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-24db Precut Sound Quality on Rockboxxed iPod?
preglow:
--- Quote from: pabouk on August 07, 2007, 06:24:22 AM ---Because the pre-cut is applied in the digital domain you lose the dynamic range by lowering the volume using pre-cut. This practically means that the quantization noise will be louder. If I remember it correctly 6 dB corresponds approximately to 1 bit of digital resolution thus by applying -24 dB you would have a digital resolution of 12 bits instead of the original 16 bits.
--- End quote ---
This assumes that Rockbox uses 16 bits precision for samples internally, and that is quite simply not true. Most codecs operate with an accuracy at or above 24 bits, so the bits that get shaved away in the precut will probably not be audible in the output anyway.
mamboman:
thanks for the advise. so i guess it doesn't discard much of the real information from the stream. just invested in an amp and listening through the lineout without the imod, the sound quality is indeed significantly improved but at the expense of portability.
pabouk:
I do not know much about Rockbox internals so correct me if I am wrong.
--- Quote from: preglow on August 07, 2007, 11:13:43 AM ---Most codecs operate with an accuracy at or above 24 bits, so the bits that get shaved away in the precut will probably not be audible in the output anyway.
--- End quote ---
I know that Rockbox operates with about 30 bit accuracy at some places but as far as I know the final output to the DAC is always truncated or dithered to the most significant 16 bits.
When you set precut to -24 dB you are approximately "shifting" the original sample value by 4 bits to the less significant bits thus the 4 most significant bits are not used by the original signal anymore. At the final stage before sending the signal to the DAC only the most significant 16 bits are taken (optionally with some information from lower bits by dithering) so in our case only 12 bits from the original signal would be used and the 4 most significant bits will be wasted (zero).
Andhyka:
Finally, I found the answers what I am looking for! ;D
http://www.wolfsonmicro.com/products/WM8983
http://www.wolfsonmicro.com/uploads/documents/en/WM8983.pdf
It turns out that Wolfson DAC chip (WM8983) on my iPod Video 80GB uses 24-bit sampling in both its ADC and DAC filters. I am not sure about other iPods/players but this seems to indicate that way. Now I can set pre-cut to its lowest lvl (-24db) and enjoy hi-fi pure sound delivered from my iPod @ 0db volume. :D
I'm still looking the answer for this question though: What sampling is used by Rockbox to deliver sound output on its final stage to Wolfson chip? Does Rockbox digitally downsample it from 32-bit back to 16-bit or 24-bit?
saratoga:
--- Quote from: Andhyka on August 07, 2007, 07:17:22 PM ---Finally, I found the answers what I am looking for! ;D
http://www.wolfsonmicro.com/products/WM8983
http://www.wolfsonmicro.com/uploads/documents/en/WM8983.pdf
It turns out that Wolfson DAC chip (WM8983) on my iPod Video 80GB uses 24-bit sampling in both its ADC and DAC filters. I am not sure about other iPods/players but this seems to indicate that way. Now I can set pre-cut to its lowest lvl (-24db) and enjoy hi-fi pure sound delivered from my iPod @ 0db volume. :D
--- End quote ---
That won't work. You'll still lose 24 dB of SNR. Anytime you adjust volume in the digital domain, you lose SNR, no exceptions. Thats why volume is adjusted on the analog side of the DAC. Don't try and defeat that, you can only make things worse.
--- Quote from: Andhyka on August 07, 2007, 07:17:22 PM ---I'm still looking the answer for this question though: What sampling is used by Rockbox to deliver sound output on its final stage to Wolfson chip? Does Rockbox digitally downsample it from 32-bit back to 16-bit or 24-bit?
--- End quote ---
Rockbox uses 44.1KHz as the sampling rate. I don't know what precision is loaded into the DAC, but I'd imagine its 16 bit.
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