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Oversampling: How and Why
#1
Can someone point me to a fairly easy to understand explanation of oversampling? Why and how?
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#2
https://en.wikipedia.org/wiki/Oversampling
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#3
(09-14-2020, 10:33 PM)malcolmwa Wrote: Can someone point me to a fairly easy to understand explanation of oversampling?  Why and how?

YouTube is a good place to look as well.
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#4
(09-15-2020, 06:05 PM)grasshopper Wrote:
(09-14-2020, 10:33 PM)malcolmwa Wrote: Can someone point me to a fairly easy to understand explanation of oversampling?  Why and how?

YouTube is a good place to look as well.
Thanks. My question comes from the preview of 7.0 which has many oversampling options.  I don't know how to use them and want to be prepared when given the options in 7.0.
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#5
The theory is that some dacs don’t do as good a job of oversampling (almost all dacs oversample internally to make filtering easier/better) as can be done on a computer and so if you convert the sample rate before sending the file to the dac it’ll sound better. The changes made in 7.0 give more control over the sample rate conversion.
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#6
Thanks swizzle. That helps a lot. Now I just need to start oversample school.
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#7
(09-16-2020, 02:19 PM)malcolmwa Wrote: Now I just need to start oversample school.

Unless for reasons of interest the only oversampling lesson you need is this:
  1. Disable oversampling and listen to a wide range of music for a week or two to get the "base".
  2. Enable oversampling at the maximum rate of your DAC (to effectively bypass the DAC oversampling) using one of the default recipies.  Listen to  a wide range of music for a week or two to decide if you prefer the sound.
  3. Repeat 2 with the other default recipies.
You will then know if you need oversampling at all (i.e. is SoX on your RPi better then onboard in your DAC), and also probably have found a recipie that someone else worked out as being good.  It would take years of study and experimentation to do better so only go that way if that's what is fun for you.
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#8
I'm familiar with only a few DAC chips or chip families that support what's called Oversampling Filter Bypass. There may be more.

Burr Brown PCM5XXX. When this chip is fed a 384kHz rate it's internal oversampling filter is automatically bypassed. Search for Table 8 in the data sheet.
https://www.ti.com/lit/ds/symlink/pcm512...52FPCM5122

ESS Sabre 9038Q2M. This chip supports an explicit setting to disable the internal oversampling FIR filter. The purpose of this setting is to allow an external oversampling filter to be used. The external filter must send 8X the source sample rate in order to maintain the correct bandwidth required by the IIR stage in the chip. For example with bypass enabled a 44.1 kHz source needs to resampled to 352.8 kHz. Enabling the bypass also disables internal volume control, master trim and soft mute...
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#9
(09-21-2020, 01:19 PM)Tim Curtis Wrote: I'm familiar with only a few DAC chips or chip families that support what's called Oversampling Filter Bypass. There may be more.

Burr Brown PCM5XXX. When this chip is fed a 384kHz rate it's internal oversampling filter is automatically bypassed.
I guess I was just lucky to have got one of these then. So upsampling on ones without a bypass will result in the DAC applying a filter even if it is fed at its native rate? I guess this is why I'll never fit in with objectivists, I don't understand the maths and have to go by what I hear.
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#10
I'm not sure what you mean by a DAC's "native rate". DAC's have a specific range of rates up to a max rate that they support.

You would need to look at the DAC's data sheet to see whether it offers an automatic, rate based filter bypass as is offered in the PCM5XXX chips. If not then its internal oversampling FIR filter will always be active.

What's the chip? I'll pull down the data sheet and see what it offers.
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