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-24db Precut Sound Quality on Rockboxxed iPod?

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Andhyka:
Umm I am unsure of how Rockbox precut works, whether it cuts the bottom half of sound or compress/normalize the whole fidelity. If the latter is what happens, I have to disagree.

Since the EQ/DSP processing done in Rockbox is through 32-bit channel, the pre-cut won't affect the SNR as the audio source is still wholly 16-bit. Yes I know it's already upscaled to 32 bit, but the fact that it is just 16-bit stretched will not affect SNR/fidelity of the original music unless I cut waaaay too much (like minus 96db - 6db * 16-bit).

But the early argument still remains, how does Rockbox cut the music? Does it use compress/normalize algorithm, or simply just cut the lower half of sound fidelity.

Any thoughts?

Hmm, I'm still hoping Rockbox feeds 24-bit audio signal to Wolfson ADC/DAC.

Here's the breakdown:

Digital Music (44khz & 16bit) -> Rockbox DSP (16bit stretched to 32bit) ->
24db Pre-Cut (16bit stretched to 28bit/32bit) -> Final Downsampling (16bit stretched to 21bit/24bit -> Wolfson chip (16bit stretched to 21bit/24bit processed by 24-bit ADC/DAC) -> Pure 16-bit signal audible through headphone

Edit: I revised the breakdown above

Llorean:
Wouldn't the math be -> 21/24 rather than 20/24, then to 14/16? I'm not really an expert on this, but if a 16 bit signal is scaled to 32bit, you actually use the other 16 bits giving twice the resolution, rather than leaving them empty, don't you? It's for more accurate processing, isn't it?

Andhyka:
I think 16/24/32 bit and decibels work exponentially, so half the volume of 32-bit is not the same as 16-bit, since 2 ^ 32-bit = 4294967296 and 2 ^ 16-bit = 65536.

But apart from the maths, I can confirm that the 16bit signal stretched to 24bit/32bit and transformed to anything above 16bit itself will not affect its fidelity/SNR. You can check it on Professional audio software like Adobe Audition.

I think let's use an easy example with 4bit/8bit/12bit signal:

1 0 1 0

if we convert this to 12bit signal:

111 000 111 000

The same data is added three times. If it's converted back to 8bit signal:

11 00 11 00

Then downscaled back to 4bit:

1 0 1 0

Its only stretched back and forth, and 16bit signal in 32bit channel is still 16bit (except for X-Fi).  :P

Llorean:
Yes, if you make absolutely no changes to the signal, obviously going from 16 bit to a linear multiple (32 bit) and back again will result in identical output. I'm not even sure why you bothered to try to explain that, as it's not relevant once you start changing the signal.

Andhyka:
Well, that explains the signal pathway I mentioned above. And it's just 16-bit data being transformed over and over until it reaches Wolfson DAC without loss of quality/fidelity, of which I hope Rockbox pre-cut EQ function compresses the whole spectrum instead of cutting the lower fidelity.

Compress vs Pre-cut:

00001100 -> 0010 (data compressed, or in another term, downscaled)

00001100 -> 1100 (the first half of data is deleted or cut, meaning it loses fidelity/SNR)

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