Getting closer to the schematics, I notice a major difference between the TR909 and the Nava... Vincent, please correct me if I am wrong.
In the original TR909 design, the sequencer drives 4174 6-bit D-latch registers, which are used as DAC with series of resistors. The 6-bit value issued of each 4171 is latched for the duration of each step. If the instrument is not triggered, the accent control signal keeps the value of the last step in which the instrument was played.
The CD4051 is not a latching demultiplexer. The value is driven on each step by the sequencer. However the value is not maintained. A capacitor between the output of the 4051 and the non-inverting inputof the accent op-amp (i.e. C89 for the Bassdrum) produces a decay on the accent signal. Moreover, if the instrument is not triggered on a step, the accent takes the maximum value instead of the previous value.
The rest of the analog design is the same.
The snapshot below shows the signal driving the accent of the bassdrum of the TR909 in red and of the Nava in blue (DC coupling).
The snapshot below shows the ENV1 of the bassdrum of the TR909 in red and of the Nava in blue (DC coupling).
The snapshot below shows the accent and trigger signals of the BD of the Nava, respectively in red and blue.
The pattern shown in the waveform is as follows: 1:ON+ACC 2:OFF 3:ON 4:OFF 5:ON+ACC 6:OFF 7:ON 8:OFF 9:ON+ACC 10:OFF 11:ON 12:OFF 13:ON+ACC 14:OFF 15:ON 16:OFF
To my analysis, there are bugs:
1. My primary question is about the use of CD4051 as non-latched demultiplexors:
Assuming that the use of the CD4051 with non-latched outputs was intentional, the select bits (MUX_A,MUX_B,MUX_C,INH_MUX1,INH_MUX2) shall actually coutinuously round to keep the value stable using the input capacitors of the accent circuits. However looking at the waveforms of these signals, this is not the behavior of the software. It appears that the outputs of each CD4051 are selected only once per step.
2. The peak level of the accent of the Nava does not match the level of the accent of the TR909.
3. The accent level shall be maintained on steps where the instrument is not played