The Yamaha DX7 keyboard seems to have a quite sophisticated DAC and I wonder how it works. It is mostly specified as 12-bit DAC running at a sampling rate of 50kHz, but this doesn't tell the whole picture it seems.
The broad bus is the main system data bus, the small busses coming in from the left source in the waveform generating digital IC. The analog audio signal exits by the bottom left and are mixed together later.
There is in fact an BA9221 12-bit DAC IC. But it does not run at 50kHz, but actually 16 times as fast, as the 16 voices the DX7 can play simultanously are time-multiplexed on this IC. This can be easily seen by scope.
However, the BA9221 reference voltage is not fixed but can be controlled by a resistor network wired to an multiplexer (IC46) by the system's digital data bus lines D0 - D2 . No clue on what this depends. The smaller sister model keyboard DX9 seems to lack this multiplexer.
The BA9221 output is fed into another resistor network whose outputs are connected by a digitally controlled switch (IC41), controlled by the digital oscillator IC via the signals SF0-SF3. These signals are individually switched by the voice multiplex frequency and seem to depend on the actual waveform played, each line exhibiting a different pattern. So we can expect this to modify the original signal in a multiplicative way I guess.
The is finally switched again by another digitally controlled switch (IC52). There are two outputs generated, these will mix down to a single signal later (not pictured), there is only one mono output. After analog volume control and amplification the signal is feed to the output jack.
So I would like to know how this exactly works and how to describe its characteristics in nowadays terms. It is obvious that a single channel 12bit signal won't reproduce the signal faithfully. And I wonder why this is so complicated and what is achieved by the all those additional parts.