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-24db Precut Sound Quality on Rockboxxed iPod?
preglow:
--- Quote from: pabouk on August 07, 2007, 01:45:46 PM ---I do not know much about Rockbox internals so correct me if I am wrong.
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).
--- End quote ---
This is basically true, but a bit confused. Let's take MP3, for example, which operates at a 29 bit accuracy. A -24 dB cut will approximately throw away the lowest four bits, and this leaves us with a 25 bit accurate signal, and the 16 highest of these bits is what is passed to the DAC (possibly after further processing). This still leaves us with 9 bits to waste before the output should suffer at all. This is the entire reason we opted to go for a 32 bit DSP path in the first place.
This is of course assuming you do some EQing to boost some bands again. If you just cut -24 dB and do nothing, then yes, the resulting signal will basically be 12 bits thanks to the level being so low overall.
saratoga:
--- Quote from: preglow on August 08, 2007, 07:01:49 AM ---
If you just cut -24 dB and do nothing, then yes, the resulting signal will basically be 12 bits thanks to the level being so low overall.
--- End quote ---
Unfortunately, this is what the original poster wants to do!
--- Quote from: Andhyka ---Edit 2: I just checked the schematics of iPod DAC, and looks like it does not have any amp, outputting line-out all the time, and uses its DAC to control volume digitally. This means the resultant signal will be compressed.
--- End quote ---
No. There has to be an amp. You can't drive headphones off of a logic gate.
--- Quote from: Andhyka ---Dynamic range compression determines the fidelity/clarity/detail of sound, and is related to 16bit/24bit/32bit.
--- End quote ---
Dynamic range compression is a type of DSP effect that you can apply to audio. It is not a metric. It does not determine anything. Its not relevant here.
--- Quote from: Andhyka ---And also, I can get the same quality with any volume as long as I put my ipod line-out on headphone amp.
--- End quote ---
You've said this a few times. I don't see why it should be true.
--- Quote from: Andhyka ---Just one last question before I bail out of this unconstructive thread ever:
Does Wolfson DAC accepts 24-bit digital signal?
If not, then I'm throwing away everything I know.
If yes, what I am proposing is "reducing the volume" through the use of wide "dynamic range compression" of 32-bit Rockbox DSP.
--- End quote ---
Did you read my previous post on this idea? As I said above, controlling the volume digitally will always reduce the SNR. Always. What you are suggesting doesn't make a lot of sense.
Andhyka:
Cheers for all the feedback (although a bit harsh), and I think I've found what I'm looking for. ;D
The reason that I bring all these matters up is because I've got highly detailed headphone MDR-SA5000 and was thinking if it sounds better with imod + headphone amp + dock connector cable.
But ofcourse, this is beyond what I can afford at the moment.
Therefore, I started this whole thread to discuss by proposing that line-out output (100% volume) can still be audible through headphone by feeding 24-bit data signal from Rockbox DSP to DAC (which then I found out this is wholly unnecessary, from you guys). Â :)
But as it turns out, the iPod has DAC chip much better than I think, most MP3 players out there. It does 24-bit signal processing in both ADC/DAC filters.
If that is true, digital signal received by DAC will be upscaled from 16-bit to 24-bit, then the volume control will compress/normalise according to the volume level specified.
Thus I can derive that volume level below 100% and above 25% DOES NOT AFFECT the compression of final output.
Don't get me wrong, the Wolfson DAC chip still compresses dynamic range of received signal according to specified volume level but it already upscales the signal to 24-bit internally (16-bit data in 24-bit channel), leaving 8-bit of interpolated fidelity.
Taken from Wolfson WM8393 white paper, 5G iPod Video DAC:
"The digital audio data is converted to oversampled bit streams in the on-chip, true 24-bit digital interpolation filters. The bitstream data enters the multi-bit, sigma-delta DACs, which convert it to high-quality analogue audio signal. The multi-bit DAC architecture reduces high frequency noise and sensitivity to clock jitter. It also uses a Dynamic Element Matching technique for high linearity and low distortion."
http://www.wolfsonmicro.com/uploads/documents/en/WM8983.pdf
I must admit though that my understanding on electrical engineering & audio manipulation are limited, and this matter may seem trivial to you, especially Rockbox developers cos they definitely knew this already years ago.
Reply to saratoga: I think iPod does have an amp but is set at static level (which volume can't be increased or decreased). iPod relies on digital volume on its DAC which results in dynamic range compression (less fidelity/clarity).
Febs:
One more time, since you seem to have great difficulty understanding this concept: Changing the volume on an ipod DOES NOT RESULT IN DYNAMIC RANGE COMPRESSION. You are mis-using that term.
Andhyka:
Exactly, it doesn't. ;)
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