02 September 2016, 08:28 | #1 |
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Queries on AGA Copper and 14-bit Audio
I read somewhere that the AGA Copper is NOT 24-bit, so I couldn't, say, take 256 shades of blue only and recreate them in a copper list to fit on a 320x256 image - or have I misheard somewhere? Almost like not all bits are used to represent the levels of R, G, B.
Also, on a lot of demos that use 14-bit audio of Paula (and there are a lot more than I thought) I've noticed that when the sound first starts or fades out there is an audible clicking or pulsing - at first I thought this was just emulation but I've heard YouTube videos of real hardware that have the same issue. And yet, not ALL 14-bit audio demos are like that, some sound perfectly fine. Maybe it's different playback routines used, some worse than others? |
02 September 2016, 10:58 | #2 |
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You can set the full 24bit RGB with the copper. Workbench is doing that, too.
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02 September 2016, 11:42 | #3 |
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02 September 2016, 13:29 | #4 |
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What they probably meant is that the copper didn't change for AGA, and can still only set 12 bits at a time - you have to load the upper 4 bits and lower 4 bits of each RGB colour register separately. There are still only 32 colour registers, and there are control bits which select which bank of colours these access, and whether they set the upper or lower bits of each palette entry.
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02 September 2016, 13:38 | #5 |
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They weren't. It's simply that it takes several copper moves to be able to change a full AGA color and tricks that needed fast color changes won't work anymore if using the full color range.
14-bit audio shouldn't click more than 8-bit audio. |
02 September 2016, 17:57 | #6 |
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Meynaf, I'm not talking about the odd click when a sample is played in a module, because with all 14-bit demos, there ARE no modules, it's literally an MP2 or WAV sample being played direct, and when the audio is quiet on some of these, you can hear a rhythmic regular clicking, it's really quiet but it's there. Thing is, some demo prods don't have it, most probably because the playback routines are better.
Can someone remind me in any case how 14-bit audio works? I heard it wasn't "true" 14-bit PCM audio, just some weird mixture of the volume per channel and 8-bit PCM audio. |
02 September 2016, 18:09 | #7 | |
son of 68k
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Quote:
There can be many reasons, however, for sure, mere 14-bit shouldn't be doing that. It's one channel playing at volume 64 (doing the top 8 bits) mixed with another playing at volume 1 (doing the bottom 6 bits). Weird mixture, but total is really 14-bit (actually slightly more due to calibration). |
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02 September 2016, 19:57 | #8 | |
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I think I need further clarification on how Amiga PCM actually works, Meynaf.
I know that it's 8-bit, so I assume that means a range of 0 to 255 with 128 being the centre line, or something like -128 to 127 with 0 being the centre line if it's signed, right? Quote:
Also, I know that there are volume settings for each channel, and I think a master for all of them, but I figured that they controlled the volume independently of whatever the waveform data was, down to the circuitry of Paula. Are you saying that the volume settings are 100% digital rather than analogue? |
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02 September 2016, 20:54 | #9 | |||
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Quote:
Quote:
To make things simple, a data of 1 with a volume of 64 is the same as a data of 64 with a volume of 1 ; therefore a data of 32 with a volume of 1 would be 1/2, and so on down to 1/64. Normally data from 0 to 16383 should be linear, even though it's not exactly so because of the properties of the D/A (on emulators it is, though). Quote:
The end result is - more or less - like if we had true 14 bit d/a. There is no master affecting all channels. I'm perhaps not the best guy to explain all this ; perhaps you could find some docs here and there. |
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02 September 2016, 21:32 | #10 | |
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Thanks for trying to explain, but I'm still not clear on this:
Quote:
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02 September 2016, 21:49 | #11 |
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The principle is easy. If you have a volume of 1 then you have 1/64th the amplitude of what you'd have with a volume of 64. It's exactly like shifting the data for 6 positions.
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02 September 2016, 22:17 | #12 |
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I get the gist of what you're saying, but I'll be satisfied to just know that, regardless of D/A properties, the data from 0 to 16383 is linear.
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04 September 2016, 20:59 | #13 |
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the data is really two sets of 8 bit data, it is two samples played at once, with one much louder than the other, so the quieter one can fill in the subtler details.
So on one channel you will have the range -128 to +127, and on the other (probably) 0 to +63. This gives a total range of -128*64 = -8192, to 127*64+63 = 8191 Last edited by Mrs Beanbag; 04 September 2016 at 21:20. |
07 September 2016, 19:48 | #14 |
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@Foebane
The 6-bit volume control is an analogue volume control, that's how Paula is able to output true 14-bit stereo audio. |
18 September 2016, 19:58 | #15 |
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I haven't posted on here for a while, but I forgot to mention in my OP here that I'm using WinUAE for my Amiga, so maybe the clicking/pulsing is caused by emulation? Unless I can find a YouTube video of real hardware with the problem, I'll have to assume it is. BUT, not all 14-bit audio demos have that clicking/pulsing problem.
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18 September 2016, 21:30 | #16 | |
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Quote:
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08 October 2016, 16:28 | #17 | |
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Nope it is digital (PWM) - that's why for sample rates higher than 56kHz volume control should be disabled (so in fact there is 65 levels attenuation not 64). btw those 6 bits attenuation are more linear (as PWM is linear by definition) than remain DAC system - not sure about real Amiga DAC linearity - maybe i should hook Amiga to Audio Precision analyzer? |
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08 October 2016, 17:56 | #18 | |
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Quote:
What i don't know is why the characteristics are the same (or very very close) for left and right outputs. |
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09 October 2016, 20:11 | #19 | |
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Quote:
(After buying Vampire 500 definitely will try measure Paula with audio analyzer). And your feedback just confirm my impression that in real life all those 14 bit tricks with stacking two 8 bit DAC is more close to 10 than 14 bits... (i assume that it should be possible to partially compensate some nonlinearity but this require self calibration and not ear but some decent ADC in feedback loop - this is how some modern DAC perform self calibration - quite common in RF ) |
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10 October 2016, 09:29 | #20 |
son of 68k
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I real life you get a little more than 14 bits, in fact. You should really have a look at how the calibration works.
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