TOPIC:

Hi dear DIY friends:

The question asked in this thread is related to the output low-pass resampling filter applicable for a wide range of MiniDSP products (2x4HD, Flex, SHD). The shape of this filter can be set for some DACs, but unfortunately not on the MiniDSP. However, can I bypass the filter by performing an upsampling "upstream" the MiniDSP? Let say I use a MiniDSP plugin that has a 96 kHz sample frequency.

Q - What happens if the MiniDSP is feed an 96 kHz input signal? 

A1 - The MiniDSP will resample anyway and will use it's DAC brick-wall low-pass filter? or
A2 - The MiniDSP will not resample since it is feed a 96 kHz signal, and not apply it's brick-wall low-pass filter?

What is the correct answer, on beforehand thanks?

Have you seen something to suggest that miniDSPs use brick-wall filters? It seems unlikely to me. The data for the SHD and Flex models below suggests they don't:

www.audiosciencereview.com/forum/index.p...-dac-streamer.18681/
www.audiosciencereview.com/forum/index.p...iew-audio-dsp.30804/

I'd have thought that the way to go about using different filters if you wanted to would be to use the digital output from an SHD, or digital Flex? 

In answer to your questions, I can't see why the input sample frequency would make any difference to the reconstruction filter applied. The internal sample rate is fixed no matter what the input so it would make sense for the same reconstruction filter to always be used.

Note I'm just commenting from a user perspective. If you really want to know exactly what is going on you could try contacting miniDSP Support, although there may be limits to what they're willing to share.
 

Yes the 2x4 HD uses a Brick-wall: audiosciencereview.com/forum/index.php?t...hd-dsp-and-dac.2674/
The new Flex is a bit shallower, and yes I can buy a digital MiniDSP. But it's not the question here:

Q: Is the DAC low-pass filter used or is it bypassed if the input signal is the same as the unit internal sample rate (96 kHz)?

 

alfegutt wrote:

Q: Is the DAC low-pass filter used or is it bypassed if the input signal is the same as the units sample rate (96 kHz)?

 

As I said above, I see no reason why the input sample rate would make any difference but you could try contacting miniDSP Support to see if you could get a definitive answer.

You're right that the filter fall-off for the 2x4 HD looks much steeper than the two newer models.

For context, do you currently own any of the miniDSPs you've asked about?

 

Ultrasonic wrote:

alfegutt wrote:

Q: Is the DAC low-pass filter used or is it bypassed if the input signal is the same as the units sample rate (96 kHz)?

 

As I said above, I see no reason why the input sample rate would make any difference but you could try contacting miniDSP Support to see if you could get a definitive answer.

You're right that the filter fall-off for the 2x4 HD looks much steeper than the two newer models.

For context, do you currently own any of the miniDSPs you've asked about?
 

The sample rate makes a world of difference, the low-pass filter is a so-called reconstruction filter used in the upsampling algorithm of DACs, but they are not applied in non-oversampling DACs (NOS). By feeding my MiniDSP 2X4HD with it's native 96 kHz from Camilla DSP (NOS setup), I hoped that I could avoid the MiniDSP DAC output filter.  And yes I have several MiniDSP products.

Anyone out there with a deeper knowledge into the topic?

alfegutt wrote:

The sample rate makes a world of difference, the low-pass filter is a so-called reconstruction filter used in the upsampling algorithm of DACs, but they are not applied in non-oversampling DACs (NOS). By feeding my MiniDSP 2X4HD with it's native 96 kHz from Camilla DSP (NOS setup), I hoped that I could avoid the MiniDSP DAC output filter.  And yes I have several MiniDSP products.

You missed my point. The internal sample rate is always the same and therefore so is what the DAC section 'sees'. That's why I don't think it's likely to make any difference what the input sample rate is.
 

If the input rate is 96 khz, and thus the same as MiniDSP 2x4 HD, it should (in theory) not be necessary for the MiniDSP to apply a lowpass brickwall filter, but only interpolate the input and output data.


 

It would be great if MiniDSP don't apply a brickwall filter in this case @alfegutt, as it would avoid severe pre-ringing.

mortengustavsen wrote:

If the input rate is 96 khz, and thus the same as MiniDSP 2x4 HD, it should (in theory) not be necessary for the MiniDSP to apply a lowpass brickwall filter, but only interpolate the input and output data.

 

I understand where you've* been coming from but stick with my guess at the likely behaviour. As you have a 2x4 HD its presumably this model's behaviour you really want to find out rather than the other models mentioned? That was why I asked about what you may own by the way.

Note that your own link shows that a very similar filter is applied for the digitised analogue input, although I suppose that could just reflect that the ADC runs at 48 kHz.

*Apologies - just spotted you weren't the OP  .

mortengustavsen wrote:

If the input rate is 96 khz, and thus the same as MiniDSP 2x4 HD, it should (in theory) not be necessary for the MiniDSP to apply a lowpass brickwall filter, but only interpolate the input and output data.



 

Thanks, you got the idea perfectly correct, and its a well known concept. But it might also be that the MiniDSP uses it's internal sampleroutine and filter anyway (as stated by Ultrasonic), hmmmm...anyone from MiniDSP that could conclude on this matter?

alfegutt wrote:

Thanks, you got the idea perfectly correct, and its a well known concept. But it might also be that the MiniDSP uses it's internal sampleroutine and filter anyway (as stated by Ultrasonic), hmmmm...anyone from MiniDSP that could conclude on this matter?

For info. I suggested contacting support as you're far more likely to get a response to your question that way rather than waiting to see if anyone from miniDSP responds here. They do sometimes but its rare. They intend this to be a user forum as per the message at the top of every forum page.

I'd be interested in the answer if you find out  . I'd love to be proved wrong for your potential benefit. 

(I did understand why you wanted this from the very start of the thread btw.)

alfegutt wrote:

Yes the 2x4 HD uses a Brick-wall: audiosciencereview.com/forum/index.php?t...hd-dsp-and-dac.2674/
The new Flex is a bit shallower, and yes I can buy a digital MiniDSP. But it's not the question here:

Q: Is the DAC low-pass filter used or is it bypassed if the input signal is the same as the unit internal sample rate (96 kHz)?



 

That measurement is incorrect and likely the result of the source, not the 2X4HD.

SHARC DSP of the miniDSP will always asynchronously resample the input to 96 kHz. Even if you send 96 kHz as an input it will resample. This is a separate operation from the DAC reconstruction filter. Neither the ASRC or the DAC reconstruction filter can be bypassed.

For reference here is what the 2X4HD actually looks like when fed with 192 kHz white noise as well as a frequency sweep to 96 kHz.

White Noise
 

Frequency Sweep


Michael

And for a more apples to apples comparison to the ASR measurement here is white noise from REW at 44.1 kHz via TOSLINK in to the 2X4HD as recorded by an ADC operating at 192 kHz. In reality this looks pretty shallow.

 

Michael

^That's good to see @mdsimon2. I was surprised that the 2x4 HD would have had as sharp a filter as the ASR data showed.

Great - thanks a lot 

This is really appreciated, I also contacted the MiniDSP development team. And they kindly explained how the ASR graphs are wrong, like it is explained in this thread.

Best regards
Alfegutt:-)