SV: Re: SV: Re: [sdiy] Good 16 bits A/D choice for mono audio recording

[Richard Wentk]

On 25 May 2007, at 22:00, Eric Brombaugh wrote:

> Karl,
>
> Good question. Studying the datasheet I see that they're quoting  
> +/-15ppm INL which is closer to 16-bit resolution than 24. A bit  
> more digging suggests that the part is really only accurate to 18  
> bits.
>
> So the answer is no - some of those 24 bits are worthless. Audio  
> rate codecs aren't much better - 24-bit ADC/DAC combinations are  
> pretty cheap these days, but unless you're picking the parts  
> carefully and surrounding them with high-quality drivers & supplies  
> you'll never get the kind of accuracy that simply counting bits  
> would suggest.

Assuming a +/- 1V signal swing, a single bit is around 0.1uV at 24 bits.

This is pretty much impossible to engineer, especially considering  
the equivalent circuit SNR has to be better than 144dB.

When recording, it's impossible to find a microphone+preamp  
combination that comes anywhere close to that. (And never mind that  
your ears have trouble hearing anything much quieter than -100dB  
anyway.)

So most so-called 24-bit converters are just specmanship. Consumer- 
grade audio hardware is more likely to have between 16-20 bits of  
useful resolution, depending on cost. Pro-grade hardware is better,  
but not perfect.

The PrismSound AD-2 ADC, which is more or less as good as it gets in  
pro circles, quotes 130dB dynamic range, but THD+n of -108dB, and  
barely less than -90dB IM distortion.

Richard