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Boss BR1200/Sigma Delta Modulation

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Boss BR1200/Sigma Delta Modulation

Postby MarkEh » Thu May 13, 2021 4:05 am

I'm a bit of a newbie with questions, and I have an old Boss BR1200 digital portastudio that uses delta sigma modulation. I'm also a bit out of my depth. My understanding is that delta sigma modulation processes audio at a 24-bit depth rate, and in using oversampling, noise-shaping, etc., stores the data at a very clean and accurate 16-bit, which is superior to straight 16-bit recording. And this "conversion" to 16-bit happens earlier in the process than with the DAW ADC. Is this correct?

I'm not sure what happens when I import 24-bit files from my DAW to the Br1200, to use the COSM effects etc (and maybe import them back into the DAW), or possibly do a final mix using the Boss faders. Does anyone know if the imported 24-bit files would be processed down to 16 bit the same way the processors record from the Boss, i.e. with noise shaping, etc.? Does any of this matter at all?

And more importantly, is there any issue with the Boss files being imported into the DAW at 16-bit, when the DAW files are 24-bit may not yet "cleaned-up" as the Boss files are at that stage?

I have also heard the BR1200 mixes at the 24-bit rate. Is this true, and how much does this matter?

Thanks, especially if you got this far.
MarkEh
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Re: Boss BR1200/Sigma Delta Modulation

Postby Hugh Robjohns » Thu May 13, 2021 11:15 am

MarkEh wrote:I have an old Boss BR1200 digital portastudio that uses delta sigma modulation.

Pretty much anything and everything working with digital audio has used converters based on delta-sigma modulation technology since the turn of the century. Your boss is not special or unusual in that respect.

My understanding is that delta sigma modulation processes audio at a 24-bit depth rate, and in using oversampling, noise-shaping, etc., stores the data at a very clean and accurate 16-bit, which is superior to straight 16-bit recording. And this "conversion" to 16-bit happens earlier in the process than with the DAW ADC. Is this correct?

Nope.

Delta-sigma modulation is just a practical engineering technique that achieves very accurate conversion between the analogue and digital domains with long word-lengths. In very simple terms it basically trades off very high sampling rates at very small word-lengths with much lower sample rates at much large word-lengths.

Most practical delta-sigma converters sample the input analogue audio at 256x the wanted sample rate -- so typically around 11MHz -- with effectively with only one bit (ie, a very short word-length) as they just measure whether the signal got bigger or smaller than last time it looked (hence delta in the name -- 'change').

They then perform a lot of clever maths that translates that data into much longer word-lengths with absolute amplitude data but at a much lower sample rate (eg.44.1kHz). And the same kind of approach is used backwards for converting digital signals to analogue -- which is where the 'oversampling' term comes in -- taking the source digital data and generating data at much higher sample rates, but with much smaller word-lengths.

Noise-shaping is a process whereby the quantising noise is spectrally shaped to make it less audible to us humans. When performed correctly, it can make a low-bit system sound as quite and clean as a high-bit system.

Delta-sigma modulation techniques are used universally these days and do a very good job. The technology used previously could just about manage reasonably accurate conversion at 16 bit word-lengths (which essentially involve accurately measuring the signal amplitude against a ruler with 65,536 graduations). But that technology just couldn't cope with the 16.7 million graduations needed for 24 bit conversion.

So back in the day, delta-sigma conversion was claimed to be better at 16 bit than the previous generation of converter technologies, and could achieve even higher resolutions too...

Some of this is explained in this feature article from a few years back:

https://www.soundonsound.com/techniques/digital-problems-practical-solutions

I'm not sure what happens when I import 24-bit files from my DAW to the Br1200, to use the COSM effects etc (and maybe import them back into the DAW), or possibly do a final mix using the Boss faders. Does anyone know if the imported 24-bit files would be processed down to 16 bit the same way the processors record from the Boss, i.e. with noise shaping, etc.?

As far as I know, the Boss operates only at 44.1kHz sample rates and 16 bit word-lengths, so I imaging 24-bit files are truncated internally to 16 bit.

Does any of this matter at all?

Nope. You're using it for an effect... if you like the sound who cares how many bits are involved?

Added to which, 16 bit audio offers way more dynamic range than you can practically achieve in most project studio setups, so it should be fine for use with your COSM effects if you optimise your signal levels appropriate.

And more importantly, is there any issue with the Boss files being imported into the DAW at 16-bit, when the DAW files are 24-bit may not yet "cleaned-up" as the Boss files are at that stage?

Truncation -- of that is what happens -- from 24 to 16 bits can generate potentially quantising distortions, but in practice I doubt you'll notice anything untoward. And if you do, then run your source 24 bit files through a redithering plugin to generate proper 16 bit files before exporting to the Boss.

I have also heard the BR1200 mixes at the 24-bit rate. Is this true, and how much does this matter?

I'm going on Tom Flint's review and he suggests it works at 16/44.1....

But whatever word-length it uses, it only matters if you don't like how it sounds...

More bits means a lower internal noise floor... but in most situations the noise floor is defined by the source recording, not the mix engine!
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