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Originally Posted by pandy71
Probably Toni knows more about magical 28867Hz
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This maximum sampling frequency is determinated by the DMA request logic from Paula to Agnus. On the first refresh slot the 4 pairs of AUDxDR and AUDxDSR signals are latched in the internal shift register. Then it's transmitted with a delay (TDM) on the DMAL line to the Agnus.
[you can find more info in the Minimig's sources].
Very interesting is the fact that the maximum sampling frequency isn't the same for every audio channel. It depends on the delay from the first refresh slot to the moment the sample data is transmitted.
The minimum sampling periods for the audio channels:
#0 : 120 (121)
#1 : 121 (122)
#2 : 122 (123)
#3 : 123 (124)
In the parenthesis is the minimal value when the DSR (dma pointer reload from location register) request is transmitted (typical case when the sample length is 1).
When the variable display mode is used (ECS/AGA feature) the scan line may be shorter (Multiscan, DblPAL, DblNTSC) and the maximum audio sampling frequency higher (56 kHz).
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- I know allocation for DMA cycles during line but what with distribution of DMA cycles during field/frame
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There is no difference. The DMA engine is scan line based.
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It is not clear for me how DMA cycles for audio are located on blanking lines
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Exactly the same as during any other scan line.
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- or maybe DMA is stopped so we have this limitation - 28867 instead 31250Hz
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It's not stopped.
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Also im curious how normal ie DMA samples are distributed - they are combined in analog way? digital? interleaved?
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Taking into account that the Paula is made using NMOS technology the sample combining has to be made in the analog domain (current mode DAC).