Audio filter force on/off switch

[TomCrazy]

The audio filter on Amiga (power led goes dim=filter off; power led lights up brighter=filter on) is software controlled as you know. I would like to modify both my C= A500 and C= A1200 with a switch with which I can put the audio filter in a "force on" or "force off" state, which will override the state that the software has currently set the filter to.
This could be useful in some games which turn the filter on. I personally HATE the audio filter and can not really understand why Commodore put it there in the first place. (If anyone knows why, then please explain.)

This is probably a very easy modification to perform, but I don't know enough about the Amiga hardware in order to be able to connect the switch to the right locations on the motherboard. And I don't have a hardware manual or pinout manual for the most common chipsets such as Paula, either.

I guess this modification has something to do with the Paula chip. Is there a certain pin on the Paula that controls if the audio filter is on or off? (For example 5V=audio filter on; 0V=audio filter off, or something like that?)
In such case, which pin would that be?

Also, do you guys have any useful links to share or maybe some other suggestions where I can find lots of hardware info on the various Amiga chipsets (like pinouts and other info) so that I could perform simple modifications like this one without having to ask on forums each time or check on the internet exactly what pinout I should access?

All advice are deeply appreciated.

[FOL]

I use the program Audio Filter and it never ever comes on.

[TomCrazy]

FOL:
Thanks for your suggestion, but what I would like to perform is a hardware hack that accesses the components which are controlling the filter and then with a switch control which state the filter is going to be in (on or off).

Using a software solution like you are suggesting might work well in Workbench, but what happens when I want to load a trackloaded game directly from floppy disk?
This is the reason why I want to be able to access the filter by hardware.

[FOL]

Originally Posted by TomCrazy

FOL:
Thanks for your suggestion, but what I would like to perform is a hardware hack that accesses the components which are controlling the filter and then with a switch control which state the filter is going to be in (on or off).

Using a software solution like you are suggesting might work well in Workbench, but what happens when I want to load a trackloaded game directly from floppy disk?
This is the reason why I want to be able to access the filter by hardware.

Floppies. Who uses floppies, .
Indeed it will only work in workbench, I stupidly assumed you used WHDLoad, .

[Photon]

Not all games are in WHDLoad (yet)! But it's the solution if you have a setup that runs it and the game you want to play is in WHDload.

There's an old expansion from 1989 that will let you bypass even the filter-off filtering I just remember it and don't know that name. So you will have to look for it on BBOAH. A friend had it, I think it connected with 3 wires to Paula directly.

But I'm pretty sure you can't hardware-switch the filter on and off without adding additional (sound-converting) hardware (edit: or filter-bypassing soldering / tracecutting). Basically because it's under software control, it's not easily patchable, it's in a register in a chip, it has control.

If there is some reset-vector CLI-command (like AddMem) that removes the filter on reset it could work. If the game doesn't set the filter.

Edit: The reason for the filter in the first place was that Commodore was memory-conscious. The very first version of Amiga had only 64K, an unexpanded A1000 was 256K, and the filter was there to allow low-samplerate samples (and wavetable type synth sounds) to sound acceptable even low rates.

[TomCrazy]

Originally Posted by FOL

Floppies. Who uses floppies, .
Indeed it will only work in workbench, I stupidly assumed you used WHDLoad, .

I have never used WHDLoad so far, but will try it someday soon.
However, back in the ol' days when I was a kid, there was no such thing as WHDLoad. When I got my first Amiga (an A500, A1200 wasn't even released at that time), I booted all my games from floppy. It took a few years before I could get my hands on an A1200 and could install some of my games to harddrive.
So who uses floppies? Well, a guy like me who wants to keep the nostalgic feeling while playing OCS/ECS games may prefer to stick with floppies.


Back on topic:
I found pinouts for Paula on Aminet, but it didn't help me much. According to some other info I found, the low-pass filter consists of a few components on the motherboard.
Here is what I just recently found:

From Wikipedia:
"The analog output is connected to a low-pass filter, which filters out high-frequency aliases when the Amiga is using a lower sampling rate. The brightness of the Amiga's power LED is used to indicate the status of the Amiga’s low-pass filter. The filter is active when the LED is at normal brightness, and deactivated when dimmed (or off on older A500 Amigas). On Amiga 1000 (and very first Amiga 500 and Amiga 2000 model), the power LED had no relation to the filter's status, a wire needed to be manually soldered between pins on the sound chip to disable the filter."

So what is the "sound chip"? Do they refer to Paula here?


A post found on another Amiga forum:
"Are you one of those persons that hear music using the amiga
frequently? Do you have your amiga connected to expensive
hifi-equipment?

Then you WILL hate the lowpass-filter,built into all amigas.
Hearing music with the filter turned on simply sounds terrible.
There is one easy step to do if you want to have the filter
turned off forever (no longer depending on the led's state).

>>> HOW TO DO

Simply remove a transistor from your Amigas mainboard.
If it is mounted using SMD (that means the resistor is
only 2-3 millimeters big and directry soldered to the
mainboard's surface) you have to use a eletronic solder-
iron (that means earthed),otherwise you can simply
cut it off.

Do this only if you have at least some experience in
soldering! Note that SMD-Parts are usually glued to
the mainboard.

To know what transistor to remove,simply look up your
amiga-model in the table below and search for the
transistor on the mainboard that has the right number.
Then remove it.

AMIGA TRANSISTOR NUMBER
----- -----------------
A500 Q301
A500+ Q301
A2000 Q202
CDTV Q4
A600 Q341
A1200 Q341
A4000 Q400

>>> TECHNICAL INFO

The transistor is usually a 2N3905/06 or BC556/727 and is
located somewhere around an IC named LF347.If you dant
want to fully remove the transistor,just isolate one pin.
For shematics see the picture...

>>> DISCLAIMER
Opening an Amiga will normally void warranty!
I am not responsible for any damage caused to your Amiga.

____ __ _____
/ \/ \/_ _/\..........mEmbEr 0f prImItIvE aRtz........
/_/_/_/_/_/\/_/\\/ ....http://home.pages.de/~MNT...pGpkeY..:
\_\_\_\_\_\/\_\/ :......1st Dan Bujinkan Ninjutsu....."


If this info is correct, I should be able to bypass the components that are filtering the sound with a single wire. I guess I can then connect this wire to a switch which will then act as a "force filter off"-switch. Two wires might be needed if there is one low-pass filter for each channel (left and right), but that's no problem since I can then use a 2-pole switch instead.
But I don't know if it's safe to perform this modification. It seems like someone on this other forum ended up with no video signal after removing the specified transistor.


"Recently I have read a hardware hack that said I can remove a transistor from the audiofilter and it will turn it off for ever. I did it, desoldered the correct one. But when I turn on my Amy it gives no video signal, the screen just flashes two times.
If I put a disk into it, it boots and works perfectly without video. Even it gives sound if I put some music."


What do you think? The guy who wrote this post must have slipped with his soldering iron or something, right? Surely you can't ruin any components that has to do with the video display just by removing a transistor?

Btw, it would be even neater if there was a way to tap in on one of the pins of Paula in order to control the filter on/off switching, because then the power led will also change its brightness level according to how I set the switch ('filter force off' or 'normal operation'; when it's in 'force off', I assume that the power led would always be dim, then).
But I guess it's not as simple as that. Maybe I will just have to go with finding the components that are filtering the sound and bypass it. I will check it I can find anything the next time I open up my A1200.

In the meantime, I welcome all advice and suggestions I can get regarding this.

Edit:
I also found this on another board, but it's for the A1000 only, not for A500 or A1200:

"Years ago, I disabled the low-pass filter on my Amiga 1000 by jumpering across the audio buffer at U5G.
I just found another method which involves removing (or clipping) C103 and C104 capacitors."

[TomCrazy]

Originally Posted by Photon

There's an old expansion from 1989 that will let you bypass even the filter-off filtering I just remember it and don't know that name. So you will have to look for it on BBOAH. A friend had it, I think it connected with 3 wires to Paula directly.

I wonder how complex this expansion from 1989 is. Only 3 wires to Paula? Doesn't sound too advanced to me. There might even be a small chance that it's possible to replicate this "expansion" with some knowledge about the architecture of Paula. I will look for it on BBOAH. Thanks for the suggestion.

Originally Posted by Photon

But I'm pretty sure you can't hardware-switch the filter on and off without adding additional (sound-converting) hardware (edit: or filter-bypassing soldering / tracecutting). Basically because it's under software control, it's not easily patchable, it's in a register in a chip, it has control.

If there is some reset-vector CLI-command (like AddMem) that removes the filter on reset it could work. If the game doesn't set the filter.

Ok, but even if it's under software control, the low-pass filter is not in the Paula itself, but consists of external components (probably involving capacitors, resistors and maybe also transistors) on the motherboard, right?
If so, then in theory, it should be possible to bypass-wire the circuit by connecting a wire before the filter and then directly to output, so the sound never travels through the low-pass filter, causing it to be always off (even when the software sets it to ON).
And yeah, I know that it might involve tracecutting in the motherboard, but it should work in theory, right?

[breech]

I'm interested in this too. Some more clues found here, an interesting read on playing mods authentically in windows.

... Paula does no interpolation of any kind. Paula's output is strictly a pulse wave,
produced on 3546895 Hz frequency ... nothing but hard-edged pulses...

... After leaving the chip, the sound enters a 6 dB/oct resistor+capacitor (RC)
low-pass filter tuned at approximately 5 kHz...

... Additionally, it is possible to engage a low-pass 12 dB/oct Butterworth filter tuned at
approximately 3.2 kHz by turning the Amiga power LED brighter with a special
protracker command.

also this:
[ Show youtube player ]

This is over my head

This is also interesting

?

[Bamiga2002]

I think the filter was kinda cool sometimes, giving the sound a distance-like feeling. A matter of taste I think.

[demolition]

Originally Posted by TomCrazy

I found pinouts for Paula on Aminet, but it didn't help me much. According to some other info I found, the low-pass filter consists of a few components on the motherboard.
But I don't know if it's safe to perform this modification. It seems like someone on this other forum ended up with no video signal after removing the specified transistor.
What do you think? The guy who wrote this post must have slipped with his soldering iron or something, right? Surely you can't ruin any components that has to do with the video display just by removing a transistor?
Btw, it would be even neater if there was a way to tap in on one of the pins of Paula in order to control the filter on/off switching, because then the power led will also change its brightness level according to how I set the switch ('filter force off' or 'normal operation'; when it's in 'force off', I assume that the power led would always be dim, then).

The signal controlling the LED and filter comes from U7. If you have a bit of skill with electronics, you could lift pin 3 and use a switch to control whether _LED is controlled by U7 or by yourself (tie _LED to VCC to disable filter).

I can't see how removing that transistor should affect the video signal. He must have removed something else or cut a trace by accident.

Edit: Btw, if you want schematics, they can be found here.

[pandy71]

Originally Posted by TomCrazy

I personally HATE the audio filter and can not really understand why Commodore put it there in the first place. (If anyone knows why, then please explain.)

In Amiga there are at least two filters, one of them can have two characteristics, explanation why first filter must be present is bellow

http://eab.abime.net/showpost.php?p=834052&postcount=18

Second filter is "antialiasing" filter (seems that Amiga 1000 have much better this filter).

Originally Posted by TomCrazy

This is probably a very easy modification to perform, but I don't know enough about the Amiga hardware in order to be able to connect the switch to the right locations on the motherboard. And I don't have a hardware manual or pinout manual for the most common chipsets such as Paula, either.

I guess this modification has something to do with the Paula chip. Is there a certain pin on the Paula that controls if the audio filter is on or off? (For example 5V=audio filter on; 0V=audio filter off, or something like that?)
In such case, which pin would that be?

Something like this and indeed it is quite easy to modify - some "smart" solution can use or simple uC or just only EXOR gate (74LS86).

Originally Posted by TomCrazy

Also, do you guys have any useful links to share or maybe some other suggestions where I can find lots of hardware info on the various Amiga chipsets (like pinouts and other info) so that I could perform simple modifications like this one without having to ask on forums each time or check on the internet exactly what pinout I should access?

All advice are deeply appreciated.

probably Service Manuals, ODD CIA, PIN 3, signal going to MC1488 (for A500+ this IC U38, PIN 2), cut trace close to U38 Pin 2, signal from ODD CIA to one inputs of the gate 7486, second input same gate to 0 or 5V (some 220Ohm resistor and switch ON/OFF can be helpful), gate output to Pin 2 of the U38. More advanced version - some small (8 pin) uC (Tiny AVR), some small program like check input port from ODD CIA, check switch (some TACT switch can be used), do EXOR on remembered state (i.e. one press filter off, next press filter ON etc), output state for filter to Pin 2 U38.

(yes i know how this sound but nowadays uC solution can be easier to build and cheaper than TTL solution)

Good luck don't brake Your Amiga

btw to produce high quality audio from Amiga two conditions must be fulfilled:
first: low pass filter with at least 50 - 60 dB attenuation for 27kHz must exist at the Paula outputs
second: each channel should have variable characteristic low pass filters (with current Paula architecture only digital implementation), filter should follow AUDxPER, filter should provide at least 50dB attenuation.

Not sure on this but perhaps Toni can implemented such functionality in one of WinUAE builds just as a proof of concept...

[hooverphonique]

as said above, the led is controlled by the TTL signal coming from the cia chip - just cut this signal somewhere between the cia and the transistor where it's most convenient (i.e. makes least "damage" to your amiga) and wire in a switch..

I used to have this mod on my old amigas (A500, A2000), but it's mostly needed with old software that doesn't turn the filter off - Nowadays I get by easily without having a manual override switch ;-)

[pandy71]

Originally Posted by hooverphonique

between the cia and the transistor where it's most convenient

TTL level is translated to +-12V! trough RS232 translator in one of MC1488!