I’ve always been curious as to what filter family they use. I see mention earlier on of brickwall though that is not mentioned in the attached nDAC document. There are so many families to choose from, before this thread I had sort of supposed it was an apodizing filter. But now I have no clue.
Naim use Analog Devices’ SHARC processor - and Naim write their own assembly code to run the SHARC DSP functions and configure its parameters such as low pass filtering and oversampling. So Naim use Analog Devices DSP that they specifically configure for their products.
Of note, which is what you might have been referring to, Naim configure the 1704K (and other TI converters I believe) to by pass their inbuilt low pass/anti aliasing filter and instead use the filtering functions in the Analog Devices SHARC processor that Naim configure for their products…
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I believe DBPoweramp only supports and decodes some of the HDCD operands - and not the more unusual ones. The DBPoweramp I believe uses a reverse engineered open source library for this. Perhaps just as well HDCD is not used much at all now
That’s my understanding already. I was mainly interested in what general methodology their implemented filter is, not so much what feature flags they enabled or on which chip. For example, do they opt for a customized filter that is in the brickwall family, apodizing family, something else entirely etc. ?
It’s academic really, but playing with other DAC (audio sources not chips) it is possible for the user to define their own oversampling and filter combination. The effects are pretty large and the combinations vast. So I personally find it interesting whether or not Naim have a filter family they think is inherently the least bad compromise. And if so, why?
The Naim DAC white paper goes into this - and has been the same since
So they use an IIR low pass Butterworth anti aliasing filter to which they have added extra poles and a standard oversampling filter technique of zero value sample insertion.
The filter DSP leans heavily on the SHARC processor and is run in 5 lines of assembly code.
The IIR part is interesting. IIR is typically poor for phase response compared to FIR, but FIR can cause more processing noise… on balance in the Naim designs they felt the IIR sounded better overall and the phase response was respectable. (By comparison Chord use FIR filters which use kernel filter samples or ‘taps’.).
I think in DACs one of they key aspects for accuracy is the analogue i2v converter, and Naim go to some effort on this to get this tracking accurately on the output of the TI DAC chips.
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Thanks Simon. i read the paper but missed this because the level of detail in the terminology was more specific than I’m used to. I had to go and look it up. So Butterworth is a class of Brickwall and FIR is a synonym for apodizing.
I like learning new things.
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There are many filter algorithms, the Butterworth filter is common in audio DSP for sampling applications as it aims to be as flat as possible in the pass band before the slope.
The steeper the slope the more instability there is in the pass band. The Butterworth performs well here but no filter algorithm is perfect so there is limit to the slope in practice (so very steepl filters don’t appear where sound quality is important), It was invented in the 1930s.
FIR stands for Finite Impulse Response as opposed to IIR - which stands for infinite Impulse response.
The latter is achieved using continuous recursive functions on the samples, and is limited by the numerical accuracy of the processor, the former is achieved by performing convolution of the sample stream against the samples of a filter response of an impulse. It is limited by the window by the size of filter samples or ‘taps’.
Both IIR and FIR filters can be constructed to be effectively apodising filters. They mean quite different things.
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Thank you, Richard. The white paper was a very interesting read. I think I am a little late to the party, especially if I also consider outboard external power supplies into the mix. I will contact a NAIM Dealer in Melbourne as my usual dealer has stopped selling Naim products.
However, this is purely theoretical as my dear wife has said no to any more HIFI equipment.
Warm regards,
Mitch in Oz.
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