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 Forum index » DIY Hardware and Software » Thomas Henry designs
SN76477 noise filter
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Pehr



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PostPosted: Sat May 24, 2008 11:53 am    Post subject: SN76477 noise filter Reply with quote  Mark this post and the followings unread

How should the noise filter in the SN-voice sound like?

I have just built one and it works good except for the filter (I think). I sounds like a high pass and if the noise init is turned low, it has just a little or no effect at all.

Does anyoe have a sound example? is it a high apss filter or low pass?

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sneakthief



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PostPosted: Sun May 25, 2008 2:26 am    Post subject: Reply with quote  Mark this post and the followings unread

that's normal behaviour on all 3 of my sn-voices.
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Peake



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PostPosted: Sun May 25, 2008 8:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here 'tis.
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Tim Servo



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PostPosted: Sun May 25, 2008 8:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey Peake,

Correct me if I'm wrong, but that sounds more like you're modulating the VC Noise Clock. The Noise Filter (R54) is a non VC passive filter and yields a more subtle change to the sound. Right? Wrong?


Tim (has been wrong once or twice...) Servo
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Peake



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PostPosted: Mon May 26, 2008 11:36 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh- I'm wrong Cool

I'd thought "filter" may have not been literal. Looks like I'll have to buy an SN-Voice board and see for myself... I was looking at ATARI POKEY specs yesterday and they use logic to mimic a high-pass filter effect- I made the ASSumption that something similar may occur in the TI chip as well, and posted away.
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State Machine
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PostPosted: Wed May 28, 2008 4:03 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Does anyoe have a sound example? is it a high apss filter or low pass?


Hi,

Just a suggestion. Could you post a sound example of what yours is doing? Then we can compare the sounds of our units Wink

Thanks,
Bill
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Pehr



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PostPosted: Thu May 29, 2008 8:47 am    Post subject: Reply with quote  Mark this post and the followings unread

I could, I'll record later today or tomorrow and post here.
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State Machine
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PostPosted: Thu May 29, 2008 9:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
could, I'll record later today or tomorrow and post here.


OK, I can make a comparison to my SN voice noise filter Cool Thanks !

Bill
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Pehr



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PostPosted: Thu May 29, 2008 9:24 am    Post subject: Reply with quote  Mark this post and the followings unread

00:00 - 00:23 Noise init MAX, sweeping the filter from low to high and back.
00:24 Canging the Noise init to center.
00:30 - 00:44 Noise init Center sweeping the filter from low to high and back.
00:51 Canging the Noise init to minimum.
00:55 - 01:12 Noise init MIN sweeping the filter from low to high and back several times.

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State Machine
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PostPosted: Thu May 29, 2008 7:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

This sounds like normal behavior to me. The noise filter acts as a Low Pass Filter (LPF). Turning the noise filter pot CW shifts the center frequency, or the filters 3 dB point, lower in frequency thus cutting off more and more energy of the noise spectrum.

The Noise INIT controls the VCO (CD4046) that provides the noise clock to the SN76477 chip. This clock is used by the chips internal digital PR Noise Gen. When you go MIN on the INIT causes the clock to drop significantly in frequency and thus the minimal effect that the filter pot has on the signal.

It's interesting to note the sound of the noise generator when the INIT is set to maximum. The generator is being clock at the highest frequency. In doing so, you will notice a distinct lower frequency pattern to the sound within the higher frequency noise. The reason this is present is because the PR generator has a specific "run length" and the pattern will repeat every so many counts using the following equation (2^n-1) where "n" is the bit width of the shift register used in the PR noise generator. So if the shift register were 10 bits, the run length would be 1023 counts of the noise clock. If the noise clock were about 1 KHz, you would hear a distinct pattern of sound at about 1 HZ .......... Very Happy I hope I explained that OK ......

Bill


Hope this helps !
Bill

Last edited by State Machine on Fri May 30, 2008 4:09 am; edited 1 time in total
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fonik



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PostPosted: Fri May 30, 2008 12:06 am    Post subject: Reply with quote  Mark this post and the followings unread

State Machine wrote:
Very Happy I hope I explaned that OK ......

as always, bill.
BTW tom wiltshire recently released his PIC based noise gen which pattern will repeat in ...err... 142 million years Shocked n

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Pehr



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PostPosted: Fri May 30, 2008 1:24 am    Post subject: Reply with quote  Mark this post and the followings unread

State Machine wrote:
It's interesting to note the sound of the noise generator when the INIT is set to maximum. The generator is being clock at the highest frequency. In doing so, you will notice a distinct lower frequency pattern to the sound within the higher frequency noise. The reason this is present is because the PR generator has a specific "run length" and the pattern will repeat every so many counts using the following equation (2^n-1) where "n" is the bit width of the shift register used in the PR noise generator. So if the shift register were 10 bits, the run length would be 1023 counts of the noise clock. If the noise clock were about 1 KHz, you would hear a distinct pattern of osund at about 1 HZ .......... Very Happy I hope I explaned that OK ......


Interesting Smile However, I'm not sure that's what I hear. There seems to be a glitch, perhaps crosstalk, from the LFO to the noise init. When I change the LFO rate, the sound of the noise changes too...

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PostPosted: Mon Jun 02, 2008 3:32 am    Post subject: Reply with quote  Mark this post and the followings unread

Anyway...

http://www.99.se/attachments/diy-bygg-reparation/16442d1212083600-modularbygge-sn-voice.mp3 Smile


.

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PostPosted: Mon Jun 02, 2008 5:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Anyway...


Cool ending . it's like the floor dropping out from under the SN Voice. It really generates some very cool bass tones. Is that random Klee sequencing going on there?

Will have to get back to this LFO interference. It might be some bleed over into the CD4046 VCO.

Bill
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Pehr



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PostPosted: Tue Jun 03, 2008 12:02 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks! Yes, it is the klee controlling the pitch, gate and mix A and C on the SN-voice Smile

So are anyone else experiencing the trouble with the LFO bleed to the noise?

Perhaps I could try and replace the PLL and see what happens.

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Last edited by Pehr on Wed Jun 04, 2008 12:51 am; edited 1 time in total
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State Machine
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PostPosted: Tue Jun 03, 2008 3:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
So are anyone else experiencing the trouble with the LFO bleed to the noise?


I will take some time this evening and pay close attention for this and let you know. Wink I will get some noise going and vary the LFO and listen for any interference.

Quote:
Perhaps I could try and replace the PPL and see what happens.


I suppose you meant the PLL Wink Unlikely that it's the cause, if it's in a socket, you can try it. better yet, if you have one from a different manufacturer, that would be better. Mine is an RCA chip but I will report what I have in mine when I give you my results.

Bill
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Pehr



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PostPosted: Wed Jun 04, 2008 12:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, PLL of course doh Razz
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PostPosted: Wed Jun 04, 2008 4:03 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Yes, PLL of course


Sure .... Very Happy I bogged down with computer repairs last evening but promise I will get a chance to check this for you within the next 1 to 2 days .... Boy, seems like we all have busy lives these days ....... Rolling Eyes Robin and I have been house hunting also ........ I really need a full finished basement to support all our inetrests Cool

OK, you will hear from me Wink

Bill
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Photon



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PostPosted: Wed Jun 04, 2008 6:19 am    Post subject: Reply with quote  Mark this post and the followings unread

State Machine wrote:
Robin and I have been house hunting also ........ I really need a full finished basement to support all our inetrests Cool


We're in the middle of buying a house right now. The days of doing this stuff in my apartment dining room are coming to an end Very Happy
Of course, now theres no money to put towards DIY! Sad

Pehr, your noise filter sweep sample sounds very similar to my SN-Voice. I'll try to compare your sample to mine soon and let you know.


Peter
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fonik



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PostPosted: Wed Jun 04, 2008 7:06 am    Post subject: Reply with quote  Mark this post and the followings unread

State Machine wrote:
Robin and I have been house hunting also ........ I really need a full finished basement to support all our inetrests Cool

you will enjoy it, believe me. dot is a high school teacher doing a lot of work at home, paper used to be almost everywhere in our flat and i was soldering right upon the computers keyboard. the house we bought this year is a true relief for both of us... so good luck, bill.

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RF



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PostPosted: Wed Jun 04, 2008 7:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

Jeese - it's moving time in the synth diy community. Jennie and I just purchased a house with a full finished basement. We have a music room - and I got a dedicated room for building stuff. Smile

Course, now I don't have time for any building....and the music eq. is still in boxes...
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State Machine
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PostPosted: Thu Jun 05, 2008 5:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Fonik, RF, Photon ........... wow , I am not alone .. House hunting is sometimes nerve racking right? But fun at the same time. Its an adventure of seeing homes and bargaining with people.

Thanks for the good wishes and good luck to you guys with your new homes ! Very Happy I am looking very much to having more living space and dedicating a finished basement area for our arts and crafts. Having the living space look more like a "living space" rather than a lab is a nice prospect. Very Happy

OK, I will get back OT ....

Bill
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