Hissing and whistling in the STe audio output

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

Post by exxos » Sun Oct 21, 2018 1:05 am

Hold up, where is the 2600hz noise in those test then ?

Can you check pin 8 of the last op amp output as well ?
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Re: Hissing and whistling in the STe audio output

Post by sandord » Sun Oct 21, 2018 9:14 am

exxos wrote:
Sun Oct 21, 2018 1:05 am
Hold up, where is the 2600hz noise in those test then ?
My thoughts exactly! Will do some more measuring today, including the cinches for comparison.

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Re: Hissing and whistling in the STe audio output

Post by exxos » Sun Oct 21, 2018 11:59 am

sandord wrote:
Sun Oct 21, 2018 9:14 am
My thoughts exactly! Will do some more measuring today, including the cinches for comparison.
Also worth noting if the psu being physically close to the amps or not.... noise could come from the psu.. so would have to by physically close to where it normally sits...
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Re: Hissing and whistling in the STe audio output

Post by sandord » Sun Oct 21, 2018 12:08 pm

exxos wrote:
Sun Oct 21, 2018 11:59 am
Also worth noting if the psu being physically close to the amps or not.... noise could come from the psu.. so would have to by physically close to where it normally sits...
Yep, I was going to mention that (my psu was sitting outside the casing) but wanted to measure the difference first.

I will probably post more results this afternoon.

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Re: Hissing and whistling in the STe audio output

Post by sandord » Sun Oct 21, 2018 4:55 pm

These are the points where I measured in the audio path (all with the PSU outside of the ST case):

ste2.png
ste2.png (204.97 KiB) Viewed 140 times

I hope I got the locations right, here's a picture for you guys to check:

ste1.png
ste1.png (1.37 MiB) Viewed 140 times

I've found a nice realtime FFT program (http://www.techmind.org/audio/specanaly.html).
The gray line is the average, the black line the current signal. I've made the screen shot with the lead disconnected so in this case, the gray signal is the real signal, the black one is de base line (wire noise).

ste3.png
ste3.png (34.15 KiB) Viewed 140 times

ste4.png
ste4.png (32.85 KiB) Viewed 140 times

ste5.png
ste5.png (32.01 KiB) Viewed 140 times

ste6.png
ste6.png (31.62 KiB) Viewed 140 times

Point D gives a rather interesting signal. I don't see the 2600Hz spike but the signal is suspicously 'clean'.

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Re: Hissing and whistling in the STe audio output

Post by exxos » Sun Oct 21, 2018 5:07 pm

Very good info. What is that MF4 part ? Noise seems to start on its output then get amplified..
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Re: Hissing and whistling in the STe audio output

Post by Stimpy » Sun Oct 21, 2018 7:18 pm

Is it an anti alias filter or something? Perhaps the clock driving it needs to be series terminated? Or maybe its just a noisy chip?

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Re: Hissing and whistling in the STe audio output

Post by sandord » Sun Oct 21, 2018 7:19 pm

Stimpy wrote:
Sun Oct 21, 2018 7:18 pm
Is it an anti alias filter or something? Perhaps the clock driving it needs to be series terminated? Or maybe its just a noisy chip?
It's a 4th order switched capacitor Butterworth lowpass filter.

See https://pdf1.alldatasheet.com/datasheet ... N-100.html

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Re: Hissing and whistling in the STe audio output

Post by sandord » Sun Oct 21, 2018 7:29 pm

Signal C is the filter out (pin 5) of the MF4.

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.

Now tracing further back into the audio path...

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Re: Hissing and whistling in the STe audio output

Post by exxos » Sun Oct 21, 2018 8:42 pm

What is on FCLK ?
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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