<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Hi Richie,<div><br></div><div>I was referring to this article in Sound on Sound: <a href="http://www.soundonsound.com/techniques/mqa-time-domain-accuracy-digital-audio-quality">http://www.soundonsound.com/techniques/mqa-time-domain-accuracy-digital-audio-quality</a>. One of the best articles I read in a long time!</div><div><br></div><div>Regards,</div><div><br></div><div>Rutger</div><div><br></div><div><br><div><div>On 21 apr 2017, at 14:49, <a href="mailto:rburnett@richieburnett.co.uk">rburnett@richieburnett.co.uk</a> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>Which issue of SOS?<br><br>On 2017-04-21 13:02, Rutger Vlek wrote:<br><blockquote type="cite">Hi Richie,<br></blockquote><blockquote type="cite">Indeed it concerns FM, mostly. And perhaps, theoretically, the low<br></blockquote><blockquote type="cite">end tightness. I recently read a great article in sound on sound about<br></blockquote><blockquote type="cite">a new HD audio codec that aims to improve aliasing as well as low<br></blockquote><blockquote type="cite">frequency phase issues and ringing that normally come with a typical<br></blockquote><blockquote type="cite">anti alias filter in or post a dac. I assume novation's approach also<br></blockquote><blockquote type="cite">improves that. Although i have no idea how therotical this is: if it<br></blockquote><blockquote type="cite">can be measured, if it makes an audible difference.<br></blockquote><blockquote type="cite">Best,<br></blockquote><blockquote type="cite">Rutger<br></blockquote><blockquote type="cite">Op 21 apr. 2017 13:13 schreef <<a href="mailto:rburnett@richieburnett.co.uk">rburnett@richieburnett.co.uk</a>>:<br></blockquote><blockquote type="cite"><blockquote type="cite">Yeah, it's an interesting way to tackle the aliasing issue. As Tom<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">and others pointed out, raising the sample rate up to 24MHz doesn't<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">completely eliminate aliasing for naïve generated sawtooths and<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">pulses, but the high sample rate means that by the time the<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">harmonics alias back into the audio spectrum they're at least 60dB<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">down, and probably more like 75dB down for the highest pitches that<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">people typically play in a music intended for human consumption<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">(4kHz/24Mhz).<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">The NCO/DDS technique isn't new. It was used in the SID, Paul<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Maddox's excellent Monowave and 002 synths, and I believe it was<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">used in the "DCO" chip in Roland's alpha-juno series of hybrid<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">synths back in 1985. I've had discussions with Tom W about this<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">off-list and did some tests on my alpha juno and we came to the<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">conclusion that the "DCO" chip in these synths generated the<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">waveforms digitally using an NCO outputting at a sample rate of<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">6MHz. So I guess that the increase of sample rate from 6MHz to<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">24Mhz in 32 years isn't unfeasible!<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">With the alpha juno you can clearly see the aliased harmonics of<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">very high pitched notes on a spectrum analyser at about -58dB<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">relative to the fundamental. However, I believe there's also a<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">trick where you can apply a *very tiny* amount of random Frequency<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Modulation to the NCO that causes imperceptible pitch modulation in<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">the audio part of the spectrum. However, it is enough to spread the<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">energy in those upper harmonics around 24MHz (that alias into the<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">audio band) so that they blend into each other and produce a lower<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">and more even white noise floor rather than a series of<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">objectionable inharmonic spikes across the audio spectrum. I would<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">imagine that Novation are doing this too, since -60dB alias<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">suppression is nothing spectacular compared to what can be achieved<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">at a sample rate of 96kHz using BLIT/BLEP/whatever on a general<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">purpose DSP.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Rutger wrote "I'm particularly interested in how high-frequency<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">interactions between NCOs are going to sound on this machine. I<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">suspect FM and RingMod will be pretty smooth, as these interactions<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">make aliasing effects more pronounced." I think you are right and<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">this is where the technique actually shines. RingMod isn't a<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">problem, but FM produces an infinite spectrum of harmonics, so<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">whatever sample rate you use it will alias to some extent, but the<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">higher the better obviously.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">The part that I thought was particularly clever was the dithering of<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">the oscillator waveforms at 24MHz sample-rate down to a single-bit<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">"bitstream" / "DSD" output within the FPGA, to mitigate the need for<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">a high-speed multi-bit DAC that can settle fast enough to handle the<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">24MHz sample-rate. Again, the dithering and noise-shaping stuff<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">isn't anything new, but it all seems to fit well together as a<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">package for their application.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">As for the reverb being done in the FPGA, that's almost surely the<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">best way. General purpose DSP chips aren't optimised for reverb<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">(except maybe the FV-1 and Wavefront chips) but you can build the<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">DSP core that you wish you had with an FPGA and make it optimised<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">for hundreds of allpass filters or whatever you want for your<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">reverb.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Definitely an interesting time for synth design :-)<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">-Richie,<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">_______________________________________________<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Synth-diy mailing list<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><a href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><a href="http://synth-diy.org/mailman/listinfo/synth-diy">http://synth-diy.org/mailman/listinfo/synth-diy</a> [1]<br></blockquote></blockquote><blockquote type="cite">Links:<br></blockquote><blockquote type="cite">------<br></blockquote><blockquote type="cite">[1] <a href="http://synth-diy.org/mailman/listinfo/synth-diy">http://synth-diy.org/mailman/listinfo/synth-diy</a><br></blockquote><blockquote type="cite">_______________________________________________<br></blockquote><blockquote type="cite">Synth-diy mailing list<br></blockquote><blockquote type="cite"><a href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a><br></blockquote><blockquote type="cite"><a href="http://synth-diy.org/mailman/listinfo/synth-diy">http://synth-diy.org/mailman/listinfo/synth-diy</a><br></blockquote></div></blockquote></div><br></div></body></html>