Look what I've found in my DX7...

[EarthmanVIII]

An analogue low pass filter after the DAC.

Here is the excertpt from the service manual:

12. DAC
A BA9221 DAC is used. The DAC converts the digital waveform data from the OPS to an actual analog waveform. The amplitude scale factor of the analog waveform is controlled via SF0 ~ SF3. This signal is then fed to the sample & hold and low-pass filter circuits from which it is sent to the output terminal. A reference voltage is applied to pin 14 of the DAC. 8 reference voltages are generated by the muPD405 1, and the total level is externally
controlled.

And a fragment of the diagram:



Uploaded with ImageShack.us

What do you think? Maybe this is why FM8 vsti doesn't fully capture the DX7 sound? Those who believe a DX7 is the same thing as a softsynth only crammed into a differently looking box, what do you think about it in this light?

First I learn that DX7 is not an FM synth but it uses PM synthesis and now it appears it is not digital but a hybrid.

[rhino]

Welcome to our crazy world.

Nearly all digital audio devices have some form of low-pass filtering in the output. This is needed to block the clock noise from the DAC.

[EarthmanVIII]

You're trying to spoil my geek moment

[piRoN]

Nearly all digital audio devices have some form of low-pass filtering in the output. This is needed to block the clock noise from the DAC.

And to block the quantise aliasing from the DAC.

[Roby31]

First I learn that DX7 is not an FM synth but it uses PM synthesis

Not really big news with DX7 using PD/PM, if I can add

[haj]

You're trying to spoil my geek moment

[nvbrkr]

I think the prices of DX7s just went up on ebay.

[SleepingInsomniac]

I think the prices of DX7s just went up on ebay.

.. i had a feeling some one was goin to post that
nice find earthman.

[Hayte]

That filter is an anti-imaging (reconstruction) filter. Its part of the DAC. You can't hold that against FM8 or any softsynth for that matter because the DAC in your soundcard also has an analog anti-imaging filter.

[Sir Ruff]

So the DX7 suddenly become totally appropriate content for the Analog Heaven mailing list. Props to the first person who posts this there!

[guitarsandsynths]

The output jack outputs an analog signal, thus the DX7 with it's LPF is a subtractive analog synthesizer. I have a few for sale for only 2,700 US dollars, but I'll also take Euros, Credit cards, gold, service or good drugs.

[Mooger5]

There may be a point, if the LPF is there to prevent high-frequency hash above 20k from "corrupting" the audioband by means of intermodulation distortion. So what would happen if you bypassed the filter completely or just lift up the cutoff point? It´s a crazy idea, but it reminds of the trend in filterless hifi dacs that claim to actually sound less digital (as in cold and sterile) without a filter. The theory is that the sound should get less constrained by being allowed more room to "breathe" thus turning livelier and punchier. Even if it´s spilling out ultrasonic garbage.

Deliberately or not, that was the case of the Linn LM-1 design. From a Hollow Sun page:

"Technically, the sonic specs weren't at all impressive. Low bit-depth and a mere 27kHz sample rate appear like a joke in today's world of 32-bit / 96kHz (or more) samples but there is no denying that the LM-1 had a great sound. Roger himself admits that the circuit design was less than perfect. However, it is this that could be the key to the Linn's sound - whereas other engineers would have used a 'brick wall' filter to remove the possibility of any unpleasant high frequency digital audio artefacts creeping in, Roger didn't which let more HF through. Any artefacts were, by and large, masked by the sound itself and so the LM-1 samples sounded better (and brighter) than the paper specs would have you suppose."

My 2 geeky cents

[cornutt]

First I learn that DX7 is not an FM synth but it uses PM synthesis

Not really big news with DX7 using PD/PM, if I can add

As long as the modulating waveform is a sine wave, PM and FM are the same thing. That's the main reason that the DX7-era FM synths only have sine wave operators. It's actually one of the main points of the Chowning patent.

[EarthmanVIII]

Do you mean DX7 is an FM synth while TX81z is FM or PM depending on what waveforms you're using?

[Synthaholic]

Both are technically PM synths but it only comes into play when using the non-sine waveforms on the TX81Z.