[sdiy] Audio Anti Aliasing Filter

Fri Dec 21 17:01:35 CET 2012

Hi!

You can find Adapters for example on Ebay:

http://www.ebay.at/sch/i.html?_from=R40&_trksid=m570&_nkw=tssop24+adapter&_sacat=See-All-Categories

Ciao
Peter

http://www.ullrich.at.tt

-----Ursprüngliche Nachricht-----
Von: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] Im Auftrag von Scott Gravenhorst
Gesendet: Freitag, 21. Dezember 2012 15:55
An: synth-diy at dropmix.xs4all.nl
Betreff: Re: [sdiy] Audio Anti Aliasing Filter

Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>I'd recommend looking at an all-in- one CODEC chip like the
>CS4270 from cirrus logic. This interfaces directly to the dspic's DCI 
>port and includes 24-bit stereo ADC and DAC. This will run at a whole 
>range of different sample rates and has all of the necessary 
>anti-aliasing and anti-imaging filters built in.
>
>It will guarantee pristine audio quality, save you a lot of design 
>effort, and leave you with more time to concentrate on developing cool 
>effects programs. You also still have the dspic ADC available to read 
>realtime control pots or external CVS, which it is much better suited 
>to.

Thanks, this looks like an option, probably the one of the highest quality options.

That Cirrus part looks interesting, but it comes only as surface mount.  And free samples are available.  I can probably solder that or my son perhaps with his 19 year old eyes, but I need to be able to experiment without etching a board.  Does anyone know of sources for some kind of pre-etched adapter boards that might take the 24 pin TSSOP arrangement?

>-Richie,
>
>Sent from my Sony Ericsson Xperia ray
>
>Scott Gravenhorst <music.maker at gte.net> wrote:
>
>>
>>Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>>>Elliptic filters are a good choice to use for anti-aliasing and 
>>>anti-imaging filters. They can be designed with an arbitrarily flat 
>>>passband and have a *very* steep transition from the passband to the 
>>>stopband. This is particularly important if you want to pass audio 
>>>cleanly up to 20khz with a 44.1kHz sample rate. With these figures an 
>>>anti-aliasing filter doesn't have much room in which to work its 
>>>magic!
>>>
>>>These days most audio ADCs have digital anti-aliasing filters built 
>>>in. Likewise audio DACs often have internal digital anti-imaging 
>>>filtering. The latter is sometimes referred to as a "reconstruction 
>>>filter." Some more info might help clarify if you could use one of 
>>>these chips.
>>>
>>>If you dont want the complexity of designing an elliptic filter, 
>>>something like a 6th or 8th order Butterworth made from cascaded 2nd 
>>>order Sallen-Key filters isn't a bad alternative. You can see this 
>>>circuit preceding and following the BBD chips in old Roland Juno 
>>>service notes. Just simulate tge circuit as-is, then scale capacitor 
>>>values to shift the cutoff frequency exactly where you want it to be.
>>
>>Ah, Ok, this is more complex than I had considered.  Here's what I want to do:
>>
>>This is an experiment which may fail. I have a dsPIC which has a
>built in 12 bit ADC which >I intend to attempt to use for audio 
>sampling of synthesizer output signals. It can easily >sample at
>44.1 kHz up to much more, but I see little reason to run it much 
>faster. Yes, I >realize that 12 bits is far from hifi. However, I'd 
>like to try it anyway for the ultimate >test - the human ear.
>I'm interested in making alias artifacts inaudible enough (good
>>enough). If it fails to sound good enough, I may have to move to
>an external ADC, >preferably 16 or more bits - I want to keep the cost 
>low, and I'd like to avoid that. The >internal ADC is far from complex, 
>from what I see in the dsPIC ADC docs, there is no anti >alias filter 
>built in. > >This is from the ADC PDF section describing external 
>connections to the ADC pins: > >"An external RC filter is sometimes 
>added for anti-aliasing of the input signal. The R >component should be 
>selected to ensure that the sampling time requirements are satisfied." 
>> >There is no mention of the type of signal that the ADC will acquire, 
>but the ADC is capable >of sampling rates up to 500 kHz. Lower sample 
>rates give me more clocks to play with for >the effects computations. 
>I've used high sample rates to reduce audible alias artifacts in
>>FPGAs, but this is impractical with a dsPIC, so I'd like to keep
>the rate reasonable. > >Since the ADC input requires being driven by a 
>low impedance output (200 ohms or less), I'll >have to buffer it 
>anyway, so making the buffer a filter makes sense to me
>- assuming I even >need the filter. > >I'm assuming that a Sallen-Key 
>would be an improvement over a simple RC filter. But given >that the 
>DAC from which I will collect the signal already has a single pole 
>filter with Fc >around 20 kHz, perhaps I don't even need to have an 
>anti alias filter on the ADC input? I'm
>>thinking that I don't even need the buffer, since the DAC
>outputs are already buffered. > >This initial experiment is part of 
>adding digital effects to a dsPIC-based Karplus-Strong >synth.
>I'd like the effects dsPIC to be portable to other designs. I know that 
>the >Karplus-Strong model could have alias artifacts generated 
>especially during the first few >milliseconds of a note start, but 
>these will quickly be attenuated by the reflection filter. > I had 
>considered transferring the audio stream to the effect dsPIC using SPI, 
>but the two >voice engine dsPICs operate from different xtals and 
>cannot be guaranteed synched. That's
>>why I'm cosidering a digital-audio-digital-audio technique. I'm
>looking for musically good >enough, not lab quality, so I think I've go 
>a lot of options. I suppose I could consider >using a single clock 
>oscillator to drive both voice engines and do this all digitally, but 
>>that isn't a solution for other kinds of synths I might want to use 
>this with, so I prefer >an audio input. > >I'd like the effect dsPIC to 
>be generic enough (in terms of it's design) so that it can be 
>>"slapped" onto other synthesizer designs without having to add more 
>stuff. > > >>Sent from my Sony Ericsson Xperia ray >> >>Scott 
>Gravenhorst <music.maker at gte.net> wrote: >> >>> >>>What are people here 
>using for audio anti-aliasing filters? >>> >>>I'm looking for something 
>simple. In my own mind, I'm thinking state >>>variable lowpass with 
>unity Q and cutoff of some 20 kHz for sample rates of
>>>>44.1 kHz and above. >>> >>>Are there better or simpler? >>>
>>>>-- ScottG
>>>>____________________________________________________________________
>>>>____
>>>>-- Scott Gravenhorst >>>-- FPGA MIDI Synth Info: 
>jovianpyx.dyndns.org:8080/public/FPGA_synth/ >>>-- FatMan Mods
>Etc.: jovianpyx.dyndns.org:8080/public/fatman/ >>>-- Some Random 
>Electronics Bits: jovianpyx.dyndns.org:8080/public/electronics/
>>>>-- When the going gets tough, the tough use the command line. 
>>>> >>>_______________________________________________
>>>>Synth-diy mailing list >>>Synth-diy at dropmix.xs4all.nl 
>>>>http://dropmix.xs4all.nl/mailman/listinfo/synth-diy >> > >--
>ScottG
>>______________________________________________________________________
>>__
>>-- Scott Gravenhorst >-- FPGA MIDI Synth Info: 
>jovianpyx.dyndns.org:8080/public/FPGA_synth/ >-- FatMan Mods
>Etc.: jovianpyx.dyndns.org:8080/public/fatman/ >-- Some Random 
>Electronics Bits: jovianpyx.dyndns.org:8080/public/electronics/
>>-- When the going gets tough, the tough use the command line. > 
>>_______________________________________________ >Synth-diy
>mailing list >Synth-diy at dropmix.xs4all.nl
>>http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>

-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
-- FPGA MIDI Synth Info: jovianpyx.dyndns.org:8080/public/FPGA_synth/
-- FatMan Mods Etc.: jovianpyx.dyndns.org:8080/public/fatman/
-- Some Random Electronics Bits: jovianpyx.dyndns.org:8080/public/electronics/
-- When the going gets tough, the tough use the command line.

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