[sdiy] TB303 Slide

Robin Whittle rw at firstpr.com.au
Sun Jun 10 10:22:52 CEST 2012


Hi Richie and others,

I just added this to my page:

  http://www.firstpr.com.au/rwi/dfish/303-slide.html

     In June 2012, the subject of TB-303 Slide and Gate times
     has arisen on the Synth DIY mailing list:

        http://dropmix.xs4all.nl/mailman/listinfo/synth-diy

     Perhaps some of what I wrote below is wrong.  I plan to review
     the discussion and update this page accordingly.

I don't have time right now to participate or read all the messages.


  - Robin


> The confusion about the TB-303 gate pulse timing can be explained as
> follows:
> 
> In 'normal mode' a gate pulse for a 16th note coming out of the micro
> goes high for 3 cycles of the tempo clock and then goes low for the
> remaining 3 cycles of the tempo clock.  This gives a 50% duty ratio for
> the gate signal where it originates from micro, but this is not what is
> seen by the outside world either at the gate output or audio output
> jacks.  Transistor Q36 turns on quickly but delays the falling edge of
> the gate signal by roughly12ms due to the action of C72.  The unequal
> propagation times for rising and falling edges noticeably stretch the
> note lengths from the internal sequencer.  12ms might not sound like
> much but it is easily noticeable on a pattern of 16th notes playing at a
> fast tempo.  12ms is about half of a tempo clock period at 120bpm, so it
> appears as if the gate output goes low shortly after three-AND-A-HALF
> tempo clock cycles.  This is the impression that a casual observer would
> get looking at only the DINSYNC clock along with gate out or audio
> output, instead of looking at the internal IO lines from the micro as
> Colin Fraser has done.  As Colin says the state changes are
> approximately synchronised to the active-going edges of the tempo clock
> only, it just happens that the subsequent circuitry delays the gate
> line's falling edges somewhat and lengthens the notes.
> 
> It's also worth noting that the gate signals from the micro are only 50%
> duty ratio in normal mode.  In the case of 'triplet mode' the gate line
> from the internal micro goes high for 5 tempo clock cycles and returns
> low for the remaining 3 tempo clock cycles.  5 out of 8 gives a duty
> ratio of 62.5% for 16th note triplets.  Then these gate pulses again are
> stretched by the unequal propagation times.
> 
> This behaviour is consistent for both of the 303's i've checked here.
> AudioRealism and Phoscyon fail to get these things right in their
> plugins though.
> 
> -Richie,
> 
> 
>>> Note in there he suggests the gate on/off times are not
>>> 50:50, but 3.5:2.5 (out of 6!) - however this 'scope shot':
>>>
>>> http://ultra303.ultrafex.de/twonotesslide.jpg
>>>
>>> seems to contradict that, showing it to be 50:50.
>>
>> It's 50:50 on my scope too.
>> Robin's info is not entirely accurate.
> 
> 
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