole chain where the individual elements interact with each other.
Alex critically listened to every single version of the modulator and reported his findings back to us. Finally, with many tweaks in the modulator state-space control system and the dithering scheme, Alex was happy with the result. The modulator is also used in his top-line DSD-MR which uses direct discrete balanced DSD conversion with no monolithic DA chips.
What we did:
FPGA integration support for DSD-S and DTR-M
DSD over PCM decoder
Up-sampling FIR filters, up to 8x
Selectable filter characteristics
1-bit 3rd and 7th order modulators
DSD64, DSD128 and DSD256 selectable modulator speeds
Encrypted raw DSD protocol codec - DTR