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Technical Paper 2. The HD Gemini Digital Engine: Arduous, but most Rewarding

Not all DACs are selected the same

AMR has spent the past few years judiciously testing, measuring and listening to each and every available HD DAC. Once we cherry-picked the chipset that was the musical, we 'threw out the textbook', literally.

We implemented the same exacting approach taken for the Philips TDA1541A in the CD-77 and the UDA1305AT in the CD-777. We discarded the textbook/datasheet schematic approach, which would have been far, far easier as it would have replicated known and tested circuitry that the DAC manufacturer guarantees will work. It is a route often taken, however it would have led to a mediocre quality of sound that was not in keeping with AMR's exacting standards.

We applied our expertise in power supply design, including our ultra-shunt regulators as well as our valve analogue stage design to extract an out-of-this-world performance from our hand-picked HD DAC chipset, very much like what we have been able to do with our CD-Processors previously.

HD Gemini Digital Engine:Twin DACs

Instead of a standard DAC chipset implementation, we developed the HD Gemini Digital Engine to satisfy the very high demands for music lovers, as it is makes music that appeals to the heart with nil fatigue, giving hours of unbridled enjoyment. The only way this could be achieved was to design from the ground-up, 2 DAC chipsets for truly optimal playback from each; High-Definition and Classic formats.

The HD Gemini Digital Engine

Source: AMR

AMR’s HD Gemini Digital Engine is a platform where the HD DAC is at the centre but it is also so much more. We use a Field Programmable Gate Array (FPGA) to handle a lot of the core digital duties, including the Intelligent Memory System and managing the Digital Signal Processing.

Essentially an FPGA is in effect, the creation of a specially-commissioned chipset with a wide range of functions; including a large memory buffer, frequency counter hardware, DSP interface and data format adjustment for the two different DACs to one’s exacting specifications.

Field Programmable Gate Arrays

Source: AMR

Or perhaps rather, is akin to programming many hundred discrete logic IC’s worth into one highly-optimised chip. This approach is technically speaking, extremely difficult and lengthy, however it allows very complex designs to be implemented, that are way past traditional methods and makes pre-made IC’s seem very ordinary.

The HD Gemini Digital Engine with both a HD 32-Bit DAC chipset and a Classic 16-Bit Multibit DAC chipset. Both are optimally configured so that each is devoted to HD and Classic music files respectively. From hearing just the first few notes from the DP-777, we believe the end has justified the means.

Ringing, the Bane of Digital Audio

With the specifically developed algorithms of the HD Gemini Digital Engine of the DP-777; Apodising, Minimum Phase and Soft Roll-Off, each lends itself to dealing with a different facet of the issue of filter ringing.

The Organic filter algorithm uniquely developed by AMR, combines the best attributes of all these methods to address ringing in the recording and the conversion process. So it should come as no surprise that DP-777 using the Organic Filter simply makes music that is ‘right’.

Special Analogue Output Stage

Once the music signal has moved outside of the HD Gemini Digital Engine, the analogue signal path is no less uncompromising. The OptiValve output stage is again, a ground-up design using the same exacting approach as the very positively received CD-777 which in turn was derived from its bigger brethren, the CD-77.

Again, there are no solid-state devices in the signal path as amplification and output buffering is performed by NOS valves. Dedicated AMR-specified transformers, boutique capacitors, even down to AMR’s very own silver leaf capacitors for analogue filters are deployed in the signal path.

The Dual-Mono Analogue Stages

Source: AMR

In every regard, AMR's starting point was to produce a Digital Conversion Component that could match the same, peerless, quality of our CD-Processors in all areas, but with exceptional functionality and capabilities that would allow it to go beyond CD; with uncompromised access to High-Definition Audio from all sources including computers, all the while keeping the same core qualities that made our CD-Processors so unique.

This is why the DP-777 Digital Processor makes music, on high-definition.

Thorsten Loesch
Director – Technology
t: +44 (0) 01704 227 204;

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