Hissing and whistling in the STe audio output

Various tweaks and fixes from video ghosting to audio fixes etc.
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sandord
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Re: Hissing and whistling in the STe audio output

Post by sandord » Mon Oct 22, 2018 3:00 pm

exxos wrote:
Mon Oct 22, 2018 2:24 pm
The "stair step" isn't relating to the filter clock exactly.. the stair step steps will be smaller the higher the frequency is.. I don't know how it would clean that up exactly, some inherent smoothing somewhere. Sample playback speed is the stair step part, the clock to the filter sets the roll off frequency... different things.. Really the roll off is to remove the HF content which can burn up op-amps or even speakers, add bad harmonics etc, so its filtered out.

I would still suggest you bypass the MF4 and see how it affects things.. we have to start somewhere..
I think that it makes sense to set the roll off frequency close to the maximum achievable nyquist frequency in order to get rid of any aliasing induced harmonics in frequencies higher than the nyquist frequency. Any frequencies beyond that point shouldn't be there for whatever reason.

Here's another quote that suggest that FCLK is not fixed:

The new GST MCU provides all the frequencies previously delivered by the GLUE and the MMU. However the new shifter (GST SHIFTER see figure 2) receives an 8MHz clock coming from a separate oscillator. This clock called SCLK (Sound Clock) allows, via some internal dividers of the GST Shifter, to generate a clock with an ajustable frequency (FCLK). This clock is used for the sound filters.

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Re: Hissing and whistling in the STe audio output

Post by sandord » Mon Oct 22, 2018 3:07 pm

Tonight I will try to play some empty wave files, one for each supported DMA frequency and we'll see if FLCK changes.

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Re: Hissing and whistling in the STe audio output

Post by exxos » Mon Oct 22, 2018 3:18 pm

sandord wrote:
Mon Oct 22, 2018 3:07 pm
Tonight I will try to play some empty wave files, one for each supported DMA frequency and we'll see if FLCK changes.
Good plan!
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Re: Hissing and whistling in the STe audio output

Post by sandord » Mon Oct 22, 2018 4:50 pm

Just tested using Ned Player (no need to play an actual file, switching frequencies immediately updates DMA hardware)

@6.26 KHz: FCLK = 250KHz
@12.52 KHz: FCLK = 500KHz
@25.04 KHz: FCLK = 1MHz
@50.07 KHz: FCLK = 2MHz

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Re: Hissing and whistling in the STe audio output

Post by exxos » Mon Oct 22, 2018 4:59 pm

So does that 2600Hz noise issue move then ?

BTW- do you see the same noise on both left and right channels or just one ?
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Re: Hissing and whistling in the STe audio output

Post by sandord » Mon Oct 22, 2018 5:01 pm

exxos wrote:
Mon Oct 22, 2018 4:59 pm
So does that 2600Hz noise issue move then ?

BTW- do you see the same noise on both left and right channels or just one ?
I don't know yet, just did a quick check and have to do other stuff right now. Will test later though!

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Re: Hissing and whistling in the STe audio output

Post by sandord » Mon Oct 22, 2018 6:37 pm

exxos wrote:
Mon Oct 22, 2018 4:59 pm
So does that 2600Hz noise issue move then ?

BTW- do you see the same noise on both left and right channels or just one ?
I can confirm that the 2600 Hz signal doesn't change, it's prevalent on all DMA playback frequencies and on both L and R channels.

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Re: Hissing and whistling in the STe audio output

Post by exxos » Mon Oct 22, 2018 6:47 pm

sandord wrote:
Mon Oct 22, 2018 6:37 pm
I can confirm that the 2600 Hz signal doesn't change, it's prevalent on all DMA playback frequencies and on both L and R channels.
Time to bypass it and see what happens then ;)
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

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Re: Hissing and whistling in the STe audio output

Post by sandord » Mon Oct 22, 2018 7:42 pm

exxos wrote:
Mon Oct 22, 2018 2:13 pm
From the datasheet (from what I remember last night) the clock was to set the roll off frequency.. So the clock is probably fixed.. So if you have 250KHz and we had /100 used, then thats 25KHz roll off which makes sense.
By the way, 250K divided by 100 actually makes 2.5K. Not quite our 2.6K but pretty close. Sadly, like I reported, the 2.6K noise doesn't go away when changing the filter clock.

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Re: Hissing and whistling in the STe audio output

Post by sandord » Mon Oct 22, 2018 7:56 pm

exxos wrote:
Mon Oct 22, 2018 6:47 pm
Time to bypass it and see what happens then ;)
Honestly, I'm a little reluctant to do that to an otherwise perfectly working STE (it's the only one I have). So I'd like to do more analysis before doing surgery.

Besides, measuring on the Filter In pin gives us much of the same signal one can expect from bypassing it altogether. Though I understand that there are ways the filter could influence the input signal.

Below you find another FFT diagram (from Audity this time) of what I measured on the Filter In and Filter Out pins of the MF4.

I have to say that I'm a little worried about the variance of results I seem to get:
sandord wrote:
Sun Oct 21, 2018 7:29 pm
I just measured filter in (pin 8) and I can see a little spike at 2600Hz as well there. I can't get a clean screen shot of this since the average line is hidden behind the current signal.
That's something I can't reproduce. As you can see below, there doesn't seem to be a 2600 Hz spike on the Filter In pin.

Screenshot (23).png
Screenshot (23).png (88.94 KiB) Viewed 116 times
Screenshot (24).png
Screenshot (24).png (81.17 KiB) Viewed 116 times

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