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Comment by richard_shelton | Hacker News Reader

In DX7 the actual sound was made with custom LSIs. I.e. there is no CPU involved. And no multiplication inside, Japanese engineers used log/exp sine tables instead. Also DX7 had sampling rate of 49096 Hz, 12-bit "floating-point" DACs etc etc. Most of software versions of DX7 don't take these nuances in account which makes some DX7 purists upset :)

If you, as a DX7 clone developer, want to make happy these 2 or 3 people too, please, use fixed point calculations, resampling, real ROM data and so on :)

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"No CPU involved" is not strictly true, as the CPU is doing the LFO. You can observe this by, for example, rapidly moving the data slider and noticing that the LFO slows down.

I considered doing a super-accurate "dirty" mode but ultimately decided against, as most of these changes would make it sound worse. One important subtlety is that after the DAC (which is just sample-and-hold) there's a Sallen-Key low-pass filter tuned at about 16kHZ, to round off the worst of the DAC artifacts. Many digital replicas (my own included) miss this, which gives a harsh brittleness to the sound.

I know dexed added multiple bit depths (this work was done after mine, and I haven't followed their fork carefully). I don't know about the other aspects of the signal chain though.

I get where you are coming from, though, and think it would be a good project for a super-dedicated contributor.

Thank you! Didn't knew that about LFO. And I just found your wiki page [1], it's very informative. So I would like direct any curious reader there :)

[1] https://github.com/google/music-synthesizer-for-android/blob...

>One important subtlety is that after the DAC (which is just sample-and-hold) there's a Sallen-Key low-pass filter tuned at about 16kHZ,

You mean IC54 and IC56? I noticed them on the circuit but didn't know it was tuned SO LOW! Wasn't the sample rate over 50KHz for each voice? 16Khz looks overkill for such sample rate. I know this is a time-division-multiplexed DAC so pehaps the time division multiplexing generates even more artifacts of their own?

I would love to see a circuit that replaces the whole DAC section with 16 separate DACs that then mix their sound on the analog domain...

I recall seeing a comparison of DX7 vs dexed patches on Youtube that mostly found audible differences in envelope lengths, not timbre. That seems like something more straightforward to fix, though there's always the potential for a caveat like it being tied to the original hardware sample rate.

>Also DX7 had sampling rate of 49096 Hz, 12-bit "floating-point" DACs etc etc.

Note that the output wasn't 12-bit. There was a 12-bit DAC chip, but additionally to those 12 bits there are additional bits that are fed to another DAC (implemented using resistors) in charge of doing "gain-ranging" (that is, adjusting the output level of that channel). Thus the dynamic range available was much higher than just 12-bit.

>Most of software versions of DX7 don't take these nuances in account which makes some DX7 purists upset. If you, as a DX7 clone developer, want to make happy these 2 or 3 people too, please, use fixed point calculations, resampling, real ROM data and so on :)

On the other hand, i play the hardware DX7 (the real thing) plus a DX7IIFD, and i'd rather have the developers produce a cleaner version of the DX7, with a much larger/longer sine table, and more nuances to the sound. On the 'net, many claim the DX1 and DX5 sounded much more cleaner (not just "less background noise" but "sweeter sound"); i wish we could aim for that.

Yes. Or we can aim for a software FM synth with analog feel in sound of Synclavier and versatility of FS1r :)

By the way, Yamaha FS1r is the only hardware FM synth that I currently have. Several years ago I've implemented singing synthesis after reading Yamaha's patent on FS1r. The synthesis was implemented on a specialized multicore chip and we even showed the result to Yamaha representatives. They liked the sound, but they were not interested in the chip. Anyway, it's a different story :)

We'll see an FPGA implementation soon enough :-)

Really? Good to know! Maybe one day a few stubborn sound designers will replace Yamaha modules from their rack with these FPGA replicas :)

I would love to have a FPGA that i could drop inside my DX7 and replaced the OPS operator IC for a more advanced (more nuanced sound, purer sine waves) version !

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