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 Forum index » DIY Hardware and Software
LF398 rate reducer / bit crusher problem
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diablojoy



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PostPosted: Fri Aug 17, 2012 3:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
i found a design but yet again i dont know how its sounds.


I would expect that all simple sample and hold circuits would give very
similar results when used in the audio range
I just tried my MFOS one , very much the same story though it isn't optimised for audio use as the hold cap is too large and the sample rate a little too slow .

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isak



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PostPosted: Fri Aug 17, 2012 3:50 am    Post subject: Reply with quote  Mark this post and the followings unread

I hope it could be done and make the bleed through go away.
We need something like ...mmm... EG ?
If we could some how have a gate in, in the crusher.
Or something similar.
Or even have a simple vca with a simple eg?
Just a thought..

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diablojoy



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PostPosted: Sat Aug 18, 2012 1:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Dont think any EG or VCA will do what we need as we are actually trying to remove something that has become inherent to the frequency spectrum of the outputted signal , we need to filter out a very specific frequency, that of the sample rate which unfortuanately is a moving target
to do that we need a notch filter that will track the clock frequency
So i started looking into an LTC1063 switching filter which gives us approx 30 db notch around the targeted frequency
would mean oscillator rate needs to be 100 x faster to clock LTC1063 filter
there is a 100:1 clock to cutoff frequency ratio
the clock would also then need to be divided by 100 [2 cascaded 4017] to clock the LF398 at the same rate it is currently which means
there is more circuitry required to remove the signal than is used to create it in the first place Haha
alaising may become an issue as well though not certain how bad that may be yet
on broad spectrum signals however there will be very noticeable effects and they maybe as bad as the original bleed through, Ahh i probably should stop calling it that as its not actually bleed through but the overlaid quantizing frequency so more sort of an FM effect really
also LTC1063 is not a cheap chip $8 to $10 and i dont have any on hand at the moment.... ordering some now
While in the end i am fairly sure i can minmise the sampling frequency from the output i am not at all sure if the result will be better just different
who knew a simple circuit for introducing quantizing distortion would get so involved Laughing
I have quite gone off the idea for a dual board so back to single it is
thinking 2 signal inputs , 2 signal attenuator controls , 1 sample rate control
one filtered output and a non filtered output . still debating on a CV'able sample rate it adds a lot of complexity for maybe not that much gain.

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bugbrand



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PostPosted: Mon Aug 20, 2012 12:11 am    Post subject: Reply with quote  Mark this post and the followings unread

There are two parts to making an audio-rate S/H quiet (BugCrusher is *just* a specific S/H design - really simple)

By the way, I gave up using AD781 - too hard to source, requires bipolar supply and is too $$$. Look at other S/H designs - simply a switch, a storage cap and a buffer amp. Really, you can do it super simple - hint, I now use the 4066 cmos switch!

It all comes down to:
i) getting the sample pulse narrow enough to sample, but not any wider. The 22pF and 22k you're mentioning is simply making a narrow pulse from the regular squarewave. Try increasing the value of the cap - eg. 33pF, 47pF
ii) decent layout - bleed can be caused by poor routing of signals, causing cross-coupling etc. As I say, the design is super simple, but to get good results you have to lay it out nicely. (same as with most work...)

Hope that helps!
Tom

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diablojoy



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PostPosted: Mon Aug 20, 2012 3:08 am    Post subject: Reply with quote  Mark this post and the followings unread

ORSM thanks tom so glad you popped in on this Shocked Cool
yes those borrowed values of 22pf and 22K made for far too a narrow
pulse when using LF398 ended up with 100pf and 100K which seem to be ok , possibly could go lower a little bit maybe 68K instead of 100K
so far i tried this on strip board to help out Isak as that was where he was at and had run into problems and then typically i got interested as well.
anyway to me at least, it now sounds like what i expected it should and
I can see on the scope it is doing what it is supposed to be doing - nice clean discete steps.
Isak asked if it would be possible to lose the sampling frequency from the output that we can hear, now i could do a board layout ok now , but i am not totally convinced what we can hear is actually bleed through of the clocking rather more of an overlaid frequency made up from the discrete steps of the samples [ not seeing any spikes or pulses in the output stepped waveform] , hence why i was looking at filtering the output.
of course i could be totally wrong about that so it would be nice to hear your thoughts on this and if you had any similar frequency show up on your original bugcrusher or any other SH used at audio rates for that matter . I know the MFOS SH does but then its not a good example as it was never really intended for audio rates but it does get there just enough to hear some extra low frequency added in that is not part of the original sampled signal but is related to the sampling frequency
cheers
denis

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isak



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PostPosted: Wed Aug 22, 2012 3:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi diablojoy

i think i found something interesting...
http://electro-music.com/forum/phpbb-files/40106vco_954.png

do you think we can use it somehow?

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diablojoy



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PostPosted: Thu Aug 23, 2012 12:58 am    Post subject: Reply with quote  Mark this post and the followings unread

maybe
need to check if the pulse width out is ok for the lf398
otherwise good simple CV control of clock rate
nice usefull down ramp output as well

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isak



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PostPosted: Sat Aug 25, 2012 12:44 am    Post subject: Reply with quote  Mark this post and the followings unread

I have found something even more interesting!
Check this out.
http://circuitworkshop.com/forum/forums.html?PHPSESSID=bad8c33acc816aeeabd39e0f8189b1c7&topic=802.0
Smile

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diablojoy



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PostPosted: Sun Aug 26, 2012 2:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

hmm i get a 404 message on that link.
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isak



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PostPosted: Mon Aug 27, 2012 1:13 am    Post subject: Reply with quote  Mark this post and the followings unread

i get that too but read the post its very interesting.
for sure you will have a sort of Idea
Cool

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diablojoy



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PostPosted: Tue Aug 28, 2012 1:13 am    Post subject: Reply with quote  Mark this post and the followings unread

finally managed to get to that forum
and now i can see where you were headed with that EG idea
of course stopping the clock when there is no input signal
means there will be silence then, but when there is an input signal
the clock frequency will still be audible, especially if the clock frequency is being varied ie by a CV at the same time
they end up seemingly making it all very complex
I do have one version almost layed out , a very basic dual board but the clock frequency will still be audible on that one i think.
I shall see how that goes first then look at trying another design with a tracking notch filtered output.

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isak



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PostPosted: Tue Aug 28, 2012 1:32 am    Post subject: Reply with quote  Mark this post and the followings unread

Very cool!
Thank you very much for the help and for not letting go!
Expecting and Waiting for your new findings!

Thanks again,
Cheers,
Isak E.

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bugbrand



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PostPosted: Tue Aug 28, 2012 3:54 am    Post subject: Reply with quote  Mark this post and the followings unread

I'll say again --- think SIMPLE (& efficient)

If you're working with bipolar supply you could also use the (great) DG4xx series chips - DG442 springs to mind.

Audio (S&H) route is:
Input buffer, switch, cap, output buffer

Switch clock route is:
Comparator, pulse conditioner (then connects to switch control input)

Two or three ICs and a handful of components!

Bests!
Tom

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diablojoy



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PostPosted: Tue Aug 28, 2012 5:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
I'll say again --- think SIMPLE (& efficient)

2 x LF398 , 1 x CD40106, 2 x TL072 plus some discretes for a dual module
so reasonably simple .
The original question involved a circuit using an LF398 and I found once i went looking i actually had about 30 x LF398 hiding in a draw dont remember when or why i bought them
so i decided to stick with that chip.
As far as a filtered version goes to me it's more of an experiment and a challenge to see if it can be done, I was asked if there was a way and i agreed to look into it. personally i like the addition of the frequencies added by the sampling rate, especially if then put through a low pass
filter and the filters cutoff frequency is CV'd.

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roglok



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PostPosted: Tue Sep 04, 2012 2:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

I also built the LF398 bugcrusher circuit and ended up with the same clock "bleed" problems that others have reported.

After reading through all the posts I could find on this topic, I finally abandoned the rate reducer idea and decided to build a stripboard version of Rene Schmitz's YASH circuit (which J3RK has referred to earlier in this thread) with my remaining LF398.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Lo and behold! The YASH, as is, does a great job as a rate reducer! I can barely hear the clock rate on the output. My guess is that not only a narrow clock pulse is needed, but also one with a low amplitude (check the 5K6 and 1K8 voltage divider before the LF398 clock input)...

The YASH doesn't have a CV input for clock rate, but I succesfully used a VCO as an external clock. When shorting the 470pF cap, the YASH becomes a Track & Hold, which can also sound pretty cool at audio rates.

jackson dancing

EDIT:
I'll try to record some samples as proof in the next days. Wink
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diablojoy



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PostPosted: Tue Sep 04, 2012 4:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

thanks roglok
good useful info especicially the 3 usec pulse width note
good figure to aim for, the lowered pulse voltage i had already sorted
i am also incorporating cv inputs for the clock rates .
I could easily still add pads for a connector for a switch to implement a track and hold mod, nice idea.

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Dogenigt



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PostPosted: Tue Feb 25, 2014 4:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

I built the bugcrusher based on LF398 and used MAX1044 for negative supply - using only 1 9V battery.

I had the same bleeding of the clock problem but managed to get rid of it somehow, but now when it's time to solder it up I can't remember what I did cause the bleeding is back.

Here's a sample of how it sounds without the annoying clock-bleed. (lovely btw!) source sound is a tape dictaphone so don't mind the noise.


RateReducer_test_tape_input.WAV
 Description:

Download
 Filename:  RateReducer_test_tape_input.WAV
 Filesize:  27.5 MB
 Downloaded:  50 Time(s)


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