Guitar / Mixer Matching and Aliasing Filter

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claywer

New member
G'day,

I am trying to use my Guitar pick-up and pre-amp (150ohm) into my Mixer as line in (little Behringer). It isn't ideal! Should I consider to purchase a DI box to get this right? Or should I instead try to build something that does the job?

Also considering to implement a BP filter for my guitar perhaps only to pass >80Hz and <15kHz. Certainly my mixer etc. can pass >20 kHz and this will cause aliasing in the A/D of the sound card.

You ideas, suggestions and input appreciated.

Clayton
 
Also

Also I think that a Common mode choke filter might help my S/N? Anybody used one for such a purpose?
 
Are you refering to an anti aliasing filter? That's part of a A/D converter. If so, you're way off base on this one. You don't waant to limit the passband of the guitar by killing upper harmonics. At 44.1 the filter is at 22KHz, 48-24KHz etc.
 
Thanks for the information on the filter. I wasn't aware that there was an internal anti-aliasing filter in my sound card, having found no mention of it in the Hoontech data sheet?

Aliasing is a "spectral wrap-around", where signals faster than the sampling frequency get translated spectrally by the A/D process - and can lead to some ugly consequences.

So an input of 21-kHz to a 20-kHz sampler will result in a digital O/P of 1-kHz effectively.

I note that my Mixer is flat to about 55-kHz and there is some guitar signal way up past 20-kHz, therefore the question of aliasing is legitimate - even if misguided with respect some of the details - I will ask Mr Hoontech..

On the question of matching, evidently the Mixer line in is more like a A (like A/V) input Z=20-kohm, whereas my guitar O/P is 150 ohms, thus a buffer of some sort is probably recommended and the DI appears to be the Industries standard, so I will give it a try!

Clayton
 
Your profile says electronic engineer, so it would appear that you should already know this.. but anyways...

The anti-aliasing filters that are part of the A/D conversion process exist to ensure that no signal aliasing will occur upon sampling. Aliasing occurs when the signal bandwidth is excessive for the sampling rate in use and sidebands are produced above/below the signal frequency. These sidebands can overlap, producing output frequencies that are not the same as the input frequencies of the signal.

Aliasing does not occur when the input frequency is equal to or less than half the sampling rate, and so derives the most fundamental rule of sampling theory, which is that the sampling rate must be at least twice the highest input frequency. This commonly known as Nyquist's Theorem.

BUT ANYWAYS.... my point is:

I have yet to see a guitar signal with an overtone above 20Khz that has a volume of any consequence.... so a 44.1 KHz sampling rate is considered more than adequate....

But if you honestly profess to hear a difference and insist that you must maintain the ultimate high-end frequency repsonse for such a, er..., bandwidth-limited instrument..., then avoid digital altogether and stick to high-end analog.......... your choice!
 
Thanks for your input. It is certainly possible to make an A/D converter and not to implement an input filter to deal with aliasing - I am not directly involved in design for this particular industry, so perhaps know enough just to be dangerous! Is it garanteed that an aliasing filter is present at the input of any and all sound cards? (Too often I have found that if you don't ask the most straight, simple and direct question, then the answer received is not the one that you want.)

Certainly the signal level according to my analysis is reducing as 20-kHz is being approached - but it is far from zero. Simple extrapoloation on my part suggests that there are components higher to this unless there is a specific multi-pole roll off. My guitar's Pre-amp is reputed flat to above 20-kHz! Thus my question of whether or not quicker signals are being dealt with prior to the A/D process.

Clayton
 
I'm not a designer either, so maybe I shouldn't generalize... but I do not know of any A/D converters for use in digital audio applications that do not use anti-aliasing filtering of some type....

But again, while a guitar preamp may spec at 20-20Khz, the instrument itself doesn't produce any significant frequencies near that upper end of the audible spectrum....

Throw up a guitar track on an SA if you don't beleive me!

I seriously don't think you have to worry about it.........!
 
One day I will bring my guitar into the office and bung it into the SA for a look see. When I get that far I will post the results. Should I bring in a little Electric and compare the 2? (Harmonic series etc. etc)

I suspect that a typical acoustic will have lots of higher F nuances above where the typical 'lectric goes, but just how far that goes will be interesting to find out!

I have a little EQ pedal amp that I used to use with just a peizo installed, it will probably do the job that I am thinking of without me having to do anything other than dust it off and buy a new battery!
 
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