16 August 2012, 08:34 | #21 | ||
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16 August 2012, 11:02 | #22 |
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under normal circumstances, the highest number of samples per second possible is 28867. This means (according to Nyquist criteria) that the highest frequency sound you can play correctly is 28867/2 = 14434.
If the volume pwm counters run at a frequency of 56KHz and you want to use volume levels other than 64, this also limits the samplerate to ~28KHz (56/2) or you would get aliasing. Considering this volume implementation, the hardware designers would be stupid to use anything but 8-bit DACs.. Last edited by hooverphonique; 16 August 2012 at 16:35. |
16 August 2012, 14:24 | #23 | ||
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If that's true, then why does 44 Khz 14 bit sound so good on double scanrate? I use a VGA dongle on my A1200 so I can use 31 Khz displays, and playing anything that's CD quality, sounds very good. According to what you're saying, it shouldn't, or am I misinterpreting what you're saying (could be )? |
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16 August 2012, 16:48 | #24 | |
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When running in double scan, paula is able to play back higher samplerates using dma. If this also affects frequency of the volume counters, I don't know. |
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16 August 2012, 17:33 | #25 | |
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The Amiga outputs samples. That is subtly different from sampling. One isn't necessarily trying to reconstruct a waveform. One instead tries to synthesize a waveform from a table of values (the Amiga hardware even supports wavetable synthesis by allowing one channel to modulate another's pitch or volume). A two sample square wave output at 1000 samples per second, for example, will generate an infinite number of odd integer harmonics above 1000Hz -- 3000Hz, 5000Hz, or 17000Hz, or 19000Hz, etc. Whether or not those frequencies are output depends on the details of the Amiga's output filter and not the sample rate. |
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16 August 2012, 19:47 | #26 |
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Yes and no.. It might be misapplication of Nyquist, I don't remember anymore, but it still holds true. How would you play back a wave of frequency 20KHz, for instance, if your max sample rate is 28KHz? By playing a lower frequency wave and then rely on harmonics? The smallest wave you can do contains 2 samples, so you cannot really create a direct wave of higher frequency than half the max sample rate..
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16 August 2012, 20:41 | #27 | |
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Okay. But I have to disagree with your suggestion that somehow harmonics aren't real frequencies that are output along with the fundamental. The question was whether or not the Amiga can produce frequencies above 14KHz using DMA and no tricks. The answer is "yes". The confusion comes from seeing the Amiga's audio strictly as a kind of sampler. It isn't just a sampler. It outputs arbitrary waveforms at variable rates up to 14K values per second per voice. It's a synthesizer. And synthesizers have a long history of relying on the higher-order harmonics of basic waveforms to develop sounds with a complex timbre. A single pure sinusoid of 20KHz is mostly useless. A lower frequency square wave and all it's harmonics, on the other hand, is very useful. Where the Amiga's audio suffers is in the control of high frequency random noise above 14KHz. |
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19 August 2012, 22:53 | #28 |
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Wow, this thread went out of hand rather quickly and prooves audio is a little understood topic in the community...
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20 August 2012, 13:37 | #29 | |
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If you have A500 and Action Replay 3, you can trace Amiga registers, easy enough. |
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20 August 2012, 13:42 | #30 | |
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20 August 2012, 20:54 | #31 |
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20 August 2012, 21:26 | #32 | |
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15 September 2012, 11:59 | #33 |
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What happens if a Protracker module uses a lower period than 124? I'm currently working on a Protracker replayer for the c64 and I found a lot Amiga modules using lower periods.
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15 September 2012, 12:49 | #34 |
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16 October 2012, 20:20 | #35 | ||
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It will sound shrill and rasping, for high-frequency instruments rather like the whistle of a referee. In other words, like crap. You can test this by starting Protracker, load a melody instrument, and then press U (or 0, depending on how high that version goes). |
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18 October 2012, 13:57 | #36 | |
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This should be quite easy to check - set two samples identical value (AUDxDAT) then modulate PWM (AUDxVOL) even square wave (but fast - maybe with Copper help), scope connected to audio output before integrator (directly to Paula pin) should show real frequency for PWM - 56kHz seems to be very low clock and IMHO will create lot intermodulation distortions. Limit 56kHz is related only to DMA not software driven audio (even HRM mention about 1.79MHz PWM audio) |
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18 October 2012, 14:04 | #37 |
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18 October 2012, 14:05 | #38 | |
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So sampling frequency for PWM is 3.58MHz/64 samples per second - ie if someone decide instead using 8 bit DAC to produce audio, 6 bit PWM as DAC can't expect sampling frequency higher than this (Nyquist criteria). To avoid problems with higher DMA audio sampling frequencies - value 65 (ie bypas PWM ) for AUDxVOL must be used - perhaps someone can verify this on ECS or AGA (create video mode when H frequency will be much faster than VGA ie something like 60kHz - this probably imply video mode with 80 pixels for lowres) Last edited by prowler; 18 October 2012 at 22:41. Reason: Back-to-back posts merged. |
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18 October 2012, 15:09 | #39 | |||
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Last edited by prowler; 18 October 2012 at 22:43. Reason: Fixed quotes. |
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18 October 2012, 20:41 | #40 | |
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Yep - never claimed anything else.
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This is circuit with two DAC's in series and one of them can be bypassed. To add more details - max frequency that can be reproduced by PWM DAC in Amiga is half 3.58MHz/64. Yes, and i never claimed that there is ECS or AGA Paula - verification in ECS or AGA system can be easier due fact that video sequencer is programmable and H line can be shorter thus H frequency higher and DMA data can be feed to Paula faster than 56ksps - then verification for internal architecture should be easier (however i think that at least in theory H line can be shortened also on OCS by access cycle to correct STR* strobe registers). And faster DMA can affect modulation when modulation one channel to another is active - (however only one byte from word is used) - limitations for internal Paula architecture can be easily exposed when H line will be really short. Last edited by pandy71; 18 October 2012 at 20:49. |
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