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Review and Measurements of Teac NT-503 Networked DAC

Discussion in 'Digital Audio Converters (DACs)' started by amirm, Oct 30, 2017.

  1. wgscott

    wgscott Active Member

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  2. A.wayne

    A.wayne Active Member

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    We are here , not there , hence my question .... What are the sonic trade offs ..
     
  3. wgscott

    wgscott Active Member

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    I know it is really fun to play obtuse on the internet, but life is too short.

    But since you are willing to generously share your wisdom and knowledge, perhaps we could start with your explanation for how upsampling differs from oversampling, both in theory and in its implementation in the Teac NT-503.

    If the differences are inaudible (and they are to me, with the possible exception of upsampling to DSD, which sounded slightly worse), there is no sonic trade-off, ipso facto. The alleged advantage of upsampling to DSD and using the high-frequency cutoff filter is it minimizes pre-ringing. Since I cannot hear demonstrably measurable pre-ringing, it offers no advantage to me.
     
    Last edited: Nov 2, 2017
  4. A.wayne

    A.wayne Active Member

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    I fail to see how I'm being obtuse, I asked if there was a sonic difference and why , thanks for finally giving me your opinion ..


    regards
     
  5. wgscott

    wgscott Active Member

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    There is nothing in there that wasn't posted previously.

    I still want to know why up-sampling is a "no-no." I don't wanna get no audiophile-arse-whooping.
     
  6. DonH56

    DonH56 Addicted to Fun and Learning Technical Expert

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    I'm not seeing all the responses... Upsampling implies some manner of interpolation between (starting) samples, so essentially some algorithm (which can get fairly complicated) is predicting what the new samples should be. The algorithm is not always going to get it right, natch, but in practice I'd guess (do not know) differences would be inaudible unless something is really hosed.
     
  7. amirm

    amirm Founder/Admin CFO (Chief Fun Officer) Staff Member

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    I don't think Wayne is playing with you. He is asking a question. Let me answer a bit because I think you spoke past each other.

    Many DACs internally run at higher speed than the sample rate of the audio. By "oversampling" this way, they get to run their core DAC at lower resolution which is easier to implement. This is because the faster you sample something, the less amount it gets to change its value. So you get away with fewer bits for the digital to audio converter. How this happens requires digging into the data sheet of the DAC silicon.

    What I tested was not that. But rather, software (DSP) algorithms inside the DAC (box, not silicon) that prior to handing the PCM data to the DAC (silicon), changes its sample rate. I tested 8X upsampling and DSD upsampling. The quality and fidelity of such upsampling is determined by the hidden algorithm inside the DAC (box). So other than measurements, we can't tell what they are doing.

    The idea of upsampling is both marketing and technical. Marketing one is the assumption by most everyone that bigger is better. And DSD is better than PCM. So when a DAC takes 44.1 Khz and converts it to 186 Khz, people "feel" better about it and likely read into it better sound.

    The technical reason to do that is that by upsampling, you no longer need to have the sharp filter that is used for 44.1 Khz to stop aliasing. So in theory, better sound can be had -- all else being equal.

    On the Teac NT-503, I briefly tested upsampling in PCM and didn't hear anything. With DSD, I thought there was some degradation but my testing was so casual that it is not worth relying on. Changing settings causes muting which makes comparisons very difficult.
     
    Last edited: Nov 2, 2017
  8. amirm

    amirm Founder/Admin CFO (Chief Fun Officer) Staff Member

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    Ah, forgot the main point. :) That oversampling in the DAC silicon is a different process than upsampling in software. When discussing, we should keep these separate for clarity even though the core signal processing operation is similar.
     
  9. wgscott

    wgscott Active Member

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    Thanks for the explanation, and apologies to @A.wayne
     
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  10. A.wayne

    A.wayne Active Member

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    Thanks Amir and separate we shall ...... :)
     
  11. Jakob1863

    Jakob1863 Active Member

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    In addition to DonH56´s post; any process that alters the sampling rate (compared to the original one) is called resampling.
    Upsampling and oversampling denote processes where the generated new sampling rate is higher than needed considering the bandwidth of the sampled original signal (which by definition has to be a bandlimited signal).

    Historically the term oversampling is linked to resampling processes that raise the sampling frequency by an integer factor (going back to the beginning of the CD era, when Philipps compensated the lack of a 16 bit DAC-IC with a 4 times oversampling and "noiseshaping" digital filter to avoid inferior technical data in comparison to Sony and other japanese manufacturers).

    Upsampling denotes a resampling process that raises the sampling frequence by an arbitrary rational factor.

    Ideally there is no new content generated during the up- and oversampling process although it is in a mathematical sense an interpolation process, but if we could meet fully the requirements of the Shannon theorem we would have the case of ideal bandlimited interpolation which means we were reconstructing the original waveform and generate new samples exactly matching this original waveform.

    In reality we can´t meet Shannon´s requirements and in addition we have quantized samples; amplitude quantization is an inherently nonlinear process and we have to calculate the new samples which means we have to consider limited wordlength, limited length in time of impulse responses of real filters, truncation and dithering and even noise shaping.
     
  12. DonH56

    DonH56 Addicted to Fun and Learning Technical Expert

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    My only nit is that oversampling does not in general mean resampling, at least to me. Resampling, which includes upsampling and may use oversampling, is sampling the data again, usually at a different rate, but sometimes just to resynchronize and isolate the incoming data stream. Oversampling can be just sampling the incoming (analog) signal (the first time) at a higher rate than needed to meet the Nyquist-Shannon theorem. In practice upsampling includes some form of interpolation, anything from linear to polynomial to various more complex functions (sinusoidal operations and such).

    One other vexing thing in ADC (and DAC) designs is that noise within the circuits (down at the transistor level) is typically both wideband and not bandlimited. For example, to accurately acquire a 20 kHz signal, your input circuits through the actual sampling switch will likely have much more bandwidth than 20 kHz. Say it's 40 kHz (probably higher but just say) and you sample at 40 kS/s; then, the noise from 20 kHz to 40 kHz is aliased to baseband (0 - 20 kHz) and adds to whatever noise was already there. Oversampling on the surface can help that by not aliasing as much noise, but remember the actual sampling circuits need even more bandwidth even if the buffers up to the sampler do not, so there will still be some additional noise.

    Finally, a reminder that by default an ADC and DAC mean something a little different to me than to the average audiophile. My context is at the device level, the acutal ADC/DAC inside the box and all the little transistors and passive elements inside the IC, not the box you plug your cables into to capture or create an analog signal. The box includes much more than just the basic ADC/DAC itself, like clock circuits, DSP/filters, and input/output buffers, though some or all of those may actually be inside the ADC chip itself.
     
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  13. mindbomb

    mindbomb Member

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    Upsampling reclocks the data, so the 8x upsampling should improve jitter performance, particularly with spdif. Usb is typically already asynchronous, so I'm not sure if that will benefit.

    I think maybe the internal power supply might be hurting the performance of the teac dac. There also is a cheaper model from teac, the ud 301, which seems like it would be pretty similar performance wise, but with less frills and cost.
     
    Last edited: Nov 13, 2017

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