Re: ADC distortion typical near 0dB??

kludge@xxxxxxxxx (Scott Dorsey) writes:

> In article <87y81e9ftg.fsf@xxxxxxxxxxxx>, <Mannr@xxxxxxxxxxxx> wrote:
> >Does anyone know if it is typical for an ADC to distort when driven near full
> >scale (0dB)?
> >
> >I've got an Edirol UA5 USB audio interface. I put a 1600Hz sine wave in the
> >line input (played from the line out of a Creative Labs Nomad Jukebox 3),
> >turned the level up to just under 0dB and recorded a few seconds of audio.
> >
> >When I looked at the spectrum (Adobe Audition 1.5!) I see several harmonics
> >(multiples of 1600Hz), and the largest one was only -40dB down or so. I
> >noticed these extra peaks went away once I lowered the input to say, -6dB.
> >So, it seems to exhibit some kind of clipping/overloading behavior, but the
> >input signal was not over. It was something like -1 or -2dB.
>
> This is bad behaviour. And it's not clipping either, it's the onset
> of distortion before the clipping point.
>

Dear Scott:

Thanks for the detailed response. Yes, I agree it is bad behaviour.

> >I looked at the circuit in this box. It is based on the AK4524 ADC. The
> >circuit seems to follow the standard design spec. The chip operates on +5V.
> >The driving opamps run on +/-5V. The input signal to the ADC seems to be
> >about 0.7V RMS at full scale.
> >
> >So, could it be that these ADCs are really not very good? Or is the
> >supporting circuitry poor? Should I expect a more expensive box, like MOTU or
> >Presonus, to do it right? I guess it pays to check these things out.
>
> This is basically the cheapest ADC circuit that it's possible to get,
> and it's preceded by cheez-whiz op-amps running at OUTRAGEOUSLY low
> rail voltages. It's a wonder you get any linearity at all with something
> like this. What you see is typical of the kind of thing I complain about
> as cost-cutting gone wild.
>

I'm just wondering if it is the Opamps or the ADC. I guess to find out I
could put a signal directly into the analog in pin on the ADC.

> Just relax and keep your levels down. And save up for a decent set of
> outboard converters. There is a reason the Prism costs more than the
> Edirol.

It seems to distort anywhere above -3dB. I would be happy to run it at -6dB
and below, say, but the problem is there is a really nice "clip" light that
comes on at -3dB. I run this thing "standlone" (without a PC) and rely on the
clip light to set levels. (I set it so the clip light just flickers every now
and then.) I would be happy if I could find a way to make the clip light to
turn on earlier, say at -6 or -9dB. Does anyone know how this clip light
works? I'm thinking it might be reading right out a digital channel
somewhere.

Oh yeah, one more question: What is the simplest ADC circuit I can make that
takes line in and outputs SPDIF optical? In particular, do I need this Cirrus
CS8402 to generate optical out? This does rate conversion and all kinds of
stuff, but can I just clock it at 44.1 and output 44.1/16 directly from the
AK4524? Again, I'm assuming that the AK4524 is really an OK chip. People
have told me the same chip is in reasonable prducts, like the "Apogee MiniME"
ADC box.

> --scott
> --
> "C'est un Nagra. C'est suisse, et tres, tres precis."

Richard
.

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