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Mixtel90

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Joined: 05/10/2019
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Posted: 07:00am 13 Jun 2023

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Fair enough, Bill. :) I'm not trying to change your mind, but I feel that it's important to explain *why* we have filters and what they do. As I said, I'm not going to test the theory just in case it's correct. :) It does *look* correct to me and specs seem to back me up.

Early CD players, in the days before oversampling and using the 44.1kHz carrier, incorporate a "brick wall" filter at about 20kHz precisely to do what Volhout's filters do (you can't produce audio at any higher than than half the carrier frequency so these filters have to be very steep to be effective). Later players oversample, doubling (or even much more) the Nyquist frequency (the effective sample rate), so the filter can have a less steep slope and still allow audio up to and even above 20kHz. Getting rid of the 44kHz isn't a new thing. It was invented as a necessity with PCM.

Incidentally, 44100 is 2x2x3x3x5x5x7x7 so it's a rather interesting number and may have been chosen because of the way in which it is calculated.
Edited 2023-06-13 17:20 by Mixtel90

Mick

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Grogster

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Joined: 31/12/2012
Location: New Zealand
Posts: 9308

Posted: 08:50am 13 Jun 2023

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Even in LPFM(Low Power FM), the audio processing is expected to pass via a 15kHz "Brick-wall" LPF of some sort. The idea is to SERIOUSLY attenuate the audio above 15kHz, so that the audio signal cannot screw with the stereo pilot tone at 19kHz, which would affect the receiver's ability to detect a stereo FM transmission.

Without said filter, any HF audio would cause the receiver to switch between stereo and mono at the will of the audio signal. Not a good result on your radio if it is swapping between stereo and mono many times per second.

Filters like this and the MM one from Volhout are a wonderful thing, and have been in use for decades, and there IS a reason for needing and having them.

Smoke makes things work. When the smoke gets out, it stops!

ice2642

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Joined: 27/05/2023
Location: Brazil
Posts: 82

Posted: 12:19am 18 Jun 2023

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Yes, I do not read the silicon chip article, just buy the board, I hope they send instructions with it.

I build the LPF very simple, and it work in simple way too :)

I intend to do another one.

Someone out there calculated the diameter, thickness and quantities of spirals to make the 4.7mf inductor ??? Did you need any iron-type stuffing or did you use air?

Best Regards,
Edited 2023-06-18 10:24 by ice2642

MMBasic 5.0707 on PicoMite VGA

Turbo46

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Joined: 24/12/2017
Location: Australia
Posts: 1611

Posted: 02:50am 18 Jun 2023

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I don't think that anyone has wound their own inductor. They have bought them. Somebody may be able to tell you where to buy the 4.7mH inductors.

This is the original filter circuit from an earlier manual. The 2k2 resistors reduce the output level to a more suitable level.

Bill

Keep safe. Live long and prosper.

stanleyella

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Joined: 25/06/2022
Location: United Kingdom
Posts: 2129

Posted: 03:14pm 18 Jun 2023

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"Someone out there calculated the diameter, thickness and quantities of spirals to make the 4.7mf inductor ??? Did you need any iron-type stuffing or did you use air?"
I got a component tester, one that measures inductors, most do so could hand wind around a 1MR resistor with varnished copper wire from a transformer but I bought a 5 pack of 4.7mH inductors ebay £2 free p&p. Why bother making?

Mixtel90

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Posted: 03:35pm 18 Jun 2023

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An air-cored 4.7mH inductor, using 0.25mm wire is about a 90mm cube. You really don't want to be winding one. :)
You can buy one here

Mick

Zilog Inside! nascom.info for Nascom & Gemini
Preliminary MMBasic docs & my PCB designs

stanleyella

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Joined: 25/06/2022
Location: United Kingdom
Posts: 2129

Posted: 03:45pm 18 Jun 2023

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Mixtel90 said An air-cored 4.7mH inductor, using 0.25mm wire is about a 90mm cube. You really don't want to be winding one. :)
You can buy one here

Ha! Ha!
https://www.ebay.co.uk/itm/223659223294?

ice2642

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Joined: 27/05/2023
Location: Brazil
Posts: 82

Posted: 06:27pm 18 Jun 2023

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Thanks, i will buy one :)

MMBasic 5.0707 on PicoMite VGA

stanleyella

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Joined: 25/06/2022
Location: United Kingdom
Posts: 2129

Posted: 07:25pm 18 Jun 2023

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I am coding a "scope" and connected the sound pin playing tone to adc pin and all I got was a square wave , no filter. I'll try with filter later

Mixtel90

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Location: United Kingdom
Posts: 6798

Posted: 07:42pm 18 Jun 2023

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If that square wave was from an audio pin then, unless you are using PLAY SOUND to get a square wave, it's probably the 44.1kHz carrier. You can't see the audio at all without a filter of some sort because it doesn't exist as audio. :)

Edit:
Ah, I'd missed that you were using a PWM signal for the test. :) That one is square if it's 50%.
Edited 2023-06-19 05:47 by Mixtel90

Mick

Zilog Inside! nascom.info for Nascom & Gemini
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stanleyella

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Joined: 25/06/2022
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Posted: 08:49pm 18 Jun 2023

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Thought so, no hint of sine wave, just carrier , looks like pwm 40KHz.
This is the filtered 10KHz tone
filter is important for sound so regards to the designer

Edited 2023-06-19 07:33 by stanleyella

ice2642

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Joined: 27/05/2023
Location: Brazil
Posts: 82

Posted: 07:28pm 27 Jun 2023

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My picomite vga arrive yesterday, and now it is done and working fine.

I am waiting now the parts for the sound

The quality of the board is very good, but the PS2 role for fixation is missing, I cut of the 2 pins, and ok, just care now when I plug the keyboard.

Stanley , your wav it's show very good.

Best Regards,

MMBasic 5.0707 on PicoMite VGA

stanleyella

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Joined: 25/06/2022
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Posts: 2129

Posted: 01:44pm 28 Jun 2023

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10KHz Tone. My simple scope code to improve. Needs sync. For 160x80 LCD.

dim count%,sync!,sync2!
Dim samples!(160)
Dim old_samples!(160)
PLAY TONE 10000,10000,60000
SETPIN GP26, AIN
'SetPin GP18,pwm1A 'this optional test signal comment out
'PWM 1,5000,50 ' 50% = square wave ,this optional test signal comment out

Pause 100
adc open 500000,1 'samples per second
cls
adc start samples!()'get new samples

for count%=0 to 159
samples!(count%)=(samples!(count%)*20)+10 'scale 3V samples to 0 to 79 pixels
next count%

for count%=0 to 159
old_samples!(count%)=samples!(count%) 'copy samples to old_samples
next count%

for count%=0 to 158
line count%,samples!(count%),count%+1,samples!(count%+1),,rgb(white) 'draw first samples
next count%
'-------- main loop
do
adc start samples!() 'get new samples
'
for count%=0 to 159
samples!(count%)=(samples!(count%)*20)+10 'scale 3V samples to 0 to 79 pixels
next count%
'
for count%=0 to 158
line count%,old_samples!(count%),count%+1,old_samples!(count%+1),,rgb(black) 'erase old_sample
line count%,samples!(count%),count%+1,samples!(count%+1),,rgb(white) 'draw new sample
next count%
'
for count%=0 to 159
old_samples!(count%)=samples!(count%) 'copy samples to old_samples
next count%
loop
end

stanleyella

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Joined: 25/06/2022
Location: United Kingdom
Posts: 2129

Posted: 04:12pm 28 Jun 2023

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vga version

mode 2
dim count%,sync!,sync2!
Dim samples!(320)
Dim old_samples!(320)
PLAY TONE 1000,5000,600000
SETPIN GP26, AIN
'SetPin GP18,pwm1A 'this optional test signal comment out
'PWM 1,5000,50 ' 50% = square wave ,this optional test signal comment out

Pause 100
adc open 500000,1 'samples per second
cls
adc start samples!()'get new samples

for count%=0 to 319
samples!(count%)=(samples!(count%)*60)+10 'scale 3V samples to 0 to 79 pixels
next count%

for count%=0 to 319
old_samples!(count%)=samples!(count%) 'copy samples to old_samples
next count%

for count%=0 to 318
line count%,samples!(count%),count%+1,samples!(count%+1),,rgb(white) 'draw first samples
next count%
'-------- main loop
do
adc start samples!() 'get new samples
'
for count%=0 to 319
samples!(count%)=(samples!(count%)*60)+10 'scale 3V samples to 0 to 79 pixels
next count%
'
for count%=0 to 318
line count%,old_samples!(count%),count%+1,old_samples!(count%+1),,rgb(black) 'erase old_sample
line count%,samples!(count%),count%+1,samples!(count%+1),,rgb(white) 'draw new sample
next count%
'
for count%=0 to 319
old_samples!(count%)=samples!(count%) 'copy samples to old_samples
next count%
pause 1000
loop
end

Volhout
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Joined: 05/03/2018
Location: Netherlands
Posts: 4247

Posted: 04:19pm 28 Jun 2023

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Tip: you can replace the for I=0 to 159 loops with MATH commands.
Math commands can perform offset and gain calculations on the whole array in one instruction
Also copy one array to another.

Volhout

P.s. I think the thread derailed a bit from it’s topic.
Edited 2023-06-29 02:23 by Volhout

PicomiteVGA PETSCII ROBOTS

Mixtel90

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Joined: 05/10/2019
Location: United Kingdom
Posts: 6798

Posted: 04:31pm 28 Jun 2023

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Don't be tempted to connect the input to a different signal source without protecting the PicoMite. This will give it some protection.

3V3
!
---
/ \
!
input-----10k-----+----ADC pin
!
---
/ \
!
!
GND

To be useful you'll probably need to use an op-amp to get some gain as a first stage. (With protection again! You can't win).

Mick

Zilog Inside! nascom.info for Nascom & Gemini
Preliminary MMBasic docs & my PCB designs

Volhout
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Location: Netherlands
Posts: 4247

Posted: 06:39am 29 Jun 2023

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@stanleyella

for count%=0 to 319
samples!(count%)=(samples!(count%)*60)+10 'scale 3V samples to 0 to 79 pixels
next count%

is the same as

math scale samples!(),60,samples!() 'multiply a() by 60
math add samples!(),10,samples!() 'add 10 to a()

and here

or count%=0 to 319
old_samples!(count%)=samples!(count%) 'copy samples to old_samples
next count%

is the same as

math add samples!(),0,old_samples!() 'copies a() to b()

Edited 2023-06-29 16:41 by Volhout

PicomiteVGA PETSCII ROBOTS

stanleyella

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Joined: 25/06/2022
Location: United Kingdom
Posts: 2129

Posted: 02:31pm 29 Jun 2023

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Thank you @Volhout. picomite non vga manual pages 131,132- Math. no mention of math add or math scale? I have a scale function.

matherp
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Joined: 11/12/2012
Location: United Kingdom
Posts: 9129

Posted: 02:35pm 29 Jun 2023

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That's because it is a command and not a function see page 108 in the latest manual

stanleyella

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Joined: 25/06/2022
Location: United Kingdom
Posts: 2129

Posted: 03:20pm 29 Jun 2023

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matherp said That's because it is a command and not a function see page 108 in the latest manual

page 104-107 in my non vga manual which I thought up to date. 10th june 23.
Ta for bringing it to my attention. I want my code to run as fast as possible sometimes. Thanks all. stan...... How many countries got stan in their name?

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