| Re: [hatari-devel] STe DMA sound |
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- To: hatari-devel <hatari-devel@xxxxxxxxxxxxxxxxxxx>
- Subject: Re: [hatari-devel] STe DMA sound
- From: David Savinkoff <dsavnkff@xxxxxxxxx>
- Date: Sat, 14 Apr 2018 13:12:07 -0600 (MDT)
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- Thread-topic: STe DMA sound
----- On Apr 14, 2018, at 11:57 AM, Christian Zietz czietz@xxxxxxx wrote:
> David Savinkoff schrieb:
>
>> Note that a real STe uses a 4th order lowpass switched capacitor filter
>> at ~20Khz cutoff frequency (Fc) ... look at the schematic (I can't remember
>> the part number) ...
>
> I always understood that the cutoff frequency of said SC filter is
> scaled according to the DMA sound sample frequency in order to provide
> proper anti-aliasing for all supported sample frequencies. Am I wrong?
You are right. The cut off frequency (Fc) is proportional to the
input sample frequency (Fs) ... and the STe DMA frequency is
changed so that Fc changes proportionately. eg.
(50 KHz dma -> 20 KHz filter Fc)
(25 KHz dma -> 10 KHz filter Fc)
The capacitor switching frequency is altered to suit the Fc.
I have the data sheet somewhere, but I'm stating things from off the
top of my head.
>
> Christian
> --
> Christian Zietz - CHZ-Soft - czietz@xxxxxxx
> WWW: http://www.chzsoft.de/
> PGP/GnuPG-Key-ID: 0x52CB97F66DA025CA / 0x6DA025CA