[sdiy] A query about the TR-909 noise generator

René Schmitz synth at schmitzbits.de
Sat Dec 21 18:04:15 CET 2019


Hi Tom and all,

IIRC if n and m (and n>m) are taps for a maximally length LFSR, then n 
and n - m are also. So the pair 31 - 18 = 13....
(and you might not find half of the pairs tabulated.)

Best,
  René

On 21.12.2019 13:31, Tom Wiltshire wrote:
> I’ve been able to simulate the LFSR and I can confirm that it is a 
> maximal length 31-bit sequence. Not 32-bit as Roland claim in the 
> service manual. Sorry Roland!
> The only list I have of two-tap sequences for a 31-bit register gives 
> the following options:
> 31 28
> 31 25
> 31 24
> 31 18
> 
> We can now add:
> 31 13
> 
> It’s clocked at ~300KHz, so it’ll be very flat. I tested the 4070 
> XOR-based clock on the breadboard to find out how fast it was. I’ve got 
> an old ST HCF4070BE and an RCA CD4070BE hanging about, and the two chips 
> vary by about 3%, plus the variation you get from the 100p cap (could 
> easily be +/-10%). Given the cycle length of the LFSR, we have 119 
> minutes of noise before the cycle repeats. Plenty long enough!
> 
> I agree Richie, it’s an unexceptional design. But you know how people 
> are about anything x0x - it *must* be magic! That leads them to go 
> looking for 4006’s to try and build one, so I wondered if an exact 
> replacement might satisfy a few people.
> 
> Tom
> 
> ==================
>         Electric Druid
> Synth & Stompbox DIY
> ==================
> 
>> On 21 Dec 2019, at 11:49, Richie Burnett <rburnett at richieburnett.co.uk 
>> <mailto:rburnett at richieburnett.co.uk>> wrote:
>>
>> ---- Tom Wiltshire wrote ----
>>
>>> I can’t find any references to the two taps they’ve used though - 
>>> could 13 and 31 be a non-maximal sequence for a 32-stage LFSR? Can 
>>> anyone confirm this?
>>
>> I'm not a good enough mathematician to spot an M-sequence polynomial 
>> on sight, but you could always simulate it in C or MATLAB or something 
>> to determine the sequence length. You know the initial state, so you 
>> just need to count the number of iterations before that state comes 
>> around again.
>>
>> FWIW, the 909 noise source is pretty textbook stuff though. It's 
>> clocked pretty fast so it's flat out to above 20kHz and the sequence 
>> is long enough that you can't hear the repeats even at such a high 
>> clock rate. There's not much quirky or interesting going on there, (in 
>> my opinion.)
>>
>> -Richie,
> 
> 
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